7 Expressions [expr]

7.1 Preamble [expr.pre]

[Note 1:

[expr] defines the syntax, order of evaluation, and meaning of expressions.39

An expression is a sequence of operators and operands that specifies a computation.

An expression can result in a value and can cause side effects.

— end note]

[Note 2:

Operators can be overloaded, that is, given meaning when applied to expressions of class type or enumeration type.

Uses of overloaded operators are transformed into function calls as described in [over.oper].

Overloaded operators obey the rules for syntax and evaluation order specified in [expr.compound], but the requirements of operand type and value category are replaced by the rules for function call.

Relations between operators, such as ++a meaning a+=1, are not guaranteed for overloaded operators .

— end note]

Subclause [expr.compound] defines the effects of operators when applied to types for which they have not been overloaded.

Operator overloading shall not modify the rules for the built-in operators, that is, for operators applied to types for which they are defined by this Standard.

However, these built-in operators participate in overload resolution, and as part of that process user-defined conversions will be considered where necessary to convert the operands to types appropriate for the built-in operator.

If a built-in operator is selected, such conversions will be applied to the operands before the operation is considered further according to the rules in [expr.compound]; see [over.match.oper], [over.built].

If during the evaluation of an expression, the result is not mathematically defined or not in the range of representable values for its type, the behavior is undefined.

[Note 3:

Treatment of division by zero, forming a remainder using a zero divisor, and all floating-point exceptions varies among machines, and is sometimes adjustable by a library function.

— end note]

[Note 4:

The implementation can regroup operators according to the usual mathematical rules only where the operators really are associative or commutative.40

For example, in the following fragment int a, b; /* ... */ a = a + 32760 + b + 5; the expression statement behaves exactly the same as a = (((a + 32760) + b) + 5); due to the associativity and precedence of these operators.

Thus, the result of the sum (a + 32760) is next added to b, and that result is then added to 5 which results in the value assigned to a.

On a machine in which overflows produce an exception and in which the range of values representable by an int is [-32768, +32767], the implementation cannot rewrite this expression as a = ((a + b) + 32765); since if the values for a and b were, respectively,

- 32754

and

- 15

, the sum a + b would produce an exception while the original expression would not; nor can the expression be rewritten as either a = ((a + 32765) + b); or a = (a + (b + 32765)); since the values for a and b might have been, respectively, 4 and

- 8

- 17

and 12.

However on a machine in which overflows do not produce an exception and in which the results of overflows are reversible, the above expression statement can be rewritten by the implementation in any of the above ways because the same result will occur.

— end note]

The values of the floating-point operands and the results of floating-point expressions may be represented in greater precision and range than that required by the type; the types are not changed thereby.41

39)39)

The precedence of operators is not directly specified, but it can be derived from the syntax.

40)40)

Overloaded operators are never assumed to be associative or commutative.

41)41)

The cast and assignment operators must still perform their specific conversions as described in [expr.type.conv], [expr.cast], [expr.static.cast] and [expr.ass].

7.2 Properties of expressions [expr.prop]

7.2.1 Value category [basic.lval]

Expressions are categorized according to the taxonomy in Figure 2 .

Figure 2: Expression category taxonomy [fig:basic.lval]

(1.1)
A glvalue is an expression whose evaluation determines the identity of an object or function.
(1.2)
A prvalue is an expression whose evaluation initializes an object or computes the value of an operand of an operator, as specified by the context in which it appears, or an expression that has type cv void.
(1.3)
An xvalue is a glvalue that denotes an object whose resources can be reused (usually because it is near the end of its lifetime).
(1.4)
An lvalue is a glvalue that is not an xvalue.
(1.5)
An rvalue is a prvalue or an xvalue.

Every expression belongs to exactly one of the fundamental categories in this taxonomy: lvalue, xvalue, or prvalue.

This property of an expression is called its value category.

[Note 1:

The discussion of each built-in operator in [expr.compound] indicates the category of the value it yields and the value categories of the operands it expects.

For example, the built-in assignment operators expect that the left operand is an lvalue and that the right operand is a prvalue and yield an lvalue as the result.

User-defined operators are functions, and the categories of values they expect and yield are determined by their parameter and return types.

— end note]

[Note 2:

Historically, lvalues and rvalues were so-called because they could appear on the left- and right-hand side of an assignment (although this is no longer generally true); glvalues are “generalized” lvalues, prvalues are “pure” rvalues, and xvalues are “eXpiring” lvalues.

Despite their names, these terms apply to expressions, not values.

— end note]

[Note 3:

An expression is an xvalue if it is:

(4.1)
a move-eligible id-expression ([expr.prim.id.unqual]),
(4.2)
the result of calling a function, whether implicitly or explicitly, whose return type is an rvalue reference to object type ([expr.call]),
(4.3)
a cast to an rvalue reference to object type ([expr.type.conv], [expr.dynamic.cast], [expr.static.cast], [expr.reinterpret.cast], [expr.const.cast], [expr.cast]),
(4.4)
a subscripting operation with an xvalue array operand ([expr.sub]),
(4.5)
a class member access expression designating a non-static data member of non-reference type in which the object expression is an xvalue ([expr.ref]), or
(4.6)
a .* pointer-to-member expression in which the first operand is an xvalue and the second operand is a pointer to data member ([expr.mptr.oper]).

In general, the effect of this rule is that named rvalue references are treated as lvalues and unnamed rvalue references to objects are treated as xvalues; rvalue references to functions are treated as lvalues whether named or not.

— end note]

[Example 1: struct A { int m; }; A&& operator+(A, A); A&& f(); A a; A&& ar = static_cast<A&&>(a);

The expressions f(), f().m, static_cast<A&&>(a), and a + a are xvalues.

The expression ar is an lvalue.

— end example]

The result of a glvalue is the entity denoted by the expression.

The result of a prvalue is the value that the expression stores into its context; a prvalue that has type cv void has no result.

A prvalue whose result is the value V is sometimes said to have or name the value V.

The result object of a prvalue is the object initialized by the prvalue; a non-discarded prvalue that is used to compute the value of an operand of a built-in operator or a prvalue that has type cv void has no result object.

[Note 4:

Except when the prvalue is the operand of a decltype-specifier, a prvalue of class or array type always has a result object.

For a discarded prvalue that has type other than cv void, a temporary object is materialized; see [expr.context].

— end note]

Whenever a glvalue appears as an operand of an operator that requires a prvalue for that operand, the lvalue-to-rvalue ([conv.lval]), array-to-pointer ([conv.array]), or function-to-pointer ([conv.func]) standard conversions are applied to convert the expression to a prvalue.

[Note 5:

An attempt to bind an rvalue reference to an lvalue is not such a context; see [dcl.init.ref].

— end note]

[Note 6:

Because cv-qualifiers are removed from the type of an expression of non-class type when the expression is converted to a prvalue, an lvalue of type const int can, for example, be used where a prvalue of type int is required.

— end note]

[Note 7:

There are no prvalue bit-fields; if a bit-field is converted to a prvalue ([conv.lval]), a prvalue of the type of the bit-field is created, which might then be promoted ([conv.prom]).

— end note]

Unless otherwise specified ([expr.reinterpret.cast], [expr.const.cast]), whenever a prvalue appears as an operand of an operator that expects a glvalue for that operand, the temporary materialization conversion is applied to convert the expression to an xvalue.

[Note 8:

The discussion of reference initialization in [dcl.init.ref] and of temporaries in [class.temporary] indicates the behavior of lvalues and rvalues in other significant contexts.

— end note]

Unless otherwise indicated ([dcl.type.decltype]), a prvalue shall always have complete type or the void type; if it has a class type or (possibly multidimensional) array of class type, that class shall not be an abstract class ([class.abstract]).

A glvalue shall not have type cv void.

[Note 9:

A glvalue can have complete or incomplete non-void type.

Class and array prvalues can have cv-qualified types; other prvalues always have cv-unqualified types.

See [expr.type].

— end note]

An lvalue is modifiable unless its type is const-qualified or is a function type.

[Note 10:

A program that attempts to modify an object through a nonmodifiable lvalue or through an rvalue is ill-formed ([expr.ass], [expr.post.incr], [expr.pre.incr]).

— end note]

An object of dynamic type

T_{obj}

is type-accessible through a glvalue of type

T_{ref}

T_{ref}

is similar ([conv.qual]) to:

(11.1)
$T_{obj}$ ,
(11.2)
a type that is the signed or unsigned type corresponding to $T_{obj}$ , or
(11.3)
a char, unsigned char, or std::byte type.

If a program attempts to access ([defns.access]) the stored value of an object through a glvalue through which it is not type-accessible, the behavior is undefined.42

If a program invokes a defaulted copy/move constructor or copy/move assignment operator for a union of type U with a glvalue argument that does not denote an object of type cv U within its lifetime, the behavior is undefined.

[Note 11:

In C, an entire object of structure type can be accessed, e.g., using assignment.

By contrast, C++ has no notion of accessing an object of class type through an lvalue of class type.

— end note]

42)42)

The intent of this list is to specify those circumstances in which an object can or cannot be aliased.

7.2.2 Type [expr.type]

If an expression initially has the type “reference to T” ([dcl.ref], [dcl.init.ref]), the type is adjusted to T prior to any further analysis; the value category of the expression is not altered.

Let X be the object or function denoted by the reference.

If a pointer to X would be valid in the context of the evaluation of the expression ([basic.fundamental]), the result designates X; otherwise, the behavior is undefined.

[Note 1:

Before the lifetime of the reference has started or after it has ended, the behavior is undefined (see [basic.life]).

— end note]

If a prvalue initially has the type “cv T”, where T is a cv-unqualified non-class, non-array type, the type of the expression is adjusted to T prior to any further analysis.

The composite pointer type of two operands p1 and p2 having types T1 and T2, respectively, where at least one is a pointer or pointer-to-member type or std::nullptr_t, is:

(3.1)
if both p1 and p2 are null pointer constants, std::nullptr_t;
(3.2)
if either p1 or p2 is a null pointer constant, T2 or T1, respectively;
(3.3)
if T1 or T2 is “pointer to cv1 void” and the other type is “pointer to cv2 T”, where T is an object type or void, “pointer to cv12 void”, where cv12 is the union of cv1 and cv2;
(3.4)
if T1 or T2 is “pointer to noexcept function” and the other type is “pointer to function”, where the function types are otherwise the same, “pointer to function”;
(3.5)
if T1 is “pointer to cv1 C1” and T2 is “pointer to cv2 C2”, where C1 is reference-related to C2 or C2 is reference-related to C1 ([dcl.init.ref]), the qualification-combined type ([conv.qual]) of T1 and T2 or the qualification-combined type of T2 and T1, respectively;
(3.6)
if T1 or T2 is “pointer to member of C1 of type function”, the other type is “pointer to member of C2 of type noexcept function”, and C1 is reference-related to C2 or C2 is reference-related to C1 ([dcl.init.ref]), where the function types are otherwise the same, “pointer to member of C2 of type function” or “pointer to member of C1 of type function”, respectively;
(3.7)
if T1 is “pointer to member of C1 of type cv1 U” and T2 is “pointer to member of C2 of type cv2 U”, for some non-function type U, where C1 is reference-related to C2 or C2 is reference-related to C1 ([dcl.init.ref]), the qualification-combined type of T2 and T1 or the qualification-combined type of T1 and T2, respectively;
(3.8)
if T1 and T2 are similar types ([conv.qual]), the qualification-combined type of T1 and T2;
(3.9)
otherwise, a program that necessitates the determination of a composite pointer type is ill-formed.

[Example 1: typedef void *p; typedef const int *q; typedef int **pi; typedef const int **pci;

The composite pointer type of p and q is “pointer to const void”; the composite pointer type of pi and pci is “pointer to const pointer to const int”.

— end example]

7.2.3 Context dependence [expr.context]

In some contexts, unevaluated operands appear ([expr.prim.req.simple], [expr.prim.req.compound], [expr.typeid], [expr.sizeof], [expr.unary.noexcept], [dcl.type.decltype], [temp.pre], [temp.concept]).

An unevaluated operand is not evaluated.

[Note 1:

In an unevaluated operand, a non-static class member can be named ([expr.prim.id]) and naming of objects or functions does not, by itself, require that a definition be provided ([basic.def.odr]).

An unevaluated operand is considered a full-expression .

— end note]

In some contexts, an expression only appears for its side effects.

Such an expression is called a discarded-value expression.

The array-to-pointer and function-to-pointer standard conversions are not applied.

The lvalue-to-rvalue conversion is applied if and only if the expression is a glvalue of volatile-qualified type and it is one of the following:

(2.1)
( expression ), where expression is one of these expressions,
(2.2)
id-expression ([expr.prim.id]),
(2.3)
subscripting,
(2.4)
class member access,
(2.5)
indirection,
(2.6)
pointer-to-member operation,
(2.7)
conditional expression where both the second and the third operands are one of these expressions, or
(2.8)
comma expression where the right operand is one of these expressions.

[Note 2:

Using an overloaded operator causes a function call; the above covers only operators with built-in meaning.

— end note]

The temporary materialization conversion ([conv.rval]) is applied if the (possibly converted) expression is a prvalue of object type.

[Note 3:

If the original expression is an lvalue of class type, it must have a volatile copy constructor to initialize the temporary object that is the result object of the temporary materialization conversion.

— end note]

The expression is evaluated and its result (if any) is discarded.

7.3 Standard conversions [conv]

7.3.1 General [conv.general]

Standard conversions are implicit conversions with built-in meaning.

[conv] enumerates the full set of such conversions.

A standard conversion sequence is a sequence of standard conversions in the following order:

(1.1)
Zero or one conversion from the following set: lvalue-to-rvalue conversion, array-to-pointer conversion, and function-to-pointer conversion.
(1.2)
Zero or one conversion from the following set: integral promotions, floating-point promotion, integral conversions, floating-point conversions, floating-integral conversions, pointer conversions, pointer-to-member conversions, and boolean conversions.
(1.3)
Zero or one function pointer conversion.
(1.4)
Zero or one qualification conversion.

[Note 1:

A standard conversion sequence can be empty, i.e., it can consist of no conversions.

— end note]

A standard conversion sequence will be applied to an expression if necessary to convert it to an expression having a required destination type and value category.

[Note 2:

Expressions with a given type will be implicitly converted to other types in several contexts:

(2.1)
When used as operands of operators.

The operator's requirements for its operands dictate the destination type ([expr.compound]).
(2.2)
When used in the condition of an if statement ([stmt.if]) or iteration statement ([stmt.iter]).

The destination type is bool.
(2.3)
When used in the expression of a switch statement ([stmt.switch]).

The destination type is integral.
(2.4)
When used as the source expression for an initialization (which includes use as an argument in a function call and use as the expression in a return statement).

The type of the entity being initialized is (generally) the destination type.

See [dcl.init], [dcl.init.ref].

— end note]

An expression E can be implicitly converted to a type T if and only if the declaration T t=E; is well-formed, for some invented temporary variable t ([dcl.init]).

Certain language constructs require that an expression be converted to a Boolean value.

An expression E appearing in such a context is said to be contextually converted to bool and is well-formed if and only if the declaration bool t(E); is well-formed, for some invented temporary variable t ([dcl.init]).

Certain language constructs require conversion to a value having one of a specified set of types appropriate to the construct.

An expression E of class type C appearing in such a context is said to be contextually implicitly converted to a specified type T and is well-formed if and only if E can be implicitly converted to a type T that is determined as follows: C is searched for non-explicit conversion functions whose return type is cv T or reference to cv T such that T is allowed by the context.

There shall be exactly one such T.

The effect of any implicit conversion is the same as performing the corresponding declaration and initialization and then using the temporary variable as the result of the conversion.

The result is an lvalue if T is an lvalue reference type or an rvalue reference to function type ([dcl.ref]), an xvalue if T is an rvalue reference to object type, and a prvalue otherwise.

The expression E is used as a glvalue if and only if the initialization uses it as a glvalue.

[Note 3:

For class types, user-defined conversions are considered as well; see [class.conv].

In general, an implicit conversion sequence ([over.best.ics]) consists of a standard conversion sequence followed by a user-defined conversion followed by another standard conversion sequence.

— end note]

[Note 4:

There are some contexts where certain conversions are suppressed.

For example, the lvalue-to-rvalue conversion is not done on the operand of the unary & operator.

Specific exceptions are given in the descriptions of those operators and contexts.

— end note]

7.3.2 Lvalue-to-rvalue conversion [conv.lval]

A glvalue of a non-function, non-array type T can be converted to a prvalue.43

If T is an incomplete type, a program that necessitates this conversion is ill-formed.

If T is a non-class type, the type of the prvalue is the cv-unqualified version of T.

Otherwise, the type of the prvalue is T.44

When an lvalue-to-rvalue conversion is applied to an expression E, and either

(2.1)
E is not potentially evaluated, or
(2.2)
the evaluation of E results in the evaluation of a member $E_{x}$ of the set of potential results of E, and $E_{x}$ names a variable x that is not odr-used by $E_{x}$ ([basic.def.odr]),

the value contained in the referenced object is not accessed.

[Example 1: struct S { int n; }; auto f() { S x { 1 }; constexpr S y { 2 }; return [&](bool b) { return (b ? y : x).n; }; } auto g = f(); int m = g(false); // undefined behavior: access of x.n outside its lifetime int n = g(true); // OK, does not access y.n — end example]

The result of the conversion is determined according to the following rules:

(3.1)
If T is cv std::nullptr_t, the result is a null pointer constant ([conv.ptr]).

[Note 1:
Since the conversion does not access the object to which the glvalue refers, there is no side effect even if T is volatile-qualified ([intro.execution]), and the glvalue can refer to an inactive member of a union ([class.union]).
— end note]
(3.2)
Otherwise, if T has a class type, the conversion copy-initializes the result object from the glvalue.
(3.3)
Otherwise, if the object to which the glvalue refers contains an invalid pointer value ([basic.stc.dynamic.deallocation]), the behavior is implementation-defined.
(3.4)
Otherwise, if the bits in the value representation of the object to which the glvalue refers are not valid for the object's type, the behavior is undefined.

[Example 2: bool f() { bool b = true; char c = 42; memcpy(&b, &c, 1); return b; // undefined behavior if 42 is not a valid value representation for bool } — end example]
(3.5)
Otherwise, the object indicated by the glvalue is read ([defns.access]).

Let V be the value contained in the object.

If T is an integer type, the prvalue result is the value of type T congruent ([basic.fundamental]) to V, and V otherwise.

[Note 2:

7.3.3 Array-to-pointer conversion [conv.array]

An lvalue or rvalue of type “array of N T” or “array of unknown bound of T” can be converted to a prvalue of type “pointer to T”.

The temporary materialization conversion ([conv.rval]) is applied.

The result is a pointer to the first element of the array.

7.3.4 Function-to-pointer conversion [conv.func]

An lvalue of function type T can be converted to a prvalue of type “pointer to T”.

The result is a pointer to the function.45

45)45)

This conversion never applies to non-static member functions because an lvalue that refers to a non-static member function cannot be obtained.

7.3.5 Temporary materialization conversion [conv.rval]

A prvalue of type T can be converted to an xvalue of type T.

This conversion initializes a temporary object ([class.temporary]) of type T from the prvalue by evaluating the prvalue with the temporary object as its result object, and produces an xvalue denoting the temporary object.

T shall be a complete type.

[Note 1:

If T is a class type (or array thereof), it must have an accessible and non-deleted destructor; see [class.dtor].

— end note]

[Example 1: struct X { int n; }; int k = X().n; // OK, X() prvalue is converted to xvalue — end example]

7.3.6 Qualification conversions [conv.qual]

A qualification-decomposition of a type T is a sequence of

c v_{i}

and

P_{i}

such that T is “

c v_{0}

P_{0}

c v_{1}

P_{1}

⋯

c v_{n - 1}

P_{n - 1}

c v_{n}

U” for n ≥ 0, where each

c v_{i}

is a set of cv-qualifiers ([basic.type.qualifier]), and each

P_{i}

is “pointer to” ([dcl.ptr]), “pointer to member of class

C_{i}

of type” ([dcl.mptr]), “array of

N_{i}

”, or “array of unknown bound of” ([dcl.array]).

P_{i}

designates an array, the cv-qualifiers

c v_{i + 1}

on the element type are also taken as the cv-qualifiers

c v_{i}

of the array.

[Example 1:

The type denoted by the type-id const int ** has three qualification-decompositions, taking U as “int”, as “pointer to const int”, and as “pointer to pointer to const int”.

— end example]

The n-tuple of cv-qualifiers after the first one in the longest qualification-decomposition of T, that is,

c v_{1}, c v_{2}, \dots, c v_{n}

, is called the cv-qualification signature of T.

Two types T1 and T2 are similar if they have qualification-decompositions with the same n such that corresponding

P_{i}

components are either the same or one is “array of

N_{i}

” and the other is “array of unknown bound of”, and the types denoted by U are the same.

The qualification-combined type of two types T1 and T2 is the type T3 similar to T1 whose qualification-decomposition is such that:

(3.1)
for every $i > 0$ , $c v_{i}^{3}$ is the union of $c v_{i}^{1}$ and $c v_{i}^{2}$ ,
(3.2)
if either $P_{i}^{1}$ or $P_{i}^{2}$ is “array of unknown bound of”, $P_{i}^{3}$ is “array of unknown bound of”, otherwise it is $P_{i}^{1}$ , and
(3.3)
if the resulting $c v_{i}^{3}$ is different from $c v_{i}^{1}$ or $c v_{i}^{2}$ , or the resulting $P_{i}^{3}$ is different from $P_{i}^{1}$ or $P_{i}^{2}$ , then const is added to every $c v_{k}^{3}$ for $0 < k < i$ ,

where

c v_{i}^{j}

and

P_{i}^{j}

are the components of the qualification-decomposition of Tj.

A prvalue of type T1 can be converted to type T2 if the qualification-combined type of T1 and T2 is T2.

[Note 1:

If a program could assign a pointer of type T** to a pointer of type const T** (that is, if line #1 below were allowed), a program could inadvertently modify a const object (as it is done on line #2).

For example, int main() { const char c = 'c'; char* pc; const char** pcc = &pc; // #1: not allowed *pcc = &c; *pc = 'C'; // #2: modifies a const object }

— end note]

[Note 2:

Given similar types T1 and T2, this construction ensures that both can be converted to the qualification-combined type of T1 and T2.

— end note]

[Note 3:

A prvalue of type “pointer to cv1 T” can be converted to a prvalue of type “pointer to cv2 T” if “cv2 T” is more cv-qualified than “cv1 T”.

A prvalue of type “pointer to member of X of type cv1 T” can be converted to a prvalue of type “pointer to member of X of type cv2 T” if “cv2 T” is more cv-qualified than “cv1 T”.

— end note]

[Note 4:

Function types (including those used in pointer-to-member-function types) are never cv-qualified ([dcl.fct]).

— end note]

7.3.7 Integral promotions [conv.prom]

For the purposes of [conv.prom], a converted bit-field is a prvalue that is the result of an lvalue-to-rvalue conversion ([conv.lval]) applied to a bit-field ([class.bit]).

A prvalue that is not a converted bit-field and has an integer type other than bool, char8_t, char16_t, char32_t, or wchar_t whose integer conversion rank ([conv.rank]) is less than the rank of int can be converted to a prvalue of type int if int can represent all the values of the source type; otherwise, the source prvalue can be converted to a prvalue of type unsigned int.

A prvalue of an unscoped enumeration type whose underlying type is not fixed can be converted to a prvalue of the first of the following types that can represent all the values of the enumeration ([dcl.enum]): int, unsigned int, long int, unsigned long int, long long int, or unsigned long long int.

If none of the types in that list can represent all the values of the enumeration, a prvalue of an unscoped enumeration type can be converted to a prvalue of the extended integer type with lowest integer conversion rank ([conv.rank]) greater than the rank of long long in which all the values of the enumeration can be represented.

If there are two such extended types, the signed one is chosen.

A prvalue of an unscoped enumeration type whose underlying type is fixed ([dcl.enum]) can be converted to a prvalue of its underlying type.

Moreover, if integral promotion can be applied to its underlying type, a prvalue of an unscoped enumeration type whose underlying type is fixed can also be converted to a prvalue of the promoted underlying type.

[Note 1:

A converted bit-field of enumeration type is treated as any other value of that type for promotion purposes.

— end note]

A converted bit-field of integral type can be converted to a prvalue of type int if int can represent all the values of the bit-field; otherwise, it can be converted to unsigned int if unsigned int can represent all the values of the bit-field.

A prvalue of type char8_t, char16_t, char32_t, or wchar_t ([basic.fundamental]) (including a converted bit-field that was not already promoted to int or unsigned int according to the rules above) can be converted to a prvalue of the first of the following types that can represent all the values of its underlying type: int, unsigned int, long int, unsigned long int, long long int, unsigned long long int, or its underlying type.

A prvalue of type bool can be converted to a prvalue of type int, with false becoming zero and true becoming one.

These conversions are called integral promotions.

7.3.8 Floating-point promotion [conv.fpprom]

A prvalue of type float can be converted to a prvalue of type double.

The value is unchanged.

This conversion is called floating-point promotion.

7.3.9 Integral conversions [conv.integral]

A prvalue of an integer type can be converted to a prvalue of another integer type.

A prvalue of an unscoped enumeration type can be converted to a prvalue of an integer type.

If the destination type is bool, see [conv.bool].

If the source type is bool, the value false is converted to zero and the value true is converted to one.

Otherwise, the result is the unique value of the destination type that is congruent to the source integer modulo

2^{N}

, where N is the width of the destination type.

The conversions allowed as integral promotions are excluded from the set of integral conversions.

7.3.10 Floating-point conversions [conv.double]

A prvalue of floating-point type can be converted to a prvalue of another floating-point type with a greater or equal conversion rank ([conv.rank]).

A prvalue of standard floating-point type can be converted to a prvalue of another standard floating-point type.

If the source value can be exactly represented in the destination type, the result of the conversion is that exact representation.

If the source value is between two adjacent destination values, the result of the conversion is an implementation-defined choice of either of those values.

Otherwise, the behavior is undefined.

The conversions allowed as floating-point promotions are excluded from the set of floating-point conversions.

7.3.11 Floating-integral conversions [conv.fpint]

A prvalue of a floating-point type can be converted to a prvalue of an integer type.

The conversion truncates; that is, the fractional part is discarded.

The behavior is undefined if the truncated value cannot be represented in the destination type.

[Note 1:

If the destination type is bool, see [conv.bool].

— end note]

A prvalue of an integer type or of an unscoped enumeration type can be converted to a prvalue of a floating-point type.

The result is exact if possible.

If the value being converted is in the range of values that can be represented but the value cannot be represented exactly, it is an implementation-defined choice of either the next lower or higher representable value.

[Note 2:

Loss of precision occurs if the integral value cannot be represented exactly as a value of the floating-point type.

— end note]

If the value being converted is outside the range of values that can be represented, the behavior is undefined.

If the source type is bool, the value false is converted to zero and the value true is converted to one.

7.3.12 Pointer conversions [conv.ptr]

A null pointer constant is an integer literal ([lex.icon]) with value zero or a prvalue of type std::nullptr_t.

A null pointer constant can be converted to a pointer type; the result is the null pointer value of that type ([basic.compound]) and is distinguishable from every other value of object pointer or function pointer type.

Such a conversion is called a null pointer conversion.

The conversion of a null pointer constant to a pointer to cv-qualified type is a single conversion, and not the sequence of a pointer conversion followed by a qualification conversion ([conv.qual]).

A null pointer constant of integral type can be converted to a prvalue of type std::nullptr_t.

[Note 1:

The resulting prvalue is not a null pointer value.

— end note]

A prvalue of type “pointer to cv T”, where T is an object type, can be converted to a prvalue of type “pointer to cv void”.

The pointer value ([basic.compound]) is unchanged by this conversion.

A prvalue v of type “pointer to cv D”, where D is a complete class type, can be converted to a prvalue of type “pointer to cv B”, where B is a base class ([class.derived]) of D.

If B is an inaccessible ([class.access]) or ambiguous ([class.member.lookup]) base class of D, a program that necessitates this conversion is ill-formed.

If v is a null pointer value, the result is a null pointer value.

Otherwise, if B is a virtual base class of D and v does not point to an object whose type is similar ([conv.qual]) to D and that is within its lifetime or within its period of construction or destruction ([class.cdtor]), the behavior is undefined.

Otherwise, the result is a pointer to the base class subobject of the derived class object.

7.3.13 Pointer-to-member conversions [conv.mem]

A null pointer constant can be converted to a pointer-to-member type; the result is the null member pointer value of that type and is distinguishable from any pointer to member not created from a null pointer constant.

Such a conversion is called a null member pointer conversion.

The conversion of a null pointer constant to a pointer to member of cv-qualified type is a single conversion, and not the sequence of a pointer-to-member conversion followed by a qualification conversion ([conv.qual]).

A prvalue of type “pointer to member of B of type cv T”, where B is a class type, can be converted to a prvalue of type “pointer to member of D of type cv T”, where D is a complete class derived ([class.derived]) from B.

If B is an inaccessible ([class.access]), ambiguous ([class.member.lookup]), or virtual ([class.mi]) base class of D, or a base class of a virtual base class of D, a program that necessitates this conversion is ill-formed.

If class D does not contain the original member and is not a base class of the class containing the original member, the behavior is undefined.

Otherwise, the result of the conversion refers to the same member as the pointer to member before the conversion took place, but it refers to the base class member as if it were a member of the derived class.

The result refers to the member in D's instance of B.

Since the result has type “pointer to member of D of type cv T”, indirection through it with a D object is valid.

The result is the same as if indirecting through the pointer to member of B with the B subobject of D.

The null member pointer value is converted to the null member pointer value of the destination type.46

46)46)

The rule for conversion of pointers to members (from pointer to member of base to pointer to member of derived) appears inverted compared to the rule for pointers to objects (from pointer to derived to pointer to base) ([conv.ptr], [class.derived]).

This inversion is necessary to ensure type safety.

Note that a pointer to member is not an object pointer or a function pointer and the rules for conversions of such pointers do not apply to pointers to members.

In particular, a pointer to member cannot be converted to a void*.

7.3.14 Function pointer conversions [conv.fctptr]

A prvalue of type “pointer to noexcept function” can be converted to a prvalue of type “pointer to function”.

The result is a pointer to the function.

A prvalue of type “pointer to member of type noexcept function” can be converted to a prvalue of type “pointer to member of type function”.

The result designates the member function.

[Example 1: void (*p)(); void (**pp)() noexcept = &p; // error: cannot convert to pointer to noexcept function struct S { typedef void (*p)(); operator p(); }; void (*q)() noexcept = S(); // error: cannot convert to pointer to noexcept function — end example]

7.3.15 Boolean conversions [conv.bool]

A prvalue of arithmetic, unscoped enumeration, pointer, or pointer-to-member type can be converted to a prvalue of type bool.

A zero value, null pointer value, or null member pointer value is converted to false; any other value is converted to true.

7.4 Usual arithmetic conversions [expr.arith.conv]

Many binary operators that expect operands of arithmetic or enumeration type cause conversions and yield result types in a similar way.

The purpose is to yield a common type, which is also the type of the result.

This pattern is called the usual arithmetic conversions, which are defined as follows:

(1.1)
The lvalue-to-rvalue conversion ([conv.lval]) is applied to each operand and the resulting prvalues are used in place of the original operands for the remainder of this section.
(1.2)
If either operand is of scoped enumeration type ([dcl.enum]), no conversions are performed; if the other operand does not have the same type, the expression is ill-formed.
(1.3)
Otherwise, if one operand is of enumeration type and the other operand is of a different enumeration type or a floating-point type, the expression is ill-formed.
(1.4)
Otherwise, if either operand is of floating-point type, the following rules are applied:
- (1.4.1)
  If both operands have the same type, no further conversion is performed.
- (1.4.2)
  Otherwise, if one of the operands is of a non-floating-point type, that operand is converted to the type of the operand with the floating-point type.
- (1.4.3)
  Otherwise, if the floating-point conversion ranks ([conv.rank]) of the types of the operands are ordered but not equal, then the operand of the type with the lesser floating-point conversion rank is converted to the type of the other operand.
- (1.4.4)
  Otherwise, if the floating-point conversion ranks of the types of the operands are equal, then the operand with the lesser floating-point conversion subrank ([conv.rank]) is converted to the type of the other operand.
- (1.4.5)
  Otherwise, the expression is ill-formed.
(1.5)
Otherwise, each operand is converted to a common type C.

The integral promotion rules ([conv.prom]) are used to determine a type T1 and type T2 for each operand.47
Then the following rules are applied to determine C:
- (1.5.1)
  If T1 and T2 are the same type, C is that type.
- (1.5.2)
  Otherwise, if T1 and T2 are both signed integer types or are both unsigned integer types, C is the type with greater rank.
- (1.5.3)
  Otherwise, let U be the unsigned integer type and S be the signed integer type.
  
  (1.5.3.1)
  If U has rank greater than or equal to the rank of S, C is U.
  (1.5.3.2)
  Otherwise, if S can represent all of the values of U, C is S.
  (1.5.3.3)
  Otherwise, C is the unsigned integer type corresponding to S.

47)47)

As a consequence, operands of type bool, char8_t, char16_t, char32_t, wchar_t, or of enumeration type are converted to some integral type.

7.5 Primary expressions [expr.prim]

7.5.1 Grammar [expr.prim.grammar]

primary-expression:
literal
this
( expression )
id-expression
lambda-expression
fold-expression
requires-expression

7.5.2 Literals [expr.prim.literal]

The type of a literal is determined based on its form as specified in [lex.literal].

A string-literal is an lvalue designating a corresponding string literal object ([lex.string]), a user-defined-literal has the same value category as the corresponding operator call expression described in [lex.ext], and any other literal is a prvalue.

7.5.3 This [expr.prim.this]

The keyword this names a pointer to the object for which an implicit object member function ([class.mfct.non.static]) is invoked or a non-static data member's initializer ([class.mem]) is evaluated.

The current class at a program point is the class associated with the innermost class scope containing that point.

[Note 1:

A lambda-expression does not introduce a class scope.

— end note]

If a declaration declares a member function or member function template of a class X, the expression this is a prvalue of type “pointer to cv-qualifier-seq X” wherever X is the current class between the optional cv-qualifier-seq and the end of the function-definition, member-declarator, or declarator.

It shall not appear within the declaration of a static or explicit object member function of the current class (although its type and value category are defined within such member functions as they are within an implicit object member function).

[Note 2:

This is because declaration matching does not occur until the complete declarator is known.

— end note]

[Note 3:

In a trailing-return-type, the class being defined is not required to be complete for purposes of class member access .

Class members declared later are not visible.

[Example 1: struct A { char g(); template<class T> auto f(T t) -> decltype(t + g()) { return t + g(); } }; template auto A::f(int t) -> decltype(t + g()); — end example]

— end note]

Otherwise, if a member-declarator declares a non-static data member ([class.mem]) of a class X, the expression this is a prvalue of type “pointer to X” wherever X is the current class within the optional default member initializer ([class.mem]).

The expression this shall not appear in any other context.

[Example 2: class Outer { int a[sizeof(*this)]; // error: not inside a member function unsigned int sz = sizeof(*this); // OK, in default member initializer void f() { int b[sizeof(*this)]; // OK struct Inner { int c[sizeof(*this)]; // error: not inside a member function of Inner }; } }; — end example]

7.5.4 Parentheses [expr.prim.paren]

A parenthesized expression (E) is a primary expression whose type, result, and value category are identical to those of E.

The parenthesized expression can be used in exactly the same contexts as those where E can be used, and with the same meaning, except as otherwise indicated.

7.5.5 Names [expr.prim.id]

7.5.5.1 General [expr.prim.id.general]

id-expression:
unqualified-id
qualified-id
pack-index-expression

An id-expression is a restricted form of a primary-expression.

[Note 1:

An id-expression can appear after . and -> operators .

— end note]

If an id-expression E denotes a non-static non-type member of some class C at a point where the current class ([expr.prim.this]) is X and

(2.1)
E is potentially evaluated or C is X or a base class of X, and
(2.2)
E is not the id-expression of a class member access expression ([expr.ref]), and
(2.3)
if E is a qualified-id, E is not the un-parenthesized operand of the unary & operator ([expr.unary.op]),

the id-expression is transformed into a class member access expression using (*this) as the object expression.

[Note 2:

If C is not X or a base class of X, the class member access expression is ill-formed.

Also, if the id-expression occurs within a static or explicit object member function, the class member access is ill-formed.

— end note]

This transformation does not apply in the template definition context ([temp.dep.type]).

If an id-expression E denotes a member M of an anonymous union ([class.union.anon]) U:

(3.1)
If U is a non-static data member, E refers to M as a member of the lookup context of the terminal name of E (after any implicit transformation to a class member access expression).

[Example 1:
o.x is interpreted as o.u.x, where u names the anonymous union member.
— end example]
(3.2)
Otherwise, E is interpreted as a class member access ([expr.ref]) that designates the member subobject M of the anonymous union variable for U.

[Note 3:
Under this interpretation, E no longer denotes a non-static data member.
— end note]

[Example 2:
N::x is interpreted as N::u.x, where u names the anonymous union variable.
— end example]

An id-expression that denotes a non-static data member or implicit object member function of a class can only be used:

(4.1)
as part of a class member access (after any implicit transformation (see above)) in which the object expression refers to the member's class or a class derived from that class, or
(4.2)
to form a pointer to member ([expr.unary.op]), or
(4.3)
if that id-expression denotes a non-static data member and it appears in an unevaluated operand.
[Example 3: struct S { int m; }; int i = sizeof(S::m); // OK int j = sizeof(S::m + 42); // OK — end example]

For an id-expression that denotes an overload set, overload resolution is performed to select a unique function ([over.match], [over.over]).

[Note 4:

A program cannot refer to a function with a trailing requires-clause whose constraint-expression is not satisfied, because such functions are never selected by overload resolution.

[Example 4: template<typename T> struct A { static void f(int) requires false; }; void g() { A<int>::f(0); // error: cannot call f void (*p1)(int) = A<int>::f; // error: cannot take the address of f decltype(A<int>::f)* p2 = nullptr; // error: the type decltype(A<int>::f) is invalid }

In each case, the constraints of f are not satisfied.

In the declaration of p2, those constraints need to be satisfied even though f is an unevaluated operand .

— end example]

— end note]

7.5.5.2 Unqualified names [expr.prim.id.unqual]

unqualified-id:
identifier
operator-function-id
conversion-function-id
literal-operator-id
~ type-name
~ computed-type-specifier
template-id

An identifier is only an id-expression if it has been suitably declared ([dcl.dcl]) or if it appears as part of a declarator-id ([dcl.decl]).

An identifier that names a coroutine parameter refers to the copy of the parameter ([dcl.fct.def.coroutine]).

[Note 1:

For operator-function-ids, see [over.oper]; for conversion-function-ids, see [class.conv.fct]; for literal-operator-ids, see [over.literal]; for template-ids, see [temp.names].

A type-name or computed-type-specifier prefixed by ~ denotes the destructor of the type so named; see [expr.prim.id.dtor].

— end note]

A component name of an unqualified-id U is

(2.1)
U if it is a name or
(2.2)
the component name of the template-id or type-name of U, if any.

[Note 2:

Other constructs that contain names to look up can have several component names ([expr.prim.id.qual], [dcl.type.simple], [dcl.type.elab], [dcl.mptr], [namespace.udecl], [temp.param], [temp.names], [temp.res]).

— end note]

The terminal name of a construct is the component name of that construct that appears lexically last.

The result is the entity denoted by the unqualified-id ([basic.lookup.unqual]).

If the unqualified-id appears in a lambda-expression at program point P and the entity is a local entity ([basic.pre]) or a variable declared by an init-capture ([expr.prim.lambda.capture]), then let S be the compound-statement of the innermost enclosing lambda-expression of P.

If naming the entity from outside of an unevaluated operand within S would refer to an entity captured by copy in some intervening lambda-expression, then let E be the innermost such lambda-expression.

(3.1)
If there is such a lambda-expression and if P is in E's function parameter scope but not its parameter-declaration-clause, then the type of the expression is the type of a class member access expression ([expr.ref]) naming the non-static data member that would be declared for such a capture in the object parameter ([dcl.fct]) of the function call operator of E.

[Note 3:
If E is not declared mutable, the type of such an identifier will typically be const qualified.
— end note]
(3.2)
Otherwise (if there is no such lambda-expression or if P either precedes E's function parameter scope or is in E's parameter-declaration-clause), the type of the expression is the type of the result.

If the entity is a template parameter object for a template parameter of type T ([temp.param]), the type of the expression is const T.

In all other cases, the type of the expression is the type of the entity.

[Note 4:

The type will be adjusted as described in [expr.type] if it is cv-qualified or is a reference type.

— end note]

The expression is an xvalue if it is move-eligible (see below); an lvalue if the entity is a function, variable, structured binding, data member, or template parameter object; and a prvalue otherwise ([basic.lval]); it is a bit-field if the identifier designates a bit-field.

[Example 1: void f() { float x, &r = x; [=]() -> decltype((x)) { // lambda returns float const& because this lambda is not mutable and // x is an lvalue decltype(x) y1; // y1 has type float decltype((x)) y2 = y1; // y2 has type float const& decltype(r) r1 = y1; // r1 has type float& decltype((r)) r2 = y2; // r2 has type float const& return y2; }; [=](decltype((x)) y) { decltype((x)) z = x; // OK, y has type float&, z has type float const& }; [=] { [](decltype((x)) y) {}; // OK, lambda takes a parameter of type float const& [x=1](decltype((x)) y) { decltype((x)) z = x; // OK, y has type int&, z has type int const& }; }; } — end example]

An implicitly movable entity is a variable of automatic storage duration that is either a non-volatile object or an rvalue reference to a non-volatile object type.

In the following contexts, an id-expression is move-eligible:

(4.1)
If the id-expression (possibly parenthesized) is the operand of a return ([stmt.return]) or co_return ([stmt.return.coroutine]) statement, and names an implicitly movable entity declared in the body or parameter-declaration-clause of the innermost enclosing function or lambda-expression, or
(4.2)
if the id-expression (possibly parenthesized) is the operand of a throw-expression ([expr.throw]), and names an implicitly movable entity that belongs to a scope that does not contain the compound-statement of the innermost lambda-expression, try-block, or function-try-block (if any) whose compound-statement or ctor-initializer contains the throw-expression.

7.5.5.3 Qualified names [expr.prim.id.qual]

qualified-id:
nested-name-specifier template

_{o p t}

unqualified-id

nested-name-specifier:
::
type-name ::
namespace-name ::
computed-type-specifier ::
nested-name-specifier identifier ::
nested-name-specifier template

_{o p t}

simple-template-id ::

The component names of a qualified-id are those of its nested-name-specifier and unqualified-id.

The component names of a nested-name-specifier are its identifier (if any) and those of its type-name, namespace-name, simple-template-id, and/or nested-name-specifier.

A nested-name-specifier is declarative if it is part of

(2.1)
a class-head-name,
(2.2)
an enum-head-name,
(2.3)
a qualified-id that is the id-expression of a declarator-id, or
(2.4)
a declarative nested-name-specifier.

A declarative nested-name-specifier shall not have a computed-type-specifier.

A declaration that uses a declarative nested-name-specifier shall be a friend declaration or inhabit a scope that contains the entity being redeclared or specialized.

The nested-name-specifier :: nominates the global namespace.

A nested-name-specifier with a computed-type-specifier nominates the type denoted by the computed-type-specifier, which shall be a class or enumeration type.

If a nested-name-specifier N is declarative and has a simple-template-id with a template argument list A that involves a template parameter, let T be the template nominated by N without A.

T shall be a class template.

(3.1)
If A is the template argument list ([temp.arg]) of the corresponding template-head H ([temp.mem]), N nominates the primary template of T; H shall be equivalent to the template-head of T ([temp.over.link]).
(3.2)
Otherwise, N nominates the partial specialization ([temp.spec.partial]) of T whose template argument list is equivalent to A ([temp.over.link]); the program is ill-formed if no such partial specialization exists.

Any other nested-name-specifier nominates the entity denoted by its type-name, namespace-name, identifier, or simple-template-id.

If the nested-name-specifier is not declarative, the entity shall not be a template.

A qualified-id shall not be of the form nested-name-specifier template

_{o p t}

~ computed-type-specifier nor of the form computed-type-specifier :: ~ type-name.

The result of a qualified-id Q is the entity it denotes ([basic.lookup.qual]).

The type of the expression is the type of the result.

The result is an lvalue if the member is

(5.1)
a function other than a non-static member function,
(5.2)
a non-static member function if Q is the operand of a unary & operator,
(5.3)
a variable,
(5.4)
a structured binding ([dcl.struct.bind]), or
(5.5)
a data member,

and a prvalue otherwise.

7.5.5.4 Pack indexing expression [expr.prim.pack.index]

pack-index-expression:
id-expression ... [ constant-expression ]

The id-expression P in a pack-index-expression shall be an identifier that denotes a pack.

The constant-expression shall be a converted constant expression ([expr.const]) of type std::size_t whose value V, termed the index, is such that

0 \leq V < sizeof...(P)

A pack-index-expression is a pack expansion ([temp.variadic]).

[Note 1:

A pack-index-expression denotes the

V^{th}

element of the pack.

— end note]

7.5.5.5 Destruction [expr.prim.id.dtor]

An id-expression that denotes the destructor of a type T names the destructor of T if T is a class type ([class.dtor]), otherwise the id-expression is said to name a pseudo-destructor.

If the id-expression names a pseudo-destructor, T shall be a scalar type and the id-expression shall appear as the right operand of a class member access ([expr.ref]) that forms the postfix-expression of a function call ([expr.call]).

[Note 1:

Such a call ends the lifetime of the object ([expr.call], [basic.life]).

— end note]

[Example 1: struct C { }; void f() { C * pc = new C; using C2 = C; pc->C::~C2(); // OK, destroys *pc C().C::~C(); // undefined behavior: temporary of type C destroyed twice using T = int; 0 .T::~T(); // OK, no effect 0.T::~T(); // error: 0.T is a user-defined-floating-point-literal ([lex.ext]) } — end example]

7.5.6 Lambda expressions [expr.prim.lambda]

7.5.6.1 General [expr.prim.lambda.general]

lambda-expression:
lambda-introducer attribute-specifier-seq

_{o p t}

lambda-declarator compound-statement
lambda-introducer < template-parameter-list > requires-clause

_{o p t}

attribute-specifier-seq

_{o p t}

lambda-declarator compound-statement

lambda-introducer:
[ lambda-capture

_{o p t}

]

lambda-declarator:
lambda-specifier-seq noexcept-specifier

_{o p t}

attribute-specifier-seq

_{o p t}

trailing-return-type

_{o p t}

noexcept-specifier attribute-specifier-seq

_{o p t}

trailing-return-type

_{o p t}

trailing-return-type

_{o p t}

( parameter-declaration-clause ) lambda-specifier-seq

_{o p t}

noexcept-specifier

_{o p t}

attribute-specifier-seq

_{o p t}

trailing-return-type

_{o p t}

requires-clause

_{o p t}

lambda-specifier:
consteval
constexpr
mutable
static

lambda-specifier-seq:
lambda-specifier lambda-specifier-seq

_{o p t}

A lambda-expression provides a concise way to create a simple function object.

[Example 1: #include <algorithm> #include <cmath> void abssort(float* x, unsigned N) { std::sort(x, x + N, [](float a, float b) { return std::abs(a) < std::abs(b); }); } — end example]

A lambda-expression is a prvalue whose result object is called the closure object.

[Note 1:

A closure object behaves like a function object .

— end note]

An ambiguity can arise because a requires-clause can end in an attribute-specifier-seq, which collides with the attribute-specifier-seq in lambda-expression.

In such cases, any attributes are treated as attribute-specifier-seq in lambda-expression.

[Note 2:

Such ambiguous cases cannot have valid semantics because the constraint expression would not have type bool.

[Example 2: auto x = []<class T> requires T::operator int [[some_attribute]] (int) { } — end example]

— end note]

A lambda-specifier-seq shall contain at most one of each lambda-specifier and shall not contain both constexpr and consteval.

If the lambda-declarator contains an explicit object parameter ([dcl.fct]), then no lambda-specifier in the lambda-specifier-seq shall be mutable or static.

The lambda-specifier-seq shall not contain both mutable and static.

If the lambda-specifier-seq contains static, there shall be no lambda-capture.

[Note 3:

The trailing requires-clause is described in [dcl.decl].

— end note]

A lambda-expression's parameter-declaration-clause is the parameter-declaration-clause of the lambda-expression's lambda-declarator, if any, or empty otherwise.

If the lambda-declarator does not include a trailing-return-type, it is considered to be -> auto.

[Note 4:

In that case, the return type is deduced from return statements as described in [dcl.spec.auto].

— end note]

[Example 3: auto x1 = [](int i) { return i; }; // OK, return type is int auto x2 = []{ return { 1, 2 }; }; // error: deducing return type from braced-init-list int j; auto x3 = [&]()->auto&& { return j; }; // OK, return type is int& — end example]

A lambda is a generic lambda if the lambda-expression has any generic parameter type placeholders ([dcl.spec.auto]), or if the lambda has a template-parameter-list.

[Example 4: auto x = [](int i, auto a) { return i; }; // OK, a generic lambda auto y = [](this auto self, int i) { return i; }; // OK, a generic lambda auto z = []<class T>(int i) { return i; }; // OK, a generic lambda — end example]

7.5.6.2 Closure types [expr.prim.lambda.closure]

The type of a lambda-expression (which is also the type of the closure object) is a unique, unnamed non-union class type, called the closure type, whose properties are described below.

The closure type is declared in the smallest block scope, class scope, or namespace scope that contains the corresponding lambda-expression.

[Note 1:

This determines the set of namespaces and classes associated with the closure type ([basic.lookup.argdep]).

The parameter types of a lambda-declarator do not affect these associated namespaces and classes.

— end note]

The closure type is not an aggregate type ([dcl.init.aggr]); it is a structural type ([temp.param]) if and only if the lambda has no lambda-capture.

An implementation may define the closure type differently from what is described below provided this does not alter the observable behavior of the program other than by changing:

(3.1)
the size and/or alignment of the closure type,
(3.2)
whether the closure type is trivially copyable ([class.prop]), or
(3.3)
whether the closure type is a standard-layout class ([class.prop]).

An implementation shall not add members of rvalue reference type to the closure type.

The closure type for a lambda-expression has a public inline function call operator (for a non-generic lambda) or function call operator template (for a generic lambda) ([over.call]) whose parameters and return type are those of the lambda-expression's parameter-declaration-clause and trailing-return-type respectively, and whose template-parameter-list consists of the specified template-parameter-list, if any.

The requires-clause of the function call operator template is the requires-clause immediately following < template-parameter-list >, if any.

The trailing requires-clause of the function call operator or operator template is the requires-clause of the lambda-declarator, if any.

[Note 2:

The function call operator template for a generic lambda can be an abbreviated function template ([dcl.fct]).

— end note]

[Example 1: auto glambda = [](auto a, auto&& b) { return a < b; }; bool b = glambda(3, 3.14); // OK auto vglambda = [](auto printer) { return [=](auto&& ... ts) { // OK, ts is a function parameter pack printer(std::forward<decltype(ts)>(ts)...); return [=]() { printer(ts ...); }; }; }; auto p = vglambda( [](auto v1, auto v2, auto v3) { std::cout << v1 << v2 << v3; } ); auto q = p(1, 'a', 3.14); // OK, outputs 1a3.14 q(); // OK, outputs 1a3.14 auto fact = [](this auto self, int n) -> int { // OK, explicit object parameter return (n <= 1) ? 1 : n * self(n-1); }; std::cout << fact(5); // OK, outputs 120 — end example]

Given a lambda with a lambda-capture, the type of the explicit object parameter, if any, of the lambda's function call operator (possibly instantiated from a function call operator template) shall be either:

(5.1)
the closure type,
(5.2)
a class type publicly and unambiguously derived from the closure type, or
(5.3)
a reference to a possibly cv-qualified such type.

[Example 2: struct C { template <typename T> C(T); }; void func(int i) { int x = [=](this auto&&) { return i; }(); // OK int y = [=](this C) { return i; }(); // error int z = [](this C) { return 42; }(); // OK } — end example]

The function call operator or operator template is a static member function or static member function template ([class.static.mfct]) if the lambda-expression's parameter-declaration-clause is followed by static.

Otherwise, it is a non-static member function or member function template ([class.mfct.non.static]) that is declared const ([class.mfct.non.static]) if and only if the lambda-expression's parameter-declaration-clause is not followed by mutable and the lambda-declarator does not contain an explicit object parameter.

It is neither virtual nor declared volatile.

Any noexcept-specifier specified on a lambda-expression applies to the corresponding function call operator or operator template.

An attribute-specifier-seq in a lambda-declarator appertains to the type of the corresponding function call operator or operator template.

An attribute-specifier-seq in a lambda-expression preceding a lambda-declarator appertains to the corresponding function call operator or operator template.

The function call operator or any given operator template specialization is a constexpr function if either the corresponding lambda-expression's parameter-declaration-clause is followed by constexpr or consteval, or it is constexpr-suitable ([dcl.constexpr]).

It is an immediate function ([dcl.constexpr]) if the corresponding lambda-expression's parameter-declaration-clause is followed by consteval.

[Example 3: auto ID = [](auto a) { return a; }; static_assert(ID(3) == 3); // OK struct NonLiteral { NonLiteral(int n) : n(n) { } int n; }; static_assert(ID(NonLiteral{3}).n == 3); // error — end example]

[Example 4: auto monoid = [](auto v) { return [=] { return v; }; }; auto add = [](auto m1) constexpr { auto ret = m1(); return [=](auto m2) mutable { auto m1val = m1(); auto plus = [=](auto m2val) mutable constexpr { return m1val += m2val; }; ret = plus(m2()); return monoid(ret); }; }; constexpr auto zero = monoid(0); constexpr auto one = monoid(1); static_assert(add(one)(zero)() == one()); // OK // Since two below is not declared constexpr, an evaluation of its constexpr member function call operator // cannot perform an lvalue-to-rvalue conversion on one of its subobjects (that represents its capture) // in a constant expression. auto two = monoid(2); assert(two() == 2); // OK, not a constant expression. static_assert(add(one)(one)() == two()); // error: two() is not a constant expression static_assert(add(one)(one)() == monoid(2)()); // OK — end example]

[Note 3:

The function call operator or operator template can be constrained ([temp.constr.decl]) by a type-constraint ([temp.param]), a requires-clause ([temp.pre]), or a trailing requires-clause ([dcl.decl]).

[Example 5: template <typename T> concept C1 = /* ... */; template <std::size_t N> concept C2 = /* ... */; template <typename A, typename B> concept C3 = /* ... */; auto f = []<typename T1, C1 T2> requires C2<sizeof(T1) + sizeof(T2)> (T1 a1, T1 b1, T2 a2, auto a3, auto a4) requires C3<decltype(a4), T2> { // T2 is constrained by a type-constraint. // T1 and T2 are constrained by a requires-clause, and // T2 and the type of a4 are constrained by a trailing requires-clause. }; — end example]

— end note]

The closure type for a non-generic lambda-expression with no lambda-capture and no explicit object parameter ([dcl.fct]) whose constraints (if any) are satisfied has a conversion function to pointer to function with C++ language linkage having the same parameter and return types as the closure type's function call operator.

The conversion is to “pointer to noexcept function” if the function call operator has a non-throwing exception specification.

If the function call operator is a static member function, then the value returned by this conversion function is a pointer to the function call operator.

Otherwise, the value returned by this conversion function is a pointer to a function F that, when invoked, has the same effect as invoking the closure type's function call operator on a default-constructed instance of the closure type.

F is a constexpr function if the function call operator is a constexpr function and is an immediate function if the function call operator is an immediate function.

For a generic lambda with no lambda-capture and no explicit object parameter ([dcl.fct]), the closure type has a conversion function template to pointer to function.

The conversion function template has the same invented template parameter list, and the pointer to function has the same parameter types, as the function call operator template.

The return type of the pointer to function shall behave as if it were a decltype-specifier denoting the return type of the corresponding function call operator template specialization.

[Note 4:

If the generic lambda has no trailing-return-type or the trailing-return-type contains a placeholder type, return type deduction of the corresponding function call operator template specialization has to be done.

The corresponding specialization is that instantiation of the function call operator template with the same template arguments as those deduced for the conversion function template.

Consider the following: auto glambda = [](auto a) { return a; }; int (*fp)(int) = glambda;

The behavior of the conversion function of glambda above is like that of the following conversion function: struct Closure { template<class T> auto operator()(T t) const { /* ... */ } template<class T> static auto lambda_call_operator_invoker(T a) { // forwards execution to operator()(a) and therefore has // the same return type deduced /* ... */ } template<class T> using fptr_t = decltype(lambda_call_operator_invoker(declval<T>())) (*)(T); template<class T> operator fptr_t<T>() const { return &lambda_call_operator_invoker; } };

— end note]

[Example 6: void f1(int (*)(int)) { } void f2(char (*)(int)) { } void g(int (*)(int)) { } // #1 void g(char (*)(char)) { } // #2 void h(int (*)(int)) { } // #3 void h(char (*)(int)) { } // #4 auto glambda = [](auto a) { return a; }; f1(glambda); // OK f2(glambda); // error: ID is not convertible g(glambda); // error: ambiguous h(glambda); // OK, calls #3 since it is convertible from ID int& (*fpi)(int*) = [](auto* a) -> auto& { return *a; }; // OK — end example]

If the function call operator template is a static member function template, then the value returned by any given specialization of this conversion function template is a pointer to the corresponding function call operator template specialization.

Otherwise, the value returned by any given specialization of this conversion function template is a pointer to a function F that, when invoked, has the same effect as invoking the generic lambda's corresponding function call operator template specialization on a default-constructed instance of the closure type.

F is a constexpr function if the corresponding specialization is a constexpr function and F is an immediate function if the function call operator template specialization is an immediate function.

[Note 5:

This will result in the implicit instantiation of the generic lambda's body.

The instantiated generic lambda's return type and parameter types need to match the return type and parameter types of the pointer to function.

— end note]

[Example 7: auto GL = [](auto a) { std::cout << a; return a; }; int (*GL_int)(int) = GL; // OK, through conversion function template GL_int(3); // OK, same as GL(3) — end example]

The conversion function or conversion function template is public, constexpr, non-virtual, non-explicit, const, and has a non-throwing exception specification .

[Example 8: auto Fwd = [](int (*fp)(int), auto a) { return fp(a); }; auto C = [](auto a) { return a; }; static_assert(Fwd(C,3) == 3); // OK // No specialization of the function call operator template can be constexpr (due to the local static). auto NC = [](auto a) { static int s; return a; }; static_assert(Fwd(NC,3) == 3); // error — end example]

The lambda-expression's compound-statement yields the function-body ([dcl.fct.def]) of the function call operator, but it is not within the scope of the closure type.

[Example 9: struct S1 { int x, y; int operator()(int); void f() { [=]()->int { return operator()(this->x + y); // equivalent to S1::operator()(this->x + (*this).y) // this has type S1* }; } }; — end example]

Further, a variable __func__ is implicitly defined at the beginning of the compound-statement of the lambda-expression, with semantics as described in [dcl.fct.def.general].

The closure type associated with a lambda-expression has no default constructor if the lambda-expression has a lambda-capture and a defaulted default constructor otherwise.

It has a defaulted copy constructor and a defaulted move constructor ([class.copy.ctor]).

It has a deleted copy assignment operator if the lambda-expression has a lambda-capture and defaulted copy and move assignment operators otherwise ([class.copy.assign]).

[Note 6:

These special member functions are implicitly defined as usual, which can result in them being defined as deleted.

— end note]

The closure type associated with a lambda-expression has an implicitly-declared destructor ([class.dtor]).

A member of a closure type shall not be explicitly instantiated, explicitly specialized, or named in a friend declaration .

7.5.6.3 Captures [expr.prim.lambda.capture]

lambda-capture:
capture-default
capture-list
capture-default , capture-list

capture-default:
&
=

capture-list:
capture
capture-list , capture

capture:
simple-capture
init-capture

simple-capture:
identifier ...

_{o p t}

& identifier ...

_{o p t}

this
* this

init-capture:
...

_{o p t}

identifier initializer
& ...

_{o p t}

identifier initializer

The body of a lambda-expression may refer to local entities of enclosing block scopes by capturing those entities, as described below.

If a lambda-capture includes a capture-default that is &, no identifier in a simple-capture of that lambda-capture shall be preceded by &.

If a lambda-capture includes a capture-default that is =, each simple-capture of that lambda-capture shall be of the form “& identifier ...

_{o p t}

”, “this”, or “* this”.

[Note 1:

The form [&,this] is redundant but accepted for compatibility with C++ 2014.

— end note]

Ignoring appearances in initializers of init-captures, an identifier or this shall not appear more than once in a lambda-capture.

[Example 1: struct S2 { void f(int i); }; void S2::f(int i) { [&, i]{ }; // OK [&, this, i]{ }; // OK, equivalent to [&, i] [&, &i]{ }; // error: i preceded by & when & is the default [=, *this]{ }; // OK [=, this]{ }; // OK, equivalent to [=] [i, i]{ }; // error: i repeated [this, *this]{ }; // error: this appears twice } — end example]

A lambda-expression shall not have a capture-default or simple-capture in its lambda-introducer unless its innermost enclosing scope is a block scope ([basic.scope.block]) or it appears within a default member initializer and its innermost enclosing scope is the corresponding class scope ([basic.scope.class]).

The identifier in a simple-capture shall denote a local entity ([basic.lookup.unqual], [basic.pre]).

The simple-captures this and * this denote the local entity *this.

An entity that is designated by a simple-capture is said to be explicitly captured.

If an identifier in a capture appears as the declarator-id of a parameter of the lambda-declarator's parameter-declaration-clause or as the name of a template parameter of the lambda-expression's template-parameter-list, the program is ill-formed.

[Example 2: void f() { int x = 0; auto g = [x](int x) { return 0; }; // error: parameter and capture have the same name auto h = [y = 0]<typename y>(y) { return 0; }; // error: template parameter and capture // have the same name } — end example]

An init-capture inhabits the lambda scope ([basic.scope.lambda]) of the lambda-expression.

An init-capture without ellipsis behaves as if it declares and explicitly captures a variable of the form “auto init-capture ;”, except that:

(6.1)
if the capture is by copy (see below), the non-static data member declared for the capture and the variable are treated as two different ways of referring to the same object, which has the lifetime of the non-static data member, and no additional copy and destruction is performed, and
(6.2)
if the capture is by reference, the variable's lifetime ends when the closure object's lifetime ends.

[Note 2:

This enables an init-capture like “x = std::move(x)”; the second “x” must bind to a declaration in the surrounding context.

— end note]

[Example 3: int x = 4; auto y = [&r = x, x = x+1]()->int { r += 2; return x+2; }(); // Updates ::x to 6, and initializes y to 7. auto z = [a = 42](int a) { return 1; }; // error: parameter and conceptual local variable have the same name auto counter = [i=0]() mutable -> decltype(i) { // OK, returns int return i++; }; — end example]

For the purposes of lambda capture, an expression potentially references local entities as follows:

(7.1)
An id-expression that names a local entity potentially references that entity; an id-expression that names one or more non-static class members and does not form a pointer to member ([expr.unary.op]) potentially references *this.

[Note 3:
This occurs even if overload resolution selects a static member function for the id-expression.
— end note]
(7.2)
A this expression potentially references *this.
(7.3)
A lambda-expression potentially references the local entities named by its simple-captures.

If an expression potentially references a local entity within a scope in which it is odr-usable ([basic.def.odr]), and the expression would be potentially evaluated if the effect of any enclosing typeid expressions ([expr.typeid]) were ignored, the entity is said to be implicitly captured by each intervening lambda-expression with an associated capture-default that does not explicitly capture it.

The implicit capture of *this is deprecated when the capture-default is =; see [depr.capture.this].

[Example 4: void f(int, const int (&)[2] = {}); // #1 void f(const int&, const int (&)[1]); // #2 void test() { const int x = 17; auto g = [](auto a) { f(x); // OK, calls #1, does not capture x }; auto g1 = [=](auto a) { f(x); // OK, calls #1, captures x }; auto g2 = [=](auto a) { int selector[sizeof(a) == 1 ? 1 : 2]{}; f(x, selector); // OK, captures x, can call #1 or #2 }; auto g3 = [=](auto a) { typeid(a + x); // captures x regardless of whether a + x is an unevaluated operand }; }

Within g1, an implementation can optimize away the capture of x as it is not odr-used.

— end example]

[Note 4:

The set of captured entities is determined syntactically, and entities are implicitly captured even if the expression denoting a local entity is within a discarded statement ([stmt.if]).

[Example 5: template<bool B> void f(int n) { [=](auto a) { if constexpr (B && sizeof(a) > 4) { (void)n; // captures n regardless of the value of B and sizeof(int) } }(0); } — end example]

— end note]

An entity is captured if it is captured explicitly or implicitly.

An entity captured by a lambda-expression is odr-used ([basic.def.odr]) by the lambda-expression.

[Note 5:

As a consequence, if a lambda-expression explicitly captures an entity that is not odr-usable, the program is ill-formed ([basic.def.odr]).

— end note]

[Example 6:

🔗

void f1(int i) { int const N = 20; auto m1 = [=]{ int const M = 30; auto m2 = [i]{ int x[N][M]; // OK, N and M are not odr-used x[0][0] = i; // OK, i is explicitly captured by m2 and implicitly captured by m1 }; }; struct s1 { int f; void work(int n) { int m = n*n; int j = 40; auto m3 = [this,m] { auto m4 = [&,j] { // error: j not odr-usable due to intervening lambda m3 int x = n; // error: n is odr-used but not odr-usable due to intervening lambda m3 x += m; // OK, m implicitly captured by m4 and explicitly captured by m3 x += i; // error: i is odr-used but not odr-usable // due to intervening function and class scopes x += f; // OK, this captured implicitly by m4 and explicitly by m3 }; }; } }; } struct s2 { double ohseven = .007; auto f() { return [this] { return [*this] { return ohseven; // OK }; }(); } auto g() { return [] { return [*this] { }; // error: *this not captured by outer lambda-expression }(); } }; — end example]

[Note 6:

Because local entities are not odr-usable within a default argument ([basic.def.odr]), a lambda-expression appearing in a default argument cannot implicitly or explicitly capture any local entity.

Such a lambda-expression can still have an init-capture if any full-expression in its initializer satisfies the constraints of an expression appearing in a default argument ([dcl.fct.default]).

— end note]

[Example 7: void f2() { int i = 1; void g1(int = ([i]{ return i; })()); // error void g2(int = ([i]{ return 0; })()); // error void g3(int = ([=]{ return i; })()); // error void g4(int = ([=]{ return 0; })()); // OK void g5(int = ([]{ return sizeof i; })()); // OK void g6(int = ([x=1]{ return x; })()); // OK void g7(int = ([x=i]{ return x; })()); // error } — end example]

An entity is captured by copy if

(10.1)
it is implicitly captured, the capture-default is =, and the captured entity is not *this, or
(10.2)
it is explicitly captured with a capture that is not of the form this, & identifier ... $_{o p t}$ , or & ... $_{o p t}$ identifier initializer.

For each entity captured by copy, an unnamed non-static data member is declared in the closure type.

The declaration order of these members is unspecified.

The type of such a data member is the referenced type if the entity is a reference to an object, an lvalue reference to the referenced function type if the entity is a reference to a function, or the type of the corresponding captured entity otherwise.

A member of an anonymous union shall not be captured by copy.

Every id-expression within the compound-statement of a lambda-expression that is an odr-use ([basic.def.odr]) of an entity captured by copy is transformed into an access to the corresponding unnamed data member of the closure type.

[Note 7:

An id-expression that is not an odr-use refers to the original entity, never to a member of the closure type.

However, such an id-expression can still cause the implicit capture of the entity.

— end note]

If *this is captured by copy, each expression that odr-uses *this is transformed to instead refer to the corresponding unnamed data member of the closure type.

[Example 8: void f(const int*); void g() { const int N = 10; [=] { int arr[N]; // OK, not an odr-use, refers to variable with automatic storage duration f(&N); // OK, causes N to be captured; &N points to // the corresponding member of the closure type }; } — end example]

An entity is captured by reference if it is implicitly or explicitly captured but not captured by copy.

It is unspecified whether additional unnamed non-static data members are declared in the closure type for entities captured by reference.

If declared, such non-static data members shall be of literal type.

[Example 9: // The inner closure type must be a literal type regardless of how reference captures are represented. static_assert([](int n) { return [&n] { return ++n; }(); }(3) == 4); — end example]

A bit-field or a member of an anonymous union shall not be captured by reference.

An id-expression within the compound-statement of a lambda-expression that is an odr-use of a reference captured by reference refers to the entity to which the captured reference is bound and not to the captured reference.

[Note 8:

The validity of such captures is determined by the lifetime of the object to which the reference refers, not by the lifetime of the reference itself.

— end note]

[Example 10: auto h(int &r) { return [&] { ++r; // Valid after h returns if the lifetime of the // object to which r is bound has not ended }; } — end example]

If a lambda-expression m2 captures an entity and that entity is captured by an immediately enclosing lambda-expression m1, then m2's capture is transformed as follows:

(14.1)
If m1 captures the entity by copy, m2 captures the corresponding non-static data member of m1's closure type; if m1 is not mutable, the non-static data member is considered to be const-qualified.
(14.2)
If m1 captures the entity by reference, m2 captures the same entity captured by m1.

[Example 11:

The nested lambda-expressions and invocations below will output 123234.

int a = 1, b = 1, c = 1; auto m1 = [a, &b, &c]() mutable { auto m2 = [a, b, &c]() mutable { std::cout << a << b << c; a = 4; b = 4; c = 4; }; a = 3; b = 3; c = 3; m2(); }; a = 2; b = 2; c = 2; m1(); std::cout << a << b << c; — end example]

When the lambda-expression is evaluated, the entities that are captured by copy are used to direct-initialize each corresponding non-static data member of the resulting closure object, and the non-static data members corresponding to the init-captures are initialized as indicated by the corresponding initializer (which may be copy- or direct-initialization).

(For array members, the array elements are direct-initialized in increasing subscript order.)

These initializations are performed in the (unspecified) order in which the non-static data members are declared.

[Note 9:

This ensures that the destructions will occur in the reverse order of the constructions.

— end note]

[Note 10:

If a non-reference entity is implicitly or explicitly captured by reference, invoking the function call operator of the corresponding lambda-expression after the lifetime of the entity has ended is likely to result in undefined behavior.

— end note]

A simple-capture containing an ellipsis is a pack expansion ([temp.variadic]).

An init-capture containing an ellipsis is a pack expansion that declares an init-capture pack ([temp.variadic]).

[Example 12: template<class... Args> void f(Args... args) { auto lm = [&, args...] { return g(args...); }; lm(); auto lm2 = [...xs=std::move(args)] { return g(xs...); }; lm2(); } — end example]

7.5.7 Fold expressions [expr.prim.fold]

A fold expression performs a fold of a pack ([temp.variadic]) over a binary operator.

fold-expression:
( cast-expression fold-operator ... )
( ... fold-operator cast-expression )
( cast-expression fold-operator ... fold-operator cast-expression )

fold-operator: one of
+ - * / % ^ & | << >>
+= -= *= /= %= ^= &= |= <<= >>= =
== != < > <= >= && || , .* ->*

An expression of the form (... op e) where op is a fold-operator is called a unary left fold.

An expression of the form (e op ...) where op is a fold-operator is called a unary right fold.

Unary left folds and unary right folds are collectively called unary folds.

In a unary fold, the cast-expression shall contain an unexpanded pack ([temp.variadic]).

An expression of the form (e1 op1 ... op2 e2) where op1 and op2 are fold-operators is called a binary fold.

In a binary fold, op1 and op2 shall be the same fold-operator, and either e1 shall contain an unexpanded pack or e2 shall contain an unexpanded pack, but not both.

If e2 contains an unexpanded pack, the expression is called a binary left fold.

If e1 contains an unexpanded pack, the expression is called a binary right fold.

[Example 1: template<typename ...Args> bool f(Args ...args) { return (true && ... && args); // OK } template<typename ...Args> bool f(Args ...args) { return (args + ... + args); // error: both operands contain unexpanded packs } — end example]

A fold expression is a pack expansion.

7.5.8 Requires expressions [expr.prim.req]

7.5.8.1 General [expr.prim.req.general]

A requires-expression provides a concise way to express requirements on template arguments that can be checked by name lookup or by checking properties of types and expressions.

requires-expression:
requires requirement-parameter-list

_{o p t}

requirement-body

requirement-parameter-list:
( parameter-declaration-clause )

requirement-body:
{ requirement-seq }

requirement-seq:
requirement requirement-seq

_{o p t}

requirement:
simple-requirement
type-requirement
compound-requirement
nested-requirement

A requires-expression is a prvalue of type bool whose value is described below.

[Example 1:

A common use of requires-expressions is to define requirements in concepts such as the one below: template<typename T> concept R = requires (T i) { typename T::type; {*i} -> std::convertible_to<const typename T::type&>; };

A requires-expression can also be used in a requires-clause ([temp.pre]) as a way of writing ad hoc constraints on template arguments such as the one below: template<typename T> requires requires (T x) { x + x; } T add(T a, T b) { return a + b; }

The first requires introduces the requires-clause, and the second introduces the requires-expression.

— end example]

A requires-expression may introduce local parameters using a parameter-declaration-clause.

A local parameter of a requires-expression shall not have a default argument.

The type of such a parameter is determined as specified for a function parameter in [dcl.fct].

These parameters have no linkage, storage, or lifetime; they are only used as notation for the purpose of defining requirements.

The parameter-declaration-clause of a requirement-parameter-list shall not terminate with an ellipsis.

[Example 2: template<typename T> concept C = requires(T t, ...) { // error: terminates with an ellipsis t; }; template<typename T> concept C2 = requires(T p[2]) { (decltype(p))nullptr; // OK, p has type “pointer to T'' }; — end example]

The substitution of template arguments into a requires-expression can result in the formation of invalid types or expressions in the immediate context of its requirements ([temp.deduct.general]) or the violation of the semantic constraints of those requirements.

In such cases, the requires-expression evaluates to false; it does not cause the program to be ill-formed.

The substitution and semantic constraint checking proceeds in lexical order and stops when a condition that determines the result of the requires-expression is encountered.

If substitution (if any) and semantic constraint checking succeed, the requires-expression evaluates to true.

[Note 1:

If a requires-expression contains invalid types or expressions in its requirements, and it does not appear within the declaration of a templated entity, then the program is ill-formed.

— end note]

If the substitution of template arguments into a requirement would always result in a substitution failure, the program is ill-formed; no diagnostic required.

[Example 3: template<typename T> concept C = requires { new decltype((void)T{}); // ill-formed, no diagnostic required }; — end example]

7.5.8.2 Simple requirements [expr.prim.req.simple]

simple-requirement:
expression ;

A simple-requirement asserts the validity of an expression.

The expression is an unevaluated operand.

[Note 1:

The enclosing requires-expression will evaluate to false if substitution of template arguments into the expression fails.

— end note]

[Example 1: template<typename T> concept C = requires (T a, T b) { a + b; // C<T> is true if a + b is a valid expression }; — end example]

A requirement that starts with a requires token is never interpreted as a simple-requirement.

[Note 2:

This simplifies distinguishing between a simple-requirement and a nested-requirement.

— end note]

7.5.8.3 Type requirements [expr.prim.req.type]

type-requirement:
typename nested-name-specifier

_{o p t}

type-name ;

A type-requirement asserts the validity of a type.

The component names of a type-requirement are those of its nested-name-specifier (if any) and type-name.

[Note 1:

The enclosing requires-expression will evaluate to false if substitution of template arguments fails.

— end note]

[Example 1: template<typename T, typename T::type = 0> struct S; template<typename T> using Ref = T&; template<typename T> concept C = requires { typename T::inner; // required nested member name typename S<T>; // required valid ([temp.names]) template-id; // fails if T::type does not exist as a type to which 0 can be implicitly converted typename Ref<T>; // required alias template substitution, fails if T is void }; — end example]

A type-requirement that names a class template specialization does not require that type to be complete ([basic.types.general]).

7.5.8.4 Compound requirements [expr.prim.req.compound]

compound-requirement:
{ expression } noexcept

_{o p t}

return-type-requirement

_{o p t}

;

return-type-requirement:
-> type-constraint

A compound-requirement asserts properties of the expression E.

The expression is an unevaluated operand.

Substitution of template arguments (if any) and verification of semantic properties proceed in the following order:

(1.1)
Substitution of template arguments (if any) into the expression is performed.
(1.2)
If the noexcept specifier is present, E shall not be a potentially-throwing expression ([except.spec]).
(1.3)
If the return-type-requirement is present, then:
- (1.3.1)
  Substitution of template arguments (if any) into the return-type-requirement is performed.
- (1.3.2)
  The immediately-declared constraint ([temp.param]) of the type-constraint for decltype((E)) shall be satisfied.
[Example 1:
Given concepts C and D, requires { { E1 } -> C; { E2 } -> D<A $_{1}$ , ⋯, A $_{n}$ >; }; is equivalent to requires { E1; requires C<decltype((E1))>; E2; requires D<decltype((E2)), A $_{1}$ , ⋯, A $_{n}$ >; }; (including in the case where n is zero).
— end example]

[Example 2: template<typename T> concept C1 = requires(T x) { {x++}; };

The compound-requirement in C1 requires that x++ is a valid expression.

It is equivalent to the simple-requirement x++;.

template<typename T> concept C2 = requires(T x) { {*x} -> std::same_as<typename T::inner>; };

The compound-requirement in C2 requires that *x is a valid expression, that typename T::inner is a valid type, and that std::same_as<decltype((*x)), typename T::inner> is satisfied.

template<typename T> concept C3 = requires(T x) { {g(x)} noexcept; };

The compound-requirement in C3 requires that g(x) is a valid expression and that g(x) is non-throwing.

— end example]

7.5.8.5 Nested requirements [expr.prim.req.nested]

nested-requirement:
requires constraint-expression ;

A nested-requirement can be used to specify additional constraints in terms of local parameters.

The constraint-expression shall be satisfied ([temp.constr.decl]) by the substituted template arguments, if any.

Substitution of template arguments into a nested-requirement does not result in substitution into the constraint-expression other than as specified in [temp.constr.constr].

[Example 1:

template<typename U> concept C = sizeof(U) == 1; template<typename T> concept D = requires (T t) { requires C<decltype (+t)>; }; D<T> is satisfied if sizeof(decltype (+t)) == 1 ([temp.constr.atomic]).

— end example]

7.6 Compound expressions [expr.compound]

7.6.1 Postfix expressions [expr.post]

7.6.1.1 General [expr.post.general]

Postfix expressions group left-to-right.

postfix-expression:
primary-expression
postfix-expression [ expression-list

_{o p t}

]
postfix-expression ( expression-list

_{o p t}

)
simple-type-specifier ( expression-list

_{o p t}

)
typename-specifier ( expression-list

_{o p t}

)
simple-type-specifier braced-init-list
typename-specifier braced-init-list
postfix-expression . template

_{o p t}

id-expression
postfix-expression -> template

_{o p t}

id-expression
postfix-expression ++
postfix-expression --
dynamic_cast < type-id > ( expression )
static_cast < type-id > ( expression )
reinterpret_cast < type-id > ( expression )
const_cast < type-id > ( expression )
typeid ( expression )
typeid ( type-id )

expression-list:
initializer-list

[Note 1:

The > token following the type-id in a dynamic_cast, static_cast, reinterpret_cast, or const_cast can be the product of replacing a >> token by two consecutive > tokens ([temp.names]).

— end note]

7.6.1.2 Subscripting [expr.sub]

A subscript expression is a postfix expression followed by square brackets containing a possibly empty, comma-separated list of initializer-clauses that constitute the arguments to the subscript operator.

The postfix-expression and the initialization of the object parameter of any applicable subscript operator function is sequenced before each expression in the expression-list and also before any default argument.

The initialization of a non-object parameter of a subscript operator function S ([over.sub]), including every associated value computation and side effect, is indeterminately sequenced with respect to that of any other non-object parameter of S.

With the built-in subscript operator, an expression-list shall be present, consisting of a single assignment-expression.

One of the expressions shall be a glvalue of type “array of T” or a prvalue of type “pointer to T” and the other shall be a prvalue of unscoped enumeration or integral type.

The result is of type “T”.

The type “T” shall be a completely-defined object type.48

The expression E1[E2] is identical (by definition) to *((E1)+(E2)), except that in the case of an array operand, the result is an lvalue if that operand is an lvalue and an xvalue otherwise.

[Note 1:

Despite its asymmetric appearance, subscripting is a commutative operation except for sequencing.

See [expr.unary] and [expr.add] for details of * and + and [dcl.array] for details of array types.

— end note]

48)48)

This is true even if the subscript operator is used in the following common idiom: &x[0].

7.6.1.3 Function call [expr.call]

A function call is a postfix expression followed by parentheses containing a possibly empty, comma-separated list of initializer-clauses which constitute the arguments to the function.

[Note 1:

If the postfix expression is a function or member function name, the appropriate function and the validity of the call are determined according to the rules in [over.match].

— end note]

The postfix expression shall have function type or function pointer type.

For a call to a non-member function or to a static member function, the postfix expression shall be either an lvalue that refers to a function (in which case the function-to-pointer standard conversion ([conv.func]) is suppressed on the postfix expression), or a prvalue of function pointer type.

If the selected function is non-virtual, or if the id-expression in the class member access expression is a qualified-id, that function is called.

Otherwise, its final overrider in the dynamic type of the object expression is called; such a call is referred to as a virtual function call.

[Note 2:

The dynamic type is the type of the object referred to by the current value of the object expression.

[class.cdtor] describes the behavior of virtual function calls when the object expression refers to an object under construction or destruction.

— end note]

[Note 3:

If a function or member function name is used, and name lookup does not find a declaration of that name, the program is ill-formed.

No function is implicitly declared by such a call.

— end note]

If the postfix-expression names a destructor or pseudo-destructor ([expr.prim.id.dtor]), the type of the function call expression is void; otherwise, the type of the function call expression is the return type of the statically chosen function (i.e., ignoring the virtual keyword), even if the type of the function actually called is different.

If the postfix-expression names a pseudo-destructor (in which case the postfix-expression is a possibly-parenthesized class member access), the function call destroys the object of scalar type denoted by the object expression of the class member access ([expr.ref], [basic.life]).

A type

T_{call}

is call-compatible with a function type

T_{func}

T_{call}

is the same type as

T_{func}

or if the type “pointer to

T_{func}

” can be converted to type “pointer to

T_{call}

” via a function pointer conversion ([conv.fctptr]).

Calling a function through an expression whose function type is not call-compatible with the type of the called function's definition results in undefined behavior.

[Note 4:

This requirement allows the case when the expression has the type of a potentially-throwing function, but the called function has a non-throwing exception specification, and the function types are otherwise the same.

— end note]

When a function is called, each parameter ([dcl.fct]) is initialized ([dcl.init], [class.copy.ctor]) with its corresponding argument.

If the function is an explicit object member function and there is an implied object argument ([over.call.func]), the list of provided arguments is preceded by the implied object argument for the purposes of this correspondence.

If there is no corresponding argument, the default argument for the parameter is used.

[Example 1: template<typename ...T> int f(int n = 0, T ...t); int x = f<int>(); // error: no argument for second function parameter — end example]

If the function is an implicit object member function, the object expression of the class member access shall be a glvalue and the implicit object parameter of the function ([over.match.funcs]) is initialized with that glvalue, converted as if by an explicit type conversion .

[Note 5:

There is no access or ambiguity checking on this conversion; the access checking and disambiguation are done as part of the (possibly implicit) class member access operator.

See [class.member.lookup], [class.access.base], and [expr.ref].

— end note]

When a function is called, the type of any parameter shall not be a class type that is either incomplete or abstract.

[Note 6:

This still allows a parameter to be a pointer or reference to such a type.

However, it prevents a passed-by-value parameter to have an incomplete or abstract class type.

— end note]

It is implementation-defined whether a parameter is destroyed when the function in which it is defined exits ([stmt.return], [except.ctor]) or at the end of the enclosing full-expression; parameters are always destroyed in the reverse order of their construction.

The initialization and destruction of each parameter occurs within the context of the full-expression ([intro.execution]) where the function call appears.

[Example 2:

The access ([class.access.general]) of the constructor, conversion functions, or destructor is checked at the point of call.

If a constructor or destructor for a function parameter throws an exception, any function-try-block ([except.pre]) of the called function with a handler that can handle the exception is not considered.

— end example]

The postfix-expression is sequenced before each expression in the expression-list and any default argument.

The initialization of a parameter, including every associated value computation and side effect, is indeterminately sequenced with respect to that of any other parameter.

[Note 7:

All side effects of argument evaluations are sequenced before the function is entered (see [intro.execution]).

— end note]

[Example 3: void f() { std::string s = "but I have heard it works even if you don't believe in it"; s.replace(0, 4, "").replace(s.find("even"), 4, "only").replace(s.find(" don't"), 6, ""); assert(s == "I have heard it works only if you believe in it"); // OK } — end example]

[Note 8:

If an operator function is invoked using operator notation, argument evaluation is sequenced as specified for the built-in operator; see [over.match.oper].

— end note]

[Example 4: struct S { S(int); }; int operator<<(S, int); int i, j; int x = S(i=1) << (i=2); int y = operator<<(S(j=1), j=2);

After performing the initializations, the value of i is 2 (see [expr.shift]), but it is unspecified whether the value of j is 1 or 2.

— end example]

The result of a function call is the result of the possibly-converted operand of the return statement ([stmt.return]) that transferred control out of the called function (if any), except in a virtual function call if the return type of the final overrider is different from the return type of the statically chosen function, the value returned from the final overrider is converted to the return type of the statically chosen function.

[Note 9:

A function can change the values of its non-const parameters, but these changes cannot affect the values of the arguments except where a parameter is of a reference type ([dcl.ref]); if the reference is to a const-qualified type, const_cast needs to be used to cast away the constness in order to modify the argument's value.

Where a parameter is of const reference type a temporary object is introduced if needed ([dcl.type], [lex.literal], [lex.string], [dcl.array], [class.temporary]).

In addition, it is possible to modify the values of non-constant objects through pointer parameters.

— end note]

A function can be declared to accept fewer arguments (by declaring default arguments) or more arguments (by using the ellipsis, ..., or a function parameter pack ([dcl.fct])) than the number of parameters in the function definition .

[Note 10:

This implies that, except where the ellipsis (...) or a function parameter pack is used, a parameter is available for each argument.

— end note]

When there is no parameter for a given argument, the argument is passed in such a way that the receiving function can obtain the value of the argument by invoking va_arg .

[Note 11:

This paragraph does not apply to arguments passed to a function parameter pack.

Function parameter packs are expanded during template instantiation ([temp.variadic]), thus each such argument has a corresponding parameter when a function template specialization is actually called.

— end note]

The lvalue-to-rvalue, array-to-pointer, and function-to-pointer standard conversions are performed on the argument expression.

An argument that has type cv std::nullptr_t is converted to type void* ([conv.ptr]).

After these conversions, if the argument does not have arithmetic, enumeration, pointer, pointer-to-member, or class type, the program is ill-formed.

Passing a potentially-evaluated argument of a scoped enumeration type ([dcl.enum]) or of a class type ([class]) having an eligible non-trivial copy constructor ([special], [class.copy.ctor]), an eligible non-trivial move constructor, or a non-trivial destructor ([class.dtor]), with no corresponding parameter, is conditionally-supported with implementation-defined semantics.

If the argument has integral or enumeration type that is subject to the integral promotions, or a floating-point type that is subject to the floating-point promotion, the value of the argument is converted to the promoted type before the call.

These promotions are referred to as the default argument promotions.

Recursive calls are permitted, except to the main function .

A function call is an lvalue if the result type is an lvalue reference type or an rvalue reference to function type, an xvalue if the result type is an rvalue reference to object type, and a prvalue otherwise.

If it is a non-void prvalue, the type of the function call expression shall be complete, except as specified in [dcl.type.decltype].

7.6.1.4 Explicit type conversion (functional notation) [expr.type.conv]

A simple-type-specifier or typename-specifier followed by a parenthesized optional expression-list or by a braced-init-list (the initializer) constructs a value of the specified type given the initializer.

If the type is a placeholder for a deduced class type, it is replaced by the return type of the function selected by overload resolution for class template deduction for the remainder of this subclause.

Otherwise, if the type contains a placeholder type, it is replaced by the type determined by placeholder type deduction ([dcl.type.auto.deduct]).

Let T denote the resulting type.

Then:

(1.1)
If the initializer is a parenthesized single expression, the type conversion expression is equivalent to the corresponding cast expression ([expr.cast]).
(1.2)
Otherwise, if T is cv void, the initializer shall be () or {} (after pack expansion, if any), and the expression is a prvalue of type void that performs no initialization.
(1.3)
Otherwise, if T is a reference type, the expression has the same effect as direct-initializing an invented variable t of type T from the initializer and then using t as the result of the expression; the result is an lvalue if T is an lvalue reference type or an rvalue reference to function type and an xvalue otherwise.
(1.4)
Otherwise, the expression is a prvalue of type T whose result object is direct-initialized ([dcl.init]) with the initializer.

If the initializer is a parenthesized optional expression-list, T shall not be an array type.

[Example 1: struct A {}; void f(A&); // #1 void f(A&&); // #2 A& g(); void h() { f(g()); // calls #1 f(A(g())); // calls #2 with a temporary object f(auto(g())); // calls #2 with a temporary object } — end example]

7.6.1.5 Class member access [expr.ref]

A postfix expression followed by a dot . or an arrow ->, optionally followed by the keyword template, and then followed by an id-expression, is a postfix expression.

[Note 1:

If the keyword template is used, the following unqualified name is considered to refer to a template ([temp.names]).

If a simple-template-id results and is followed by a ::, the id-expression is a qualified-id.

— end note]

For the first option (dot), if the id-expression names a static member or an enumerator, the first expression is a discarded-value expression ([expr.context]); if the id-expression names a non-static data member, the first expression shall be a glvalue.

For the second option (arrow), the first expression shall be a prvalue having pointer type.

The expression E1->E2 is converted to the equivalent form (*(E1)).E2; the remainder of [expr.ref] will address only the first option (dot).49

The postfix expression before the dot is evaluated;50 the result of that evaluation, together with the id-expression, determines the result of the entire postfix expression.

Abbreviating postfix-expression.id-expression as E1.E2, E1 is called the object expression.

If the object expression is of scalar type, E2 shall name the pseudo-destructor of that same type (ignoring cv-qualifications) and E1.E2 is a prvalue of type “function of () returning void”.

[Note 2:

This value can only be used for a notional function call ([expr.prim.id.dtor]).

— end note]

Otherwise, the object expression shall be of class type.

The class type shall be complete unless the class member access appears in the definition of that class.

[Note 3:

The program is ill-formed if the result differs from that when the class is complete ([class.member.lookup]).

— end note]

[Note 4:

[basic.lookup.qual] describes how names are looked up after the . and -> operators.

— end note]

If E2 is a bit-field, E1.E2 is a bit-field.

The type and value category of E1.E2 are determined as follows.

In the remainder of [expr.ref], cq represents either const or the absence of const and vq represents either volatile or the absence of volatile.

cv represents an arbitrary set of cv-qualifiers, as defined in [basic.type.qualifier].

If E2 is declared to have type “reference to T”, then E1.E2 is an lvalue of type T.

If E2 is a static data member, E1.E2 designates the object or function to which the reference is bound, otherwise E1.E2 designates the object or function to which the corresponding reference member of E1 is bound.

Otherwise, one of the following rules applies.

(7.1)
If E2 is a static data member and the type of E2 is T, then E1.E2 is an lvalue; the expression designates the named member of the class.

The type of E1.E2 is T.
(7.2)
If E2 is a non-static data member and the type of E1 is “cq1 vq1 X”, and the type of E2 is “cq2 vq2 T”, the expression designates the corresponding member subobject of the object designated by the first expression.

If E1 is an lvalue, then E1.E2 is an lvalue; otherwise E1.E2 is an xvalue.

Let the notation vq12 stand for the “union” of vq1 and vq2; that is, if vq1 or vq2 is volatile, then vq12 is volatile.

Similarly, let the notation cq12 stand for the “union” of cq1 and cq2; that is, if cq1 or cq2 is const, then cq12 is const.

If E2 is declared to be a mutable member, then the type of E1.E2 is “vq12 T”.

If E2 is not declared to be a mutable member, then the type of E1.E2 is “cq12 vq12 T”.
(7.3)
If E2 is an overload set, the expression shall be the (possibly-parenthesized) left-hand operand of a member function call ([expr.call]), and function overload resolution ([over.match]) is used to select the function to which E2 refers.

The type of E1.E2 is the type of E2 and E1.E2 refers to the function referred to by E2.
- (7.3.1)
  If E2 refers to a static member function, E1.E2 is an lvalue.
- (7.3.2)
  Otherwise (when E2 refers to a non-static member function), E1.E2 is a prvalue.
  
  [Note 5:
  Any redundant set of parentheses surrounding the expression is ignored ([expr.prim.paren]).
  — end note]
(7.4)
If E2 is a nested type, the expression E1.E2 is ill-formed.
(7.5)
If E2 is a member enumerator and the type of E2 is T, the expression E1.E2 is a prvalue of type T whose value is the value of the enumerator.

If E2 is a non-static member, the program is ill-formed if the class of which E2 is directly a member is an ambiguous base ([class.member.lookup]) of the naming class ([class.access.base]) of E2.

[Note 6:

The program is also ill-formed if the naming class is an ambiguous base of the class type of the object expression; see [class.access.base].

— end note]

If E2 is a non-static member and the result of E1 is an object whose type is not similar ([conv.qual]) to the type of E1, the behavior is undefined.

[Example 1: struct A { int i; }; struct B { int j; }; struct D : A, B {}; void f() { D d; static_cast<B&>(d).j; // OK, object expression designates the B subobject of d reinterpret_cast<B&>(d).j; // undefined behavior } — end example]

49)49)

Note that (*(E1)) is an lvalue.

50)50)

If the class member access expression is evaluated, the subexpression evaluation happens even if the result is unnecessary to determine the value of the entire postfix expression, for example if the id-expression denotes a static member.

7.6.1.6 Increment and decrement [expr.post.incr]

The value of a postfix ++ expression is the value obtained by applying the lvalue-to-rvalue conversion ([conv.lval]) to its operand.

[Note 1:

The value obtained is a copy of the original value.

— end note]

The operand shall be a modifiable lvalue.

The type of the operand shall be an arithmetic type other than cv bool, or a pointer to a complete object type.

An operand with volatile-qualified type is deprecated; see [depr.volatile.type].

The value of the operand object is modified ([defns.access]) as if it were the operand of the prefix ++ operator ([expr.pre.incr]).

The value computation of the ++ expression is sequenced before the modification of the operand object.

With respect to an indeterminately-sequenced function call, the operation of postfix ++ is a single evaluation.

[Note 2:

Therefore, a function call cannot intervene between the lvalue-to-rvalue conversion and the side effect associated with any single postfix ++ operator.

— end note]

The result is a prvalue.

The type of the result is the cv-unqualified version of the type of the operand.

The operand of postfix -- is decremented analogously to the postfix ++ operator.

[Note 3:

For prefix increment and decrement, see [expr.pre.incr].

— end note]

7.6.1.7 Dynamic cast [expr.dynamic.cast]

The result of the expression dynamic_cast<T>(v) is the result of converting the expression v to type T.

T shall be a pointer or reference to a complete class type, or “pointer to cv void”.

The dynamic_cast operator shall not cast away constness ([expr.const.cast]).

If T is a pointer type, v shall be a prvalue of a pointer to complete class type, and the result is a prvalue of type T.

If T is an lvalue reference type, v shall be an lvalue of a complete class type, and the result is an lvalue of the type referred to by T.

If T is an rvalue reference type, v shall be a glvalue having a complete class type, and the result is an xvalue of the type referred to by T.

If the type of v is the same as T (ignoring cv-qualifications), the result is v (converted if necessary).

If T is “pointer to cv1 B” and v has type “pointer to cv2 D” such that B is a base class of D, the result is a pointer to the unique B subobject of the D object pointed to by v, or a null pointer value if v is a null pointer value.

Similarly, if T is “reference to cv1 B” and v has type cv2 D such that B is a base class of D, the result is the unique B subobject of the D object referred to by v.51

In both the pointer and reference cases, the program is ill-formed if B is an inaccessible or ambiguous base class of D.

[Example 1: struct B { }; struct D : B { }; void foo(D* dp) { B* bp = dynamic_cast<B*>(dp); // equivalent to B* bp = dp; } — end example]

Otherwise, v shall be a pointer to or a glvalue of a polymorphic type .

If v is a null pointer value, the result is a null pointer value.

If v has type “pointer to cv U” and v does not point to an object whose type is similar ([conv.qual]) to U and that is within its lifetime or within its period of construction or destruction ([class.cdtor]), the behavior is undefined.

If v is a glvalue of type U and v does not refer to an object whose type is similar to U and that is within its lifetime or within its period of construction or destruction, the behavior is undefined.

If T is “pointer to cv void”, then the result is a pointer to the most derived object pointed to by v.

Otherwise, a runtime check is applied to see if the object pointed or referred to by v can be converted to the type pointed or referred to by T.

Let C be the class type to which T points or refers.

The runtime check logically executes as follows:

(9.1)
If, in the most derived object pointed (referred) to by v, v points (refers) to a public base class subobject of a C object, and if only one object of type C is derived from the subobject pointed (referred) to by v, the result points (refers) to that C object.
(9.2)
Otherwise, if v points (refers) to a public base class subobject of the most derived object, and the type of the most derived object has a base class, of type C, that is unambiguous and public, the result points (refers) to the C subobject of the most derived object.
(9.3)
Otherwise, the runtime check fails.

The value of a failed cast to pointer type is the null pointer value of the required result type.

A failed cast to reference type throws an exception of a type that would match a handler of type std::bad_cast .

[Example 2: class A { virtual void f(); }; class B { virtual void g(); }; class D : public virtual A, private B { }; void g() { D d; B* bp = (B*)&d; // cast needed to break protection A* ap = &d; // public derivation, no cast needed D& dr = dynamic_cast<D&>(*bp); // fails ap = dynamic_cast<A*>(bp); // fails bp = dynamic_cast<B*>(ap); // fails ap = dynamic_cast<A*>(&d); // succeeds bp = dynamic_cast<B*>(&d); // ill-formed (not a runtime check) } class E : public D, public B { }; class F : public E, public D { }; void h() { F f; A* ap = &f; // succeeds: finds unique A D* dp = dynamic_cast<D*>(ap); // fails: yields null; f has two D subobjects E* ep = (E*)ap; // error: cast from virtual base E* ep1 = dynamic_cast<E*>(ap); // succeeds } — end example]

[Note 1:

Subclause [class.cdtor] describes the behavior of a dynamic_cast applied to an object under construction or destruction.

— end note]

51)51)

The most derived object ([intro.object]) pointed or referred to by v can contain other B objects as base classes, but these are ignored.

7.6.1.8 Type identification [expr.typeid]

The result of a typeid expression is an lvalue of static type const std::type_info ([type.info]) and dynamic type const std::type_info or const name where name is an implementation-defined class publicly derived from std::type_info which preserves the behavior described in [type.info].52

The lifetime of the object referred to by the lvalue extends to the end of the program.

Whether or not the destructor is called for the std::type_info object at the end of the program is unspecified.

If the type of the expression or type-id operand is a (possibly cv-qualified) class type or a reference to (possibly cv-qualified) class type, that class shall be completely defined.

If an expression operand of typeid is a possibly-parenthesized unary-expression whose unary-operator is * and whose operand evaluates to a null pointer value ([basic.compound]), the typeid expression throws an exception ([except.throw]) of a type that would match a handler of type std::bad_typeid ([bad.typeid]).

[Note 1:

In other contexts, evaluating such a unary-expression results in undefined behavior ([expr.unary.op]).

— end note]

When typeid is applied to a glvalue whose type is a polymorphic class type ([class.virtual]), the result refers to a std::type_info object representing the type of the most derived object ([intro.object]) (that is, the dynamic type) to which the glvalue refers.

When typeid is applied to an expression other than a glvalue of a polymorphic class type, the result refers to a std::type_info object representing the static type of the expression.

Lvalue-to-rvalue, array-to-pointer, and function-to-pointer conversions are not applied to the expression.

If the expression is a prvalue, the temporary materialization conversion is applied.

The expression is an unevaluated operand .

When typeid is applied to a type-id, the result refers to a std::type_info object representing the type of the type-id.

If the type of the type-id is a reference to a possibly cv-qualified type, the result of the typeid expression refers to a std::type_info object representing the cv-unqualified referenced type.

[Note 2:

The type-id cannot denote a function type with a cv-qualifier-seq or a ref-qualifier ([dcl.fct]).

— end note]

If the type of the expression or type-id is a cv-qualified type, the result of the typeid expression refers to a std::type_info object representing the cv-unqualified type.

[Example 1: class D { /* ... */ }; D d1; const D d2; typeid(d1) == typeid(d2); // yields true typeid(D) == typeid(const D); // yields true typeid(D) == typeid(d2); // yields true typeid(D) == typeid(const D&); // yields true — end example]

The type std::type_info ([type.info]) is not predefined; if a standard library declaration ([typeinfo.syn], [std.modules]) of std::type_info does not precede ([basic.lookup.general]) a typeid expression, the program is ill-formed.

[Note 3:

Subclause [class.cdtor] describes the behavior of typeid applied to an object under construction or destruction.

— end note]

52)52)

The recommended name for such a class is extended_type_info.

7.6.1.9 Static cast [expr.static.cast]

The result of the expression static_cast<T>(v) is the result of converting the expression v to type T.

The static_cast operator shall not cast away constness ([expr.const.cast]).

An lvalue of type “cv1 B”, where B is a class type, can be cast to type “reference to cv2 D”, where D is a complete class derived ([class.derived]) from B, if cv2 is the same cv-qualification as, or greater cv-qualification than, cv1.

If B is a virtual base class of D or a base class of a virtual base class of D, or if no valid standard conversion from “pointer to D” to “pointer to B” exists ([conv.ptr]), the program is ill-formed.

An xvalue of type “cv1 B” can be cast to type “rvalue reference to cv2 D” with the same constraints as for an lvalue of type “cv1 B”.

If the object of type “cv1 B” is actually a base class subobject of an object of type D, the result refers to the enclosing object of type D.

Otherwise, the behavior is undefined.

[Example 1: struct B { }; struct D : public B { }; D d; B &br = d; static_cast<D&>(br); // produces lvalue denoting the original d object — end example]

An lvalue of type T1 can be cast to type “rvalue reference to T2” if T2 is reference-compatible with T1 ([dcl.init.ref]).

If the value is not a bit-field, the result refers to the object or the specified base class subobject thereof; otherwise, the lvalue-to-rvalue conversion is applied to the bit-field and the resulting prvalue is used as the operand of the static_cast for the remainder of this subclause.

If T2 is an inaccessible ([class.access]) or ambiguous ([class.member.lookup]) base class of T1, a program that necessitates such a cast is ill-formed.

Any expression can be explicitly converted to type cv void, in which case the operand is a discarded-value expression ([expr.prop]).

[Note 1:

Such a static_cast has no result as it is a prvalue of type void; see [basic.lval].

— end note]

[Note 2:

However, if the value is in a temporary object ([class.temporary]), the destructor for that object is not executed until the usual time, and the value of the object is preserved for the purpose of executing the destructor.

— end note]

Otherwise, an expression E can be explicitly converted to a type T if there is an implicit conversion sequence ([over.best.ics]) from E to T, if overload resolution for a direct-initialization ([dcl.init]) of an object or reference of type T from E would find at least one viable function ([over.match.viable]), or if T is an aggregate type ([dcl.init.aggr]) having a first element x and there is an implicit conversion sequence from E to the type of x.

If T is a reference type, the effect is the same as performing the declaration and initialization T t(E); for some invented temporary variable t ([dcl.init]) and then using the temporary variable as the result of the conversion.

Otherwise, the result object is direct-initialized from E.

[Note 3:

The conversion is ill-formed when attempting to convert an expression of class type to an inaccessible or ambiguous base class.

— end note]

[Note 4:

If T is “array of unknown bound of U”, this direct-initialization defines the type of the expression as U[1].

— end note]

Otherwise, the inverse of a standard conversion sequence ([conv]) not containing an lvalue-to-rvalue ([conv.lval]), array-to-pointer ([conv.array]), function-to-pointer ([conv.func]), null pointer ([conv.ptr]), null member pointer ([conv.mem]), boolean ([conv.bool]), or function pointer ([conv.fctptr]) conversion, can be performed explicitly using static_cast.

A program is ill-formed if it uses static_cast to perform the inverse of an ill-formed standard conversion sequence.

[Example 2: struct B { }; struct D : private B { }; void f() { static_cast<D*>((B*)0); // error: B is a private base of D static_cast<int B::*>((int D::*)0); // error: B is a private base of D } — end example]

The lvalue-to-rvalue ([conv.lval]), array-to-pointer ([conv.array]), and function-to-pointer ([conv.func]) conversions are applied to the operand.

Such a static_cast is subject to the restriction that the explicit conversion does not cast away constness ([expr.const.cast]), and the following additional rules for specific cases:

A value of a scoped enumeration type ([dcl.enum]) can be explicitly converted to an integral type; the result is the same as that of converting to the enumeration's underlying type and then to the destination type.

A value of a scoped enumeration type can also be explicitly converted to a floating-point type; the result is the same as that of converting from the original value to the floating-point type.

A value of integral or enumeration type can be explicitly converted to a complete enumeration type.

If the enumeration type has a fixed underlying type, the value is first converted to that type by integral promotion ([conv.prom]) or integral conversion ([conv.integral]), if necessary, and then to the enumeration type.

If the enumeration type does not have a fixed underlying type, the value is unchanged if the original value is within the range of the enumeration values ([dcl.enum]), and otherwise, the behavior is undefined.

A value of floating-point type can also be explicitly converted to an enumeration type.

The resulting value is the same as converting the original value to the underlying type of the enumeration ([conv.fpint]), and subsequently to the enumeration type.

A prvalue of floating-point type can be explicitly converted to any other floating-point type.

If the source value can be exactly represented in the destination type, the result of the conversion has that exact representation.

If the source value is between two adjacent destination values, the result of the conversion is an implementation-defined choice of either of those values.

Otherwise, the behavior is undefined.

A prvalue of type “pointer to cv1 B”, where B is a class type, can be converted to a prvalue of type “pointer to cv2 D”, where D is a complete class derived from B, if cv2 is the same cv-qualification as, or greater cv-qualification than, cv1.

The null pointer value ([basic.compound]) is converted to the null pointer value of the destination type.

If the prvalue of type “pointer to cv1 B” points to a B that is actually a base class subobject of an object of type D, the resulting pointer points to the enclosing object of type D.

Otherwise, the behavior is undefined.

A prvalue of type “pointer to member of D of type cv1 T” can be converted to a prvalue of type “pointer to member of B of type cv2 T”, where D is a complete class type and B is a base class of D, if cv2 is the same cv-qualification as, or greater cv-qualification than, cv1.

[Note 5:

Function types (including those used in pointer-to-member-function types) are never cv-qualified ([dcl.fct]).

— end note]

If no valid standard conversion from “pointer to member of B of type T” to “pointer to member of D of type T” exists ([conv.mem]), the program is ill-formed.

The null member pointer value is converted to the null member pointer value of the destination type.

If class B contains the original member, or is a base class of the class containing the original member, the resulting pointer to member points to the original member.

Otherwise, the behavior is undefined.

[Note 6:

Although class B need not contain the original member, the dynamic type of the object with which indirection through the pointer to member is performed must contain the original member; see [expr.mptr.oper].

— end note]

A prvalue of type “pointer to cv1 void” can be converted to a prvalue of type “pointer to cv2 T”, where T is an object type and cv2 is the same cv-qualification as, or greater cv-qualification than, cv1.

If the original pointer value represents the address A of a byte in memory and A does not satisfy the alignment requirement of T, then the resulting pointer value ([basic.compound]) is unspecified.

Otherwise, if the original pointer value points to an object a, and there is an object b of type similar to T that is pointer-interconvertible with a, the result is a pointer to b.

Otherwise, the pointer value is unchanged by the conversion.

[Example 3: T* p1 = new T; const T* p2 = static_cast<const T*>(static_cast<void*>(p1)); bool b = p1 == p2; // b will have the value true. — end example]

No other conversion can be performed using static_cast.

7.6.1.10 Reinterpret cast [expr.reinterpret.cast]

The result of the expression reinterpret_cast<T>(v) is the result of converting the expression v to type T.

If T is an lvalue reference type or an rvalue reference to function type, the result is an lvalue; if T is an rvalue reference to object type, the result is an xvalue; otherwise, the result is a prvalue and the lvalue-to-rvalue, array-to-pointer, and function-to-pointer standard conversions are performed on the expression v.

Conversions that can be performed explicitly using reinterpret_cast are listed below.

No other conversion can be performed explicitly using reinterpret_cast.

The reinterpret_cast operator shall not cast away constness ([expr.const.cast]).

An expression of integral, enumeration, pointer, or pointer-to-member type can be explicitly converted to its own type; such a cast yields the value of its operand.

[Note 1:

The mapping performed by reinterpret_cast might, or might not, produce a representation different from the original value.

— end note]

A pointer can be explicitly converted to any integral type large enough to hold all values of its type.

The mapping function is implementation-defined.

[Note 2:

It is intended to be unsurprising to those who know the addressing structure of the underlying machine.

— end note]

A value of type std::nullptr_t can be converted to an integral type; the conversion has the same meaning and validity as a conversion of (void*)0 to the integral type.

[Note 3:

A reinterpret_cast cannot be used to convert a value of any type to the type std::nullptr_t.

— end note]

A value of integral type or enumeration type can be explicitly converted to a pointer.

A pointer converted to an integer of sufficient size (if any such exists on the implementation) and back to the same pointer type will have its original value ([basic.compound]); mappings between pointers and integers are otherwise implementation-defined.

A function pointer can be explicitly converted to a function pointer of a different type.

[Note 4:

The effect of calling a function through a pointer to a function type ([dcl.fct]) that is not the same as the type used in the definition of the function is undefined ([expr.call]).

— end note]

Except that converting a prvalue of type “pointer to T1” to the type “pointer to T2” (where T1 and T2 are function types) and back to its original type yields the original pointer value, the result of such a pointer conversion is unspecified.

An object pointer can be explicitly converted to an object pointer of a different type.53

When a prvalue v of object pointer type is converted to the object pointer type “pointer to cv T”, the result is static_cast<cv T*>(static_cast<cv void*>(v)).

[Note 5:

Converting a pointer of type “pointer to T1” that points to an object of type T1 to the type “pointer to T2” (where T2 is an object type and the alignment requirements of T2 are no stricter than those of T1) and back to its original type yields the original pointer value.

— end note]

Converting a function pointer to an object pointer type or vice versa is conditionally-supported.

The meaning of such a conversion is implementation-defined, except that if an implementation supports conversions in both directions, converting a prvalue of one type to the other type and back, possibly with different cv-qualification, shall yield the original pointer value.

The null pointer value ([basic.compound]) is converted to the null pointer value of the destination type.

[Note 6:

A null pointer constant of type std::nullptr_t cannot be converted to a pointer type, and a null pointer constant of integral type is not necessarily converted to a null pointer value.

— end note]

A prvalue of type “pointer to member of X of type T1” can be explicitly converted to a prvalue of a different type “pointer to member of Y of type T2” if T1 and T2 are both function types or both object types.54

The null member pointer value ([conv.mem]) is converted to the null member pointer value of the destination type.

The result of this conversion is unspecified, except in the following cases:

(10.1)
Converting a prvalue of type “pointer to member function” to a different pointer-to-member-function type and back to its original type yields the original pointer-to-member value.
(10.2)
Converting a prvalue of type “pointer to data member of X of type T1” to the type “pointer to data member of Y of type T2” (where the alignment requirements of T2 are no stricter than those of T1) and back to its original type yields the original pointer-to-member value.

If v is a glvalue of type T1, designating an object or function x, it can be cast to the type “reference to T2” if an expression of type “pointer to T1” can be explicitly converted to the type “pointer to T2” using a reinterpret_cast.

The result is that of *reinterpret_cast<T2 *>(p) where p is a pointer to x of type “pointer to T1”.

[Note 7:

No temporary is materialized ([conv.rval]) or created, no copy is made, and no constructors ([class.ctor]) or conversion functions ([class.conv]) are called.55

— end note]

53)53)

The types can have different cv-qualifiers, subject to the overall restriction that a reinterpret_cast cannot cast away constness.

54)54)

T1 and T2 can have different cv-qualifiers, subject to the overall restriction that a reinterpret_cast cannot cast away constness.

55)55)

This is sometimes referred to as a type pun when the result refers to the same object as the source glvalue.

7.6.1.11 Const cast [expr.const.cast]

The result of the expression const_cast<T>(v) is of type T.

If T is an lvalue reference to object type, the result is an lvalue; if T is an rvalue reference to object type, the result is an xvalue; otherwise, the result is a prvalue and the lvalue-to-rvalue, array-to-pointer, and function-to-pointer standard conversions are performed on the expression v.

The temporary materialization conversion is not performed on v, other than as specified below.

Conversions that can be performed explicitly using const_cast are listed below.

No other conversion shall be performed explicitly using const_cast.

[Note 1:

Subject to the restrictions in this subclause, an expression can be cast to its own type using a const_cast operator.

— end note]

For two similar object pointer or pointer to data member types T1 and T2 ([conv.qual]), a prvalue of type T1 can be explicitly converted to the type T2 using a const_cast if, considering the qualification-decompositions of both types, each

P_{i}^{1}

is the same as

P_{i}^{2}

for all i.

If v is a null pointer or null member pointer, the result is a null pointer or null member pointer, respectively.

Otherwise, the result points to or past the end of the same object, or points to the same member, respectively, as v.

For two object types T1 and T2, if a pointer to T1 can be explicitly converted to the type “pointer to T2” using a const_cast, then the following conversions can also be made:

(4.1)
an lvalue of type T1 can be explicitly converted to an lvalue of type T2 using the cast const_cast<T2&>;
(4.2)
a glvalue of type T1 can be explicitly converted to an xvalue of type T2 using the cast const_cast<T2&&>; and
(4.3)
if T1 is a class or array type, a prvalue of type T1 can be explicitly converted to an xvalue of type T2 using the cast const_cast<T2&&>.
The temporary materialization conversion is performed on v.

The result refers to the same object as the (possibly converted) operand.

[Example 1: typedef int *A[3]; // array of 3 pointer to int typedef const int *const CA[3]; // array of 3 const pointer to const int auto &&r2 = const_cast<A&&>(CA{}); // OK, temporary materialization conversion is performed — end example]

[Note 2:

Depending on the type of the object, a write operation through the pointer, lvalue or pointer to data member resulting from a const_cast that casts away a const-qualifier56 can produce undefined behavior ([dcl.type.cv]).

— end note]

A conversion from a type T1 to a type T2 casts away constness if T1 and T2 are different, there is a qualification-decomposition ([conv.qual]) of T1 yielding n such that T2 has a qualification-decomposition of the form

c v_{0}^{2}

P_{0}^{2}

c v_{1}^{2}

P_{1}^{2}

⋯

c v_{n - 1}^{2}

P_{n - 1}^{2}

c v_{n}^{2}

U_{2}

, and there is no qualification conversion that converts T1 to

c v_{0}^{2}

P_{0}^{1}

c v_{1}^{2}

P_{1}^{1}

⋯

c v_{n - 1}^{2}

P_{n - 1}^{1}

c v_{n}^{2}

U_{1}

Casting from an lvalue of type T1 to an lvalue of type T2 using an lvalue reference cast or casting from an expression of type T1 to an xvalue of type T2 using an rvalue reference cast casts away constness if a cast from a prvalue of type “pointer to T1” to the type “pointer to T2” casts away constness.

[Note 3:

Some conversions which involve only changes in cv-qualification cannot be done using const_cast.

For instance, conversions between pointers to functions are not covered because such conversions lead to values whose use causes undefined behavior.

For the same reasons, conversions between pointers to member functions, and in particular, the conversion from a pointer to a const member function to a pointer to a non-const member function, are not covered.

— end note]

56)56)

const_cast is not limited to conversions that cast away a const-qualifier.

7.6.2 Unary expressions [expr.unary]

7.6.2.1 General [expr.unary.general]

Expressions with unary operators group right-to-left.

unary-expression:
postfix-expression
unary-operator cast-expression
++ cast-expression
-- cast-expression
await-expression
sizeof unary-expression
sizeof ( type-id )
sizeof ... ( identifier )
alignof ( type-id )
noexcept-expression
new-expression
delete-expression

unary-operator: one of
* & + - ! ~

7.6.2.2 Unary operators [expr.unary.op]

The unary * operator performs indirection.

Its operand shall be a prvalue of type “pointer to T”, where T is an object or function type.

The operator yields an lvalue of type T.

If the operand points to an object or function, the result denotes that object or function; otherwise, the behavior is undefined except as specified in [expr.typeid].

[Note 1:

Indirection through a pointer to an incomplete type (other than cv void) is valid.

The lvalue thus obtained can be used in limited ways (to initialize a reference, for example); this lvalue must not be converted to a prvalue, see [conv.lval].

— end note]

Each of the following unary operators yields a prvalue.

The operand of the unary & operator shall be an lvalue of some type T.

(3.1)
If the operand is a qualified-id naming a non-static or variant member m of some class C, other than an explicit object member function, the result has type “pointer to member of class C of type T” and designates C::m.
(3.2)
Otherwise, the result has type “pointer to T” and points to the designated object ([intro.memory]) or function ([basic.compound]).

If the operand names an explicit object member function ([dcl.fct]), the operand shall be a qualified-id.

[Note 2:
In particular, taking the address of a variable of type “cv T” yields a pointer of type “pointer to cv T”.
— end note]

[Example 1: struct A { int i; }; struct B : A { }; ... &B::i ... // has type int A::* int a; int* p1 = &a; int* p2 = p1 + 1; // defined behavior bool b = p2 > p1; // defined behavior, with value true — end example]

[Note 3:

A pointer to member formed from a mutable non-static data member ([dcl.stc]) does not reflect the mutable specifier associated with the non-static data member.

— end note]

A pointer to member is only formed when an explicit & is used and its operand is a qualified-id not enclosed in parentheses.

[Note 4:

That is, the expression &(qualified-id), where the qualified-id is enclosed in parentheses, does not form an expression of type “pointer to member”.

Neither does qualified-id, because there is no implicit conversion from a qualified-id for a non-static member function to the type “pointer to member function” as there is from an lvalue of function type to the type “pointer to function” ([conv.func]).

Nor is &unqualified-id a pointer to member, even within the scope of the unqualified-id's class.

— end note]

If & is applied to an lvalue of incomplete class type and the complete type declares operator&(), it is unspecified whether the operator has the built-in meaning or the operator function is called.

The operand of & shall not be a bit-field.

[Note 5:

The address of an overload set ([over]) can be taken only in a context that uniquely determines which function is referred to (see [over.over]).

Since the context can affect whether the operand is a static or non-static member function, the context can also affect whether the expression has type “pointer to function” or “pointer to member function”.

— end note]

The operand of the unary + operator shall be a prvalue of arithmetic, unscoped enumeration, or pointer type and the result is the value of the argument.

Integral promotion is performed on integral or enumeration operands.

The type of the result is the type of the promoted operand.

The operand of the unary - operator shall be a prvalue of arithmetic or unscoped enumeration type and the result is the negative of its operand.

Integral promotion is performed on integral or enumeration operands.

The negative of an unsigned quantity is computed by subtracting its value from

2^{n}

, where n is the number of bits in the promoted operand.

The type of the result is the type of the promoted operand.

[Note 6:

The result is the two's complement of the operand (where operand and result are considered as unsigned).

— end note]

The operand of the logical negation operator ! is contextually converted to bool ([conv]); its value is true if the converted operand is false and false otherwise.

The type of the result is bool.

The operand of the ~ operator shall be a prvalue of integral or unscoped enumeration type.

Integral promotions are performed.

The type of the result is the type of the promoted operand.

Given the coefficients

x_{i}

of the base-2 representation ([basic.fundamental]) of the promoted operand x, the coefficient

r_{i}

of the base-2 representation of the result r is 1 if

x_{i}

is 0, and 0 otherwise.

[Note 7:

The result is the ones' complement of the operand (where operand and result are considered as unsigned).

— end note]

There is an ambiguity in the grammar when ~ is followed by a type-name or computed-type-specifier.

The ambiguity is resolved by treating ~ as the operator rather than as the start of an unqualified-id naming a destructor.

[Note 8:

Because the grammar does not permit an operator to follow the ., ->, or :: tokens, a ~ followed by a type-name or computed-type-specifier in a member access expression or qualified-id is unambiguously parsed as a destructor name.

— end note]

7.6.2.3 Increment and decrement [expr.pre.incr]

The operand of prefix ++ or -- shall not be of type cv bool.

An operand with volatile-qualified type is deprecated; see [depr.volatile.type].

The expression ++x is otherwise equivalent to x+=1 and the expression --x is otherwise equivalent to x-=1 ([expr.ass]).

[Note 1:

For postfix increment and decrement, see [expr.post.incr].

— end note]

7.6.2.4 Await [expr.await]

The co_await expression is used to suspend evaluation of a coroutine ([dcl.fct.def.coroutine]) while awaiting completion of the computation represented by the operand expression.

Suspending the evaluation of a coroutine transfers control to its caller or resumer.

await-expression:
co_await cast-expression

An await-expression shall appear only as a potentially-evaluated expression within the compound-statement of a function-body or lambda-expression, in either case outside of a handler ([except.pre]).

In a declaration-statement or in the simple-declaration (if any) of an init-statement, an await-expression shall appear only in an initializer of that declaration-statement or simple-declaration.

An await-expression shall not appear in a default argument ([dcl.fct.default]).

An await-expression shall not appear in the initializer of a block variable with static or thread storage duration.

A context within a function where an await-expression can appear is called a suspension context of the function.

Evaluation of an await-expression involves the following auxiliary types, expressions, and objects:

(3.1)
p is an lvalue naming the promise object ([dcl.fct.def.coroutine]) of the enclosing coroutine and P is the type of that object.
(3.2)
Unless the await-expression was implicitly produced by a yield-expression ([expr.yield]), an initial await expression, or a final await expression ([dcl.fct.def.coroutine]), a search is performed for the name await_transform in the scope of P ([class.member.lookup]).

If this search is performed and finds at least one declaration, then a is p.await_transform(cast-expression); otherwise, a is the cast-expression.
(3.3)
o is determined by enumerating the applicable operator co_await functions for an argument a ([over.match.oper]), and choosing the best one through overload resolution ([over.match]).

If overload resolution is ambiguous, the program is ill-formed.

If no viable functions are found, o is a.

Otherwise, o is a call to the selected function with the argument a.

If o would be a prvalue, the temporary materialization conversion ([conv.rval]) is applied.
(3.4)
e is an lvalue referring to the result of evaluating the (possibly-converted) o.
(3.5)
h is an object of type std::coroutine_handle<P> referring to the enclosing coroutine.
(3.6)
await-ready is the expression e.await_ready(), contextually converted to bool.
(3.7)
await-suspend is the expression e.await_suspend(h), which shall be a prvalue of type void, bool, or std::coroutine_handle<Z> for some type Z.
(3.8)
await-resume is the expression e.await_resume().

The await-expression has the same type and value category as the await-resume expression.

The await-expression evaluates the (possibly-converted) o expression and the await-ready expression, then:

(5.1)
If the result of await-ready is false, the coroutine is considered suspended.
Then:
- (5.1.1)
  If the type of await-suspend is std::coroutine_handle<Z>, await-suspend.resume() is evaluated.
  
  [Note 1:
  This resumes the coroutine referred to by the result of await-suspend.
  
  Any number of coroutines can be successively resumed in this fashion, eventually returning control flow to the current coroutine caller or resumer ([dcl.fct.def.coroutine]).
  — end note]
- (5.1.2)
  Otherwise, if the type of await-suspend is bool, await-suspend is evaluated, and the coroutine is resumed if the result is false.
- (5.1.3)
  Otherwise, await-suspend is evaluated.
If the evaluation of await-suspend exits via an exception, the exception is caught, the coroutine is resumed, and the exception is immediately rethrown ([except.throw]).

Otherwise, control flow returns to the current coroutine caller or resumer ([dcl.fct.def.coroutine]) without exiting any scopes ([stmt.jump]).

The point in the coroutine immediately prior to control returning to its caller or resumer is a coroutine suspend point.
(5.2)
If the result of await-ready is true, or when the coroutine is resumed other than by rethrowing an exception from await-suspend, the await-resume expression is evaluated, and its result is the result of the await-expression.

[Note 2:

With respect to sequencing, an await-expression is indivisible ([intro.execution]).

— end note]

[Example 1: template <typename T> struct my_future { /* ... */ bool await_ready(); void await_suspend(std::coroutine_handle<>); T await_resume(); }; template <class Rep, class Period> auto operator co_await(std::chrono::duration<Rep, Period> d) { struct awaiter { std::chrono::system_clock::duration duration; /* ... */ awaiter(std::chrono::system_clock::duration d) : duration(d) {} bool await_ready() const { return duration.count() <= 0; } void await_resume() {} void await_suspend(std::coroutine_handle<> h) { /* ... */ } }; return awaiter{d}; } using namespace std::chrono; my_future<int> h(); my_future<void> g() { std::cout << "just about to go to sleep...\n"; co_await 10ms; std::cout << "resumed\n"; co_await h(); } auto f(int x = co_await h()); // error: await-expression outside of function suspension context int a[] = { co_await h() }; // error: await-expression outside of function suspension context — end example]

7.6.2.5 Sizeof [expr.sizeof]

The sizeof operator yields the number of bytes occupied by a non-potentially-overlapping object of the type of its operand.

The operand is either an expression, which is an unevaluated operand, or a parenthesized type-id.

The sizeof operator shall not be applied to an expression that has function or incomplete type, to the parenthesized name of such types, or to a glvalue that designates a bit-field.

The result of sizeof applied to any of the narrow character types is 1.

The result of sizeof applied to any other fundamental type ([basic.fundamental]) is implementation-defined.

[Note 1:

In particular, the values of sizeof(bool), sizeof(char16_t), sizeof(char32_t), and sizeof(wchar_t) are implementation-defined.57

— end note]

[Note 2:

See [intro.memory] for the definition of byte and [basic.types.general] for the definition of object representation.

— end note]

When applied to a reference type, the result is the size of the referenced type.

When applied to a class, the result is the number of bytes in an object of that class including any padding required for placing objects of that type in an array.

The result of applying sizeof to a potentially-overlapping subobject is the size of the type, not the size of the subobject.58

When applied to an array, the result is the total number of bytes in the array.

This implies that the size of an array of n elements is n times the size of an element.

The lvalue-to-rvalue ([conv.lval]), array-to-pointer ([conv.array]), and function-to-pointer ([conv.func]) standard conversions are not applied to the operand of sizeof.

If the operand is a prvalue, the temporary materialization conversion is applied.

The identifier in a sizeof... expression shall name a pack.

The sizeof... operator yields the number of elements in the pack ([temp.variadic]).

A sizeof... expression is a pack expansion ([temp.variadic]).

[Example 1: template<class... Types> struct count { static constexpr std::size_t value = sizeof...(Types); }; — end example]

The result of sizeof and sizeof... is a prvalue of type std::size_t.

[Note 3:

A sizeof expression is an integral constant expression ([expr.const]).

The type std::size_t is defined in the standard header <cstddef> ([cstddef.syn], [support.types.layout]).

— end note]

57)57)

sizeof(bool) is not required to be 1.

58)58)

The actual size of a potentially-overlapping subobject can be less than the result of applying sizeof to the subobject, due to virtual base classes and less strict padding requirements on potentially-overlapping subobjects.

7.6.2.6 Alignof [expr.alignof]

An alignof expression yields the alignment requirement of its operand type.

The operand shall be a type-id representing a complete object type, or an array thereof, or a reference to one of those types.

The result is a prvalue of type std::size_t.

[Note 1:

An alignof expression is an integral constant expression ([expr.const]).

The type std::size_t is defined in the standard header <cstddef> ([cstddef.syn], [support.types.layout]).

— end note]

When alignof is applied to a reference type, the result is the alignment of the referenced type.

When alignof is applied to an array type, the result is the alignment of the element type.

7.6.2.7 noexcept operator [expr.unary.noexcept]

noexcept-expression:
noexcept ( expression )

The operand of the noexcept operator is an unevaluated operand ([expr.context]).

If the operand is a prvalue, the temporary materialization conversion ([conv.rval]) is applied.

The result of the noexcept operator is a prvalue of type bool.

The result is false if the full-expression of the operand is potentially-throwing ([except.spec]), and true otherwise.

[Note 1:

A noexcept-expression is an integral constant expression ([expr.const]).

— end note]

7.6.2.8 New [expr.new]

The new-expression attempts to create an object of the type-id or new-type-id ([dcl.name]) to which it is applied.

The type of that object is the allocated type.

This type shall be a complete object type ([basic.types.general]), but not an abstract class type ([class.abstract]) or array thereof ([intro.object]).

[Note 1:

Because references are not objects, references cannot be created by new-expressions.

— end note]

[Note 2:

The type-id can be a cv-qualified type, in which case the object created by the new-expression has a cv-qualified type.

— end note]

new-expression:
::

_{o p t}

new new-placement

_{o p t}

new-type-id new-initializer

_{o p t}

_{o p t}

new new-placement

_{o p t}

( type-id ) new-initializer

_{o p t}

new-placement:
( expression-list )

new-type-id:
type-specifier-seq new-declarator

_{o p t}

new-declarator:
ptr-operator new-declarator

_{o p t}

noptr-new-declarator

noptr-new-declarator:
[ expression

_{o p t}

] attribute-specifier-seq

_{o p t}

noptr-new-declarator [ constant-expression ] attribute-specifier-seq

_{o p t}

new-initializer:
( expression-list

_{o p t}

)
braced-init-list

If a placeholder type or a placeholder for a deduced class type ([dcl.type.class.deduct]) appears in the type-specifier-seq of a new-type-id or type-id of a new-expression, the allocated type is deduced as follows: Let init be the new-initializer, if any, and T be the new-type-id or type-id of the new-expression, then the allocated type is the type deduced for the variable x in the invented declaration ([dcl.spec.auto]): T x init ;

[Example 1: new auto(1); // allocated type is int auto x = new auto('a'); // allocated type is char, x is of type char* template<class T> struct A { A(T, T); }; auto y = new A{1, 2}; // allocated type is A<int> — end example]

The new-type-id in a new-expression is the longest possible sequence of new-declarators.

[Note 3:

This prevents ambiguities between the declarator operators &, &&, *, and [] and their expression counterparts.

— end note]

[Example 2: new int * i; // syntax error: parsed as (new int*) i, not as (new int)*i

The * is the pointer declarator and not the multiplication operator.

— end example]

[Note 4:

Parentheses in a new-type-id of a new-expression can have surprising effects.

[Example 3:

new int(*[10])(); // error is ill-formed because the binding is (new int) (*[10])(); // error

Instead, the explicitly parenthesized version of the new operator can be used to create objects of compound types ([basic.compound]):

new (int (*[10])()); allocates an array of 10 pointers to functions (taking no argument and returning int).

— end example]

— end note]

The attribute-specifier-seq in a noptr-new-declarator appertains to the associated array type.

Every constant-expression in a noptr-new-declarator shall be a converted constant expression ([expr.const]) of type std::size_t and its value shall be greater than zero.

[Example 4:

Given the definition int n = 42, new float[n][5] is well-formed (because n is the expression of a noptr-new-declarator), but new float[5][n] is ill-formed (because n is not a constant expression).

Furthermore, new float[0] is well-formed (because 0 is the expression of a noptr-new-declarator, where a value of zero results in the allocation of an array with no elements), but new float[n][0] is ill-formed (because 0 is the constant-expression of a noptr-new-declarator, where only values greater than zero are allowed).

— end example]

If the type-id or new-type-id denotes an array type of unknown bound ([dcl.array]), the new-initializer shall not be omitted; the allocated object is an array with n elements, where n is determined from the number of initial elements supplied in the new-initializer ([dcl.init.aggr], [dcl.init.string]).

If the expression in a noptr-new-declarator is present, it is implicitly converted to std::size_t.

The value of the expression is invalid if

(8.1)
the expression is of non-class type and its value before converting to std::size_t is less than zero;
(8.2)
the expression is of class type and its value before application of the second standard conversion ([over.ics.user])59 is less than zero;
(8.3)
its value is such that the size of the allocated object would exceed the implementation-defined limit; or
(8.4)
the new-initializer is a braced-init-list and the number of array elements for which initializers are provided (including the terminating '\0' in a string-literal ([lex.string])) exceeds the number of elements to initialize.

If the value of the expression is invalid after converting to std::size_t:

(8.5)
if the expression is a potentially-evaluated core constant expression, the program is ill-formed;
(8.6)
otherwise, an allocation function is not called; instead
- (8.6.1)
  if the allocation function that would have been called has a non-throwing exception specification ([except.spec]), the value of the new-expression is the null pointer value of the required result type;
- (8.6.2)
  otherwise, the new-expression terminates by throwing an exception of a type that would match a handler ([except.handle]) of type std::bad_array_new_length .

When the value of the expression is zero, the allocation function is called to allocate an array with no elements.

If the allocated type is an array, the new-initializer is a braced-init-list, and the expression is potentially-evaluated and not a core constant expression, the semantic constraints of copy-initializing a hypothetical element of the array from an empty initializer list are checked ([dcl.init.list]).

[Note 5:

The array can contain more elements than there are elements in the braced-init-list, requiring initialization of the remainder of the array elements from an empty initializer list.

— end note]

Objects created by a new-expression have dynamic storage duration ([basic.stc.dynamic]).

[Note 6:

The lifetime of such an object is not necessarily restricted to the scope in which it is created.

— end note]

When the allocated type is “array of N T” (that is, the noptr-new-declarator syntax is used or the new-type-id or type-id denotes an array type), the new-expression yields a prvalue of type “pointer to T” that points to the initial element (if any) of the array.

Otherwise, let T be the allocated type; the new-expression is a prvalue of type “pointer to T” that points to the object created.

[Note 7:

Both new int and new int[10] have type int* and the type of new int[i][10] is int (*)[10].

— end note]

A new-expression may obtain storage for the object by calling an allocation function ([basic.stc.dynamic.allocation]).

If the new-expression terminates by throwing an exception, it may release storage by calling a deallocation function .

If the allocated type is a non-array type, the allocation function's name is operator new and the deallocation function's name is operator delete.

If the allocated type is an array type, the allocation function's name is operator new[] and the deallocation function's name is operator delete[].

[Note 8:

An implementation is expected to provide default definitions for the global allocation functions ([basic.stc.dynamic], [new.delete.single], [new.delete.array]).

A C++ program can provide alternative definitions of these functions ([replacement.functions]) and/or class-specific versions ([class.free]).

The set of allocation and deallocation functions that can be called by a new-expression can include functions that do not perform allocation or deallocation; for example, see [new.delete.placement].

— end note]

If the new-expression does not begin with a unary :: operator and the allocated type is a class type T or array thereof, a search is performed for the allocation function's name in the scope of T ([class.member.lookup]).

Otherwise, or if nothing is found, the allocation function's name is looked up by searching for it in the global scope.

An implementation is allowed to omit a call to a replaceable global allocation function ([new.delete.single], [new.delete.array]).

When it does so, the storage is instead provided by the implementation or provided by extending the allocation of another new-expression.

During an evaluation of a constant expression, a call to a replaceable allocation function is always omitted ([expr.const]).

The implementation may extend the allocation of a new-expression e1 to provide storage for a new-expression e2 if the following would be true were the allocation not extended:

(16.1)
the evaluation of e1 is sequenced before the evaluation of e2, and
(16.2)
e2 is evaluated whenever e1 obtains storage, and
(16.3)
both e1 and e2 invoke the same replaceable global allocation function, and
(16.4)
if the allocation function invoked by e1 and e2 is throwing, any exceptions thrown in the evaluation of either e1 or e2 would be first caught in the same handler, and
(16.5)
the pointer values produced by e1 and e2 are operands to evaluated delete-expressions, and
(16.6)
the evaluation of e2 is sequenced before the evaluation of the delete-expression whose operand is the pointer value produced by e1.

[Example 5: void can_merge(int x) { // These allocations are safe for merging: std::unique_ptr<char[]> a{new (std::nothrow) char[8]}; std::unique_ptr<char[]> b{new (std::nothrow) char[8]}; std::unique_ptr<char[]> c{new (std::nothrow) char[x]}; g(a.get(), b.get(), c.get()); } void cannot_merge(int x) { std::unique_ptr<char[]> a{new char[8]}; try { // Merging this allocation would change its catch handler. std::unique_ptr<char[]> b{new char[x]}; } catch (const std::bad_alloc& e) { std::cerr << "Allocation failed: " << e.what() << std::endl; throw; } } — end example]

When a new-expression calls an allocation function and that allocation has not been extended, the new-expression passes the amount of space requested to the allocation function as the first argument of type std::size_t.

That argument shall be no less than the size of the object being created; it may be greater than the size of the object being created only if the object is an array and the allocation function is not a non-allocating form ([new.delete.placement]).

For arrays of char, unsigned char, and std::byte, the difference between the result of the new-expression and the address returned by the allocation function shall be an integral multiple of the strictest fundamental alignment requirement of any object type whose size is no greater than the size of the array being created.

[Note 9:

Because allocation functions are assumed to return pointers to storage that is appropriately aligned for objects of any type with fundamental alignment, this constraint on array allocation overhead permits the common idiom of allocating character arrays into which objects of other types will later be placed.

— end note]

When a new-expression calls an allocation function and that allocation has been extended, the size argument to the allocation call shall be no greater than the sum of the sizes for the omitted calls as specified above, plus the size for the extended call had it not been extended, plus any padding necessary to align the allocated objects within the allocated memory.

The new-placement syntax is used to supply additional arguments to an allocation function; such an expression is called a placement new-expression.

Overload resolution is performed on a function call created by assembling an argument list.

The first argument is the amount of space requested, and has type std::size_t.

If the type of the allocated object has new-extended alignment, the next argument is the type's alignment, and has type std::align_val_t.

If the new-placement syntax is used, the initializer-clauses in its expression-list are the succeeding arguments.

If no matching function is found then

(20.1)
if the allocated object type has new-extended alignment, the alignment argument is removed from the argument list;
(20.2)
otherwise, an argument that is the type's alignment and has type std::align_val_t is added into the argument list immediately after the first argument;

and then overload resolution is performed again.

[Example 6:

(21.1)
new T results in one of the following calls: operator new(sizeof(T)) operator new(sizeof(T), std::align_val_t(alignof(T)))
(21.2)
new(2,f) T results in one of the following calls: operator new(sizeof(T), 2, f) operator new(sizeof(T), std::align_val_t(alignof(T)), 2, f)
(21.3)
new T[5] results in one of the following calls: operator new[](sizeof(T) * 5 + x) operator new[](sizeof(T) * 5 + x, std::align_val_t(alignof(T)))
(21.4)
new(2,f) T[5] results in one of the following calls: operator new[](sizeof(T) * 5 + x, 2, f) operator new[](sizeof(T) * 5 + x, std::align_val_t(alignof(T)), 2, f)

Here, each instance of x is a non-negative unspecified value representing array allocation overhead; the result of the new-expression will be offset by this amount from the value returned by operator new[].

This overhead may be applied in all array new-expressions, including those referencing a placement allocation function, except when referencing the library function operator new[](std::size_t, void*).

The amount of overhead may vary from one invocation of new to another.

— end example]

[Note 10:

Unless an allocation function has a non-throwing exception specification, it indicates failure to allocate storage by throwing a std::bad_alloc exception ([basic.stc.dynamic.allocation], [except], [bad.alloc]); it returns a non-null pointer otherwise.

If the allocation function has a non-throwing exception specification, it returns null to indicate failure to allocate storage and a non-null pointer otherwise.

— end note]

If the allocation function is a non-allocating form ([new.delete.placement]) that returns null, the behavior is undefined.

Otherwise, if the allocation function returns null, initialization shall not be done, the deallocation function shall not be called, and the value of the new-expression shall be null.

[Note 11:

When the allocation function returns a value other than null, it must be a pointer to a block of storage in which space for the object has been reserved.

The block of storage is assumed to be appropriately aligned ([basic.align]) and of the requested size.

The address of the created object will not necessarily be the same as that of the block if the object is an array.

— end note]

A new-expression that creates an object of type T initializes that object as follows:

(24.1)
If the new-initializer is omitted, the object is default-initialized ([dcl.init]).

[Note 12:
If no initialization is performed, the object has an indeterminate value.
— end note]
(24.2)
Otherwise, the new-initializer is interpreted according to the initialization rules of [dcl.init] for direct-initialization.

The invocation of the allocation function is sequenced before the evaluations of expressions in the new-initializer.

Initialization of the allocated object is sequenced before the value computation of the new-expression.

If the new-expression creates an array of objects of class type, the destructor is potentially invoked ([class.dtor]).

If any part of the object initialization described above60 terminates by throwing an exception and a suitable deallocation function can be found, the deallocation function is called to free the memory in which the object was being constructed, after which the exception continues to propagate in the context of the new-expression.

If no unambiguous matching deallocation function can be found, propagating the exception does not cause the object's memory to be freed.

[Note 13:

This is appropriate when the called allocation function does not allocate memory; otherwise, it is likely to result in a memory leak.

— end note]

If the new-expression does not begin with a unary :: operator and the allocated type is a class type T or an array thereof, a search is performed for the deallocation function's name in the scope of T.

Otherwise, or if nothing is found, the deallocation function's name is looked up by searching for it in the global scope.

A declaration of a placement deallocation function matches the declaration of a placement allocation function if it has the same number of parameters and, after parameter transformations ([dcl.fct]), all parameter types except the first are identical.

If the lookup finds a single matching deallocation function, that function will be called; otherwise, no deallocation function will be called.

If the lookup finds a usual deallocation function and that function, considered as a placement deallocation function, would have been selected as a match for the allocation function, the program is ill-formed.

For a non-placement allocation function, the normal deallocation function lookup is used to find the matching deallocation function ([expr.delete]).

In any case, the matching deallocation function (if any) shall be non-deleted and accessible from the point where the new-expression appears.

[Example 7: struct S { // Placement allocation function: static void* operator new(std::size_t, std::size_t); // Usual (non-placement) deallocation function: static void operator delete(void*, std::size_t); }; S* p = new (0) S; // error: non-placement deallocation function matches // placement allocation function — end example]

If a new-expression calls a deallocation function, it passes the value returned from the allocation function call as the first argument of type void*.

If a placement deallocation function is called, it is passed the same additional arguments as were passed to the placement allocation function, that is, the same arguments as those specified with the new-placement syntax.

If the implementation is allowed to introduce a temporary object or make a copy of any argument as part of the call to the allocation function, it is unspecified whether the same object is used in the call to both the allocation and deallocation functions.

59)59)

If the conversion function returns a signed integer type, the second standard conversion converts to the unsigned type std::size_t and thus thwarts any attempt to detect a negative value afterwards.

60)60)

This can include evaluating a new-initializer and/or calling a constructor.

7.6.2.9 Delete [expr.delete]

The delete-expression operator destroys a most derived object or array created by a new-expression.

delete-expression:
::

_{o p t}

delete cast-expression
::

_{o p t}

delete [ ] cast-expression

The first alternative is a single-object delete expression, and the second is an array delete expression.

Whenever the delete keyword is immediately followed by empty square brackets, it shall be interpreted as the second alternative.61

If the operand is of class type, it is contextually implicitly converted ([conv]) to a pointer to object type and the converted operand is used in place of the original operand for the remainder of this subclause.

Otherwise, it shall be a prvalue of pointer to object type.

The delete-expression has type void.

In a single-object delete expression, the value of the operand of delete may be a null pointer value, a pointer value that resulted from a previous non-array new-expression, or a pointer to a base class subobject of an object created by such a new-expression.

If not, the behavior is undefined.

In an array delete expression, the value of the operand of delete may be a null pointer value or a pointer value that resulted from a previous array new-expression whose allocation function was not a non-allocating form ([new.delete.placement]).62

If not, the behavior is undefined.

[Note 1:

This means that the syntax of the delete-expression must match the type of the object allocated by new, not the syntax of the new-expression.

— end note]

[Note 2:

A pointer to a const type can be the operand of a delete-expression; it is not necessary to cast away the constness ([expr.const.cast]) of the pointer expression before it is used as the operand of the delete-expression.

— end note]

In a single-object delete expression, if the static type of the object to be deleted is not similar ([conv.qual]) to its dynamic type and the selected deallocation function (see below) is not a destroying operator delete, the static type shall be a base class of the dynamic type of the object to be deleted and the static type shall have a virtual destructor or the behavior is undefined.

In an array delete expression, if the dynamic type of the object to be deleted is not similar to its static type, the behavior is undefined.

If the object being deleted has incomplete class type at the point of deletion, the program is ill-formed.

If the value of the operand of the delete-expression is not a null pointer value and the selected deallocation function (see below) is not a destroying operator delete, evaluating the delete-expression invokes the destructor (if any) for the object or the elements of the array being deleted.

The destructor shall be accessible from the point where the delete-expression appears.

In the case of an array, the elements are destroyed in order of decreasing address (that is, in reverse order of the completion of their constructor; see [class.base.init]).

If the value of the operand of the delete-expression is not a null pointer value, then:

(6.1)
If the allocation call for the new-expression for the object to be deleted was not omitted and the allocation was not extended ([expr.new]), the delete-expression shall call a deallocation function .

The value returned from the allocation call of the new-expression shall be passed as the first argument to the deallocation function.
(6.2)
Otherwise, if the allocation was extended or was provided by extending the allocation of another new-expression, and the delete-expression for every other pointer value produced by a new-expression that had storage provided by the extended new-expression has been evaluated, the delete-expression shall call a deallocation function.

The value returned from the allocation call of the extended new-expression shall be passed as the first argument to the deallocation function.
(6.3)
Otherwise, the delete-expression will not call a deallocation function.

[Note 3:

The deallocation function is called regardless of whether the destructor for the object or some element of the array throws an exception.

— end note]

If the value of the operand of the delete-expression is a null pointer value, it is unspecified whether a deallocation function will be called as described above.

If a deallocation function is called, it is operator delete for a single-object delete expression or operator delete[] for an array delete expression.

[Note 4:

An implementation provides default definitions of the global deallocation functions ([new.delete.single], [new.delete.array]).

A C++ program can provide alternative definitions of these functions ([replacement.functions]), and/or class-specific versions ([class.free]).

— end note]

If the keyword delete in a delete-expression is not preceded by the unary :: operator and the type of the operand is a pointer to a (possibly cv-qualified) class type T or (possibly multidimensional) array thereof:

(8.1)
For a single-object delete expression, if the operand is a pointer to cv T and T has a virtual destructor, the deallocation function is the one selected at the point of definition of the dynamic type's virtual destructor ([class.dtor]).
(8.2)
Otherwise, a search is performed for the deallocation function's name in the scope of T.

Otherwise, or if nothing is found, the deallocation function's name is looked up by searching for it in the global scope.

In any case, any declarations other than of usual deallocation functions ([basic.stc.dynamic.deallocation]) are discarded.

[Note 5:

If only a placement deallocation function is found in a class, the program is ill-formed because the lookup set is empty ([basic.lookup]).

— end note]

The deallocation function to be called is selected as follows:

(9.1)
If any of the deallocation functions is a destroying operator delete, all deallocation functions that are not destroying operator deletes are eliminated from further consideration.
(9.2)
If the type has new-extended alignment, a function with a parameter of type std::align_val_t is preferred; otherwise a function without such a parameter is preferred.

If any preferred functions are found, all non-preferred functions are eliminated from further consideration.
(9.3)
If exactly one function remains, that function is selected and the selection process terminates.
(9.4)
If the deallocation functions belong to a class scope, the one without a parameter of type std::size_t is selected.
(9.5)
If the type is complete and if, for an array delete expression only, the operand is a pointer to a class type with a non-trivial destructor or a (possibly multidimensional) array thereof, the function with a parameter of type std::size_t is selected.
(9.6)
Otherwise, it is unspecified whether a deallocation function with a parameter of type std::size_t is selected.

Unless the deallocation function is selected at the point of definition of the dynamic type's virtual destructor, the selected deallocation function shall be accessible from the point where the delete-expression appears.

For a single-object delete expression, the deleted object is the object A pointed to by the operand if the static type of A does not have a virtual destructor, and the most-derived object of A otherwise.

[Note 6:

If the deallocation function is not a destroying operator delete and the deleted object is not the most derived object in the former case, the behavior is undefined, as stated above.

— end note]

For an array delete expression, the deleted object is the array object.

When a delete-expression is executed, the selected deallocation function shall be called with the address of the deleted object in a single-object delete expression, or the address of the deleted object suitably adjusted for the array allocation overhead ([expr.new]) in an array delete expression, as its first argument.

[Note 7:

Any cv-qualifiers in the type of the deleted object are ignored when forming this argument.

— end note]

If a destroying operator delete is used, an unspecified value is passed as the argument corresponding to the parameter of type std::destroying_delete_t.

If a deallocation function with a parameter of type std::align_val_t is used, the alignment of the type of the deleted object is passed as the corresponding argument.

If a deallocation function with a parameter of type std::size_t is used, the size of the deleted object in a single-object delete expression, or of the array plus allocation overhead in an array delete expression, is passed as the corresponding argument.

[Note 8:

If this results in a call to a replaceable deallocation function, and either the first argument was not the result of a prior call to a replaceable allocation function or the second or third argument was not the corresponding argument in said call, the behavior is undefined ([new.delete.single], [new.delete.array]).

— end note]

61)61)

A lambda-expression with a lambda-introducer that consists of empty square brackets can follow the delete keyword if the lambda-expression is enclosed in parentheses.

62)62)

For nonzero-length arrays, this is the same as a pointer to the first element of the array created by that new-expression.

Zero-length arrays do not have a first element.

7.6.3 Explicit type conversion (cast notation) [expr.cast]

The result of the expression (T) cast-expression is of type T.

The result is an lvalue if T is an lvalue reference type or an rvalue reference to function type and an xvalue if T is an rvalue reference to object type; otherwise the result is a prvalue.

[Note 1:

If T is a non-class type that is cv-qualified, the cv-qualifiers are discarded when determining the type of the resulting prvalue; see [expr.prop].

— end note]

An explicit type conversion can be expressed using functional notation ([expr.type.conv]), a type conversion operator (dynamic_cast, static_cast, reinterpret_cast, const_cast), or the cast notation.

cast-expression:
unary-expression
( type-id ) cast-expression

Any type conversion not mentioned below and not explicitly defined by the user ([class.conv]) is ill-formed.

The conversions performed by

(4.1)
a const_cast ([expr.const.cast]),
(4.2)
a static_cast ([expr.static.cast]),
(4.3)
a static_cast followed by a const_cast,
(4.4)
a reinterpret_cast ([expr.reinterpret.cast]), or
(4.5)
a reinterpret_cast followed by a const_cast,

can be performed using the cast notation of explicit type conversion.

The same semantic restrictions and behaviors apply, with the exception that in performing a static_cast in the following situations the conversion is valid even if the base class is inaccessible:

(4.6)
a pointer to an object of derived class type or an lvalue or rvalue of derived class type may be explicitly converted to a pointer or reference to an unambiguous base class type, respectively;
(4.7)
a pointer to member of derived class type may be explicitly converted to a pointer to member of an unambiguous non-virtual base class type;
(4.8)
a pointer to an object of an unambiguous non-virtual base class type, a glvalue of an unambiguous non-virtual base class type, or a pointer to member of an unambiguous non-virtual base class type may be explicitly converted to a pointer, a reference, or a pointer to member of a derived class type, respectively.

If a conversion can be interpreted in more than one of the ways listed above, the interpretation that appears first in the list is used, even if a cast resulting from that interpretation is ill-formed.

If a static_cast followed by a const_cast is used and the conversion can be interpreted in more than one way as such, the conversion is ill-formed.

[Example 1: struct A { }; struct I1 : A { }; struct I2 : A { }; struct D : I1, I2 { }; A* foo( D* p ) { return (A*)( p ); // ill-formed static_cast interpretation } int*** ptr = 0; auto t = (int const*const*const*)ptr; // OK, const_cast interpretation struct S { operator const int*(); operator volatile int*(); }; int *p = (int*)S(); // error: two possible interpretations using static_cast followed by const_cast — end example]

The operand of a cast using the cast notation can be a prvalue of type “pointer to incomplete class type”.

The destination type of a cast using the cast notation can be “pointer to incomplete class type”.

If both the operand and destination types are class types and one or both are incomplete, it is unspecified whether the static_cast or the reinterpret_cast interpretation is used, even if there is an inheritance relationship between the two classes.

[Note 2:

For example, if the classes were defined later in the translation unit, a multi-pass compiler could validly interpret a cast between pointers to the classes as if the class types were complete at the point of the cast.

— end note]

7.6.4 Pointer-to-member operators [expr.mptr.oper]

The pointer-to-member operators ->* and .* group left-to-right.

pm-expression:
cast-expression
pm-expression .* cast-expression
pm-expression ->* cast-expression

The binary operator .* binds its second operand, which shall be a prvalue of type “pointer to member of T” to its first operand, which shall be a glvalue of class T or of a class of which T is an unambiguous and accessible base class.

The result is an object or a function of the type specified by the second operand.

The binary operator ->* binds its second operand, which shall be a prvalue of type “pointer to member of T” to its first operand, which shall be of type “pointer to U” where U is either T or a class of which T is an unambiguous and accessible base class.

The expression E1->*E2 is converted into the equivalent form (*(E1)).*E2.

Abbreviating pm-expression.*cast-expression as E1.*E2, E1 is called the object expression.

If the result of E1 is an object whose type is not similar to the type of E1, or whose most derived object does not contain the member to which E2 refers, the behavior is undefined.

The expression E1 is sequenced before the expression E2.

The restrictions on cv-qualification, and the manner in which the cv-qualifiers of the operands are combined to produce the cv-qualifiers of the result, are the same as the rules for E1.E2 given in [expr.ref].

[Note 1:

It is not possible to use a pointer to member that refers to a mutable member to modify a const class object.

For example, struct S { S() : i(0) { } mutable int i; }; void f() { const S cs; int S::* pm = &S::i; // pm refers to mutable member S::i cs.*pm = 88; // error: cs is a const object }

— end note]

If the result of .* or ->* is a function, then that result can be used only as the operand for the function call operator ().

[Example 1:

(ptr_to_obj->*ptr_to_mfct)(10); calls the member function denoted by ptr_to_mfct for the object pointed to by ptr_to_obj.

— end example]

In a .* expression whose object expression is an rvalue, the program is ill-formed if the second operand is a pointer to member function whose ref-qualifier is &, unless its cv-qualifier-seq is const.

In a .* expression whose object expression is an lvalue, the program is ill-formed if the second operand is a pointer to member function whose ref-qualifier is &&.

The result of a .* expression whose second operand is a pointer to a data member is an lvalue if the first operand is an lvalue and an xvalue otherwise.

The result of a .* expression whose second operand is a pointer to a member function is a prvalue.

If the second operand is the null member pointer value, the behavior is undefined.

7.6.5 Multiplicative operators [expr.mul]

The multiplicative operators *, /, and % group left-to-right.

multiplicative-expression:
pm-expression
multiplicative-expression * pm-expression
multiplicative-expression / pm-expression
multiplicative-expression % pm-expression

The operands of * and / shall have arithmetic or unscoped enumeration type; the operands of % shall have integral or unscoped enumeration type.

The usual arithmetic conversions are performed on the operands and determine the type of the result.

The binary * operator indicates multiplication.

The binary / operator yields the quotient, and the binary % operator yields the remainder from the division of the first expression by the second.

If the second operand of / or % is zero, the behavior is undefined.

For integral operands, the / operator yields the algebraic quotient with any fractional part discarded;63 if the quotient a/b is representable in the type of the result, (a/b)*b + a%b is equal to a; otherwise, the behavior of both a/b and a%b is undefined.

63)63)

This is often called truncation towards zero.

7.6.6 Additive operators [expr.add]

The additive operators + and - group left-to-right.

Each operand shall be a prvalue.

If both operands have arithmetic or unscoped enumeration type, the usual arithmetic conversions ([expr.arith.conv]) are performed.

Otherwise, if one operand has arithmetic or unscoped enumeration type, integral promotion is applied ([conv.prom]) to that operand.

A converted or promoted operand is used in place of the corresponding original operand for the remainder of this section.

additive-expression:
multiplicative-expression
additive-expression + multiplicative-expression
additive-expression - multiplicative-expression

For addition, either both operands shall have arithmetic type, or one operand shall be a pointer to a completely-defined object type and the other shall have integral type.

For subtraction, one of the following shall hold:

(2.1)
both operands have arithmetic type; or
(2.2)
both operands are pointers to cv-qualified or cv-unqualified versions of the same completely-defined object type; or
(2.3)
the left operand is a pointer to a completely-defined object type and the right operand has integral type.

The result of the binary + operator is the sum of the operands.

The result of the binary - operator is the difference resulting from the subtraction of the second operand from the first.

When an expression J that has integral type is added to or subtracted from an expression P of pointer type, the result has the type of P.

(4.1)
If P evaluates to a null pointer value and J evaluates to 0, the result is a null pointer value.
(4.2)
Otherwise, if P points to a (possibly-hypothetical) array element i of an array object x with n elements ([dcl.array]),64 the expressions P + J and J + P (where J has the value j) point to the (possibly-hypothetical) array element $i + j$ of x if $0 \leq i + j \leq n$ and the expression P - J points to the (possibly-hypothetical) array element $i - j$ of x if $0 \leq i - j \leq n$ .
(4.3)
Otherwise, the behavior is undefined.

[Note 1:

Adding a value other than 0 or 1 to a pointer to a base class subobject, a member subobject, or a complete object results in undefined behavior.

— end note]

When two pointer expressions P and Q are subtracted, the type of the result is an implementation-defined signed integral type; this type shall be the same type that is defined as std::ptrdiff_t in the <cstddef> header ([support.types.layout]).

(5.1)
If P and Q both evaluate to null pointer values, the result is 0.
(5.2)
Otherwise, if P and Q point to, respectively, array elements i and j of the same array object x, the expression P - Q has the value $i - j$ .

[Note 2:
If the value $i - j$ is not in the range of representable values of type std::ptrdiff_t, the behavior is undefined ([expr.pre]).
— end note]
(5.3)
Otherwise, the behavior is undefined.

For addition or subtraction, if the expressions P or Q have type “pointer to cv T”, where T and the array element type are not similar, the behavior is undefined.

[Example 1: int arr[5] = {1, 2, 3, 4, 5}; unsigned int *p = reinterpret_cast<unsigned int*>(arr + 1); unsigned int k = *p; // OK, value of k is 2 ([conv.lval]) unsigned int *q = p + 1; // undefined behavior: p points to an int, not an unsigned int object — end example]

64)64)

As specified in [basic.compound], an object that is not an array element is considered to belong to a single-element array for this purpose and a pointer past the last element of an array of n elements is considered to be equivalent to a pointer to a hypothetical array element n for this purpose.

7.6.7 Shift operators [expr.shift]

The shift operators << and >> group left-to-right.

shift-expression:
additive-expression
shift-expression << additive-expression
shift-expression >> additive-expression

The operands shall be prvalues of integral or unscoped enumeration type and integral promotions are performed.

The type of the result is that of the promoted left operand.

The behavior is undefined if the right operand is negative, or greater than or equal to the width of the promoted left operand.

The value of E1 << E2 is the unique value congruent to

E1 \times 2^{E2}

modulo

2^{N}

, where N is the width of the type of the result.

[Note 1:

E1 is left-shifted E2 bit positions; vacated bits are zero-filled.

— end note]

The value of E1 >> E2 is

E1 / 2^{E2}

, rounded towards negative infinity.

[Note 2:

E1 is right-shifted E2 bit positions.

Right-shift on signed integral types is an arithmetic right shift, which performs sign-extension.

— end note]

The expression E1 is sequenced before the expression E2.

7.6.8 Three-way comparison operator [expr.spaceship]

The three-way comparison operator groups left-to-right.

compare-expression:
shift-expression
compare-expression <=> shift-expression

The expression p <=> q is a prvalue indicating whether p is less than, equal to, greater than, or incomparable with q.

If one of the operands is of type bool and the other is not, the program is ill-formed.

If both operands have arithmetic types, or one operand has integral type and the other operand has unscoped enumeration type, the usual arithmetic conversions are applied to the operands.

Then:

(4.1)
If a narrowing conversion is required, other than from an integral type to a floating-point type, the program is ill-formed.
(4.2)
Otherwise, if the operands have integral type, the result is of type std::strong_ordering.

The result is std::strong_ordering::equal if both operands are arithmetically equal, std::strong_ordering::less if the first operand is arithmetically less than the second operand, and std::strong_ordering::greater otherwise.
(4.3)
Otherwise, the operands have floating-point type, and the result is of type std::partial_ordering.

The expression a <=> b yields std::partial_ordering::less if a is less than b, std::partial_ordering::greater if a is greater than b, std::partial_ordering::equivalent if a is equivalent to b, and std::partial_ordering::unordered otherwise.

If both operands have the same enumeration type E, the operator yields the result of converting the operands to the underlying type of E and applying <=> to the converted operands.

If at least one of the operands is of object pointer type and the other operand is of object pointer or array type, array-to-pointer conversions ([conv.array]), pointer conversions ([conv.ptr]), and qualification conversions are performed on both operands to bring them to their composite pointer type ([expr.type]).

After the conversions, the operands shall have the same type.

[Note 1:

If both of the operands are arrays, array-to-pointer conversions are not applied.

— end note]

In this case, p <=> q is of type std::strong_ordering and the result is defined by the following rules:

(6.1)
If two pointer operands p and q compare equal ([expr.eq]), p <=> q yields std::strong_ordering::equal;
(6.2)
otherwise, if p and q compare unequal, p <=> q yields std::strong_ordering::less if q compares greater than p and std::strong_ordering::greater if p compares greater than q ([expr.rel]);
(6.3)
otherwise, the result is unspecified.

Otherwise, the program is ill-formed.

The three comparison category types ([cmp.categories]) (the types std::strong_ordering, std::weak_ordering, and std::partial_ordering) are not predefined; if a standard library declaration ([compare.syn], [std.modules]) of such a class type does not precede ([basic.lookup.general]) a use of that type — even an implicit use in which the type is not named (e.g., via the auto specifier ([dcl.spec.auto]) in a defaulted three-way comparison ([class.spaceship]) or use of the built-in operator) — the program is ill-formed.

7.6.9 Relational operators [expr.rel]

The relational operators group left-to-right.

[Example 1:

a<b<c means (a<b)<c and not (a<b)&&(b<c).

— end example]

relational-expression:
compare-expression
relational-expression < compare-expression
relational-expression > compare-expression
relational-expression <= compare-expression
relational-expression >= compare-expression

The lvalue-to-rvalue ([conv.lval]) and function-to-pointer ([conv.func]) standard conversions are performed on the operands.

If one of the operands is a pointer, the array-to-pointer conversion ([conv.array]) is performed on the other operand.

The converted operands shall have arithmetic, enumeration, or pointer type.

The operators < (less than), > (greater than), <= (less than or equal to), and >= (greater than or equal to) all yield false or true.

The type of the result is bool.

The usual arithmetic conversions ([expr.arith.conv]) are performed on operands of arithmetic or enumeration type.

If both converted operands are pointers, pointer conversions ([conv.ptr]), function pointer conversions ([conv.fctptr]), and qualification conversions ([conv.qual]) are performed to bring them to their composite pointer type .

After conversions, the operands shall have the same type.

The result of comparing unequal pointers to objects65 is defined in terms of a partial order consistent with the following rules:

(4.1)
If two pointers point to different elements of the same array, or to subobjects thereof, the pointer to the element with the higher subscript is required to compare greater.
(4.2)
If two pointers point to different non-static data members of the same object, or to subobjects of such members, recursively, the pointer to the later declared member is required to compare greater provided neither member is a subobject of zero size and their class is not a union.
(4.3)
Otherwise, neither pointer is required to compare greater than the other.

If two operands p and q compare equal ([expr.eq]), p<=q and p>=q both yield true and p<q and p>q both yield false.

Otherwise, if a pointer to object p compares greater than a pointer q, p>=q, p>q, q<=p, and q<p all yield true and p<=q, p<q, q>=p, and q>p all yield false.

Otherwise, the result of each of the operators is unspecified.

[Note 1:

A relational operator applied to unequal function pointers yields an unspecified result.

A pointer value of type “pointer to cv void” can point to an object ([basic.compound]).

— end note]

If both operands (after conversions) are of arithmetic or enumeration type, each of the operators shall yield true if the specified relationship is true and false if it is false.

65)65)

7.6.10 Equality operators [expr.eq]

equality-expression:
relational-expression
equality-expression == relational-expression
equality-expression != relational-expression

The == (equal to) and the != (not equal to) operators group left-to-right.

The lvalue-to-rvalue ([conv.lval]) and function-to-pointer ([conv.func]) standard conversions are performed on the operands.

If one of the operands is a pointer or a null pointer constant ([conv.ptr]), the array-to-pointer conversion ([conv.array]) is performed on the other operand.

The converted operands shall have scalar type.

The operators == and != both yield true or false, i.e., a result of type bool.

In each case below, the operands shall have the same type after the specified conversions have been applied.

If at least one of the converted operands is a pointer, pointer conversions, function pointer conversions, and qualification conversions are performed on both operands to bring them to their composite pointer type .

Comparing pointers is defined as follows:

(3.1)
If one pointer represents the address of a complete object, and another pointer represents the address one past the last element of a different complete object,66 the result of the comparison is unspecified.
(3.2)
Otherwise, if the pointers are both null, both point to the same function, or both represent the same address, they compare equal.
(3.3)
Otherwise, the pointers compare unequal.

If at least one of the operands is a pointer to member, pointer-to-member conversions ([conv.mem]), function pointer conversions ([conv.fctptr]), and qualification conversions ([conv.qual]) are performed on both operands to bring them to their composite pointer type ([expr.type]).

Comparing pointers to members is defined as follows:

(4.1)
If two pointers to members are both the null member pointer value, they compare equal.
(4.2)
If only one of two pointers to members is the null member pointer value, they compare unequal.
(4.3)
If either is a pointer to a virtual member function, the result is unspecified.
(4.4)
If one refers to a member of class C1 and the other refers to a member of a different class C2, where neither is a base class of the other, the result is unspecified.

[Example 1: struct A {}; struct B : A { int x; }; struct C : A { int x; }; int A::*bx = (int(A::*))&B::x; int A::*cx = (int(A::*))&C::x; bool b1 = (bx == cx); // unspecified — end example]
(4.5)
If both refer to (possibly different) members of the same union, they compare equal.
(4.6)
Otherwise, two pointers to members compare equal if they would refer to the same member of the same most derived object or the same subobject if indirection with a hypothetical object of the associated class type were performed, otherwise they compare unequal.

[Example 2: struct B { int f(); }; struct L : B { }; struct R : B { }; struct D : L, R { }; int (B::*pb)() = &B::f; int (L::*pl)() = pb; int (R::*pr)() = pb; int (D::*pdl)() = pl; int (D::*pdr)() = pr; bool x = (pdl == pdr); // false bool y = (pb == pl); // true — end example]

Two operands of type std::nullptr_t or one operand of type std::nullptr_t and the other a null pointer constant compare equal.

If two operands compare equal, the result is true for the == operator and false for the != operator.

If two operands compare unequal, the result is false for the == operator and true for the != operator.

Otherwise, the result of each of the operators is unspecified.

If both operands are of arithmetic or enumeration type, the usual arithmetic conversions ([expr.arith.conv]) are performed on both operands; each of the operators shall yield true if the specified relationship is true and false if it is false.

66)66)

As specified in [basic.compound], an object that is not an array element is considered to belong to a single-element array for this purpose.

7.6.11 Bitwise AND operator [expr.bit.and]

and-expression:
equality-expression
and-expression & equality-expression

The & operator groups left-to-right.

The operands shall be of integral or unscoped enumeration type.

The usual arithmetic conversions ([expr.arith.conv]) are performed.

Given the coefficients

x_{i}

and

y_{i}

of the base-2 representation ([basic.fundamental]) of the converted operands x and y, the coefficient

r_{i}

of the base-2 representation of the result r is 1 if both

x_{i}

and

y_{i}

are 1, and 0 otherwise.

[Note 1:

The result is the bitwise and function of the operands.

— end note]

7.6.12 Bitwise exclusive OR operator [expr.xor]

exclusive-or-expression:
and-expression
exclusive-or-expression ^ and-expression

The ^ operator groups left-to-right.

The operands shall be of integral or unscoped enumeration type.

The usual arithmetic conversions ([expr.arith.conv]) are performed.

Given the coefficients

x_{i}

and

y_{i}

of the base-2 representation ([basic.fundamental]) of the converted operands x and y, the coefficient

r_{i}

of the base-2 representation of the result r is 1 if either (but not both) of

x_{i}

and

y_{i}

is 1, and 0 otherwise.

[Note 1:

The result is the bitwise exclusive or function of the operands.

— end note]

7.6.13 Bitwise inclusive OR operator [expr.or]

inclusive-or-expression:
exclusive-or-expression
inclusive-or-expression | exclusive-or-expression

The | operator groups left-to-right.

The operands shall be of integral or unscoped enumeration type.

The usual arithmetic conversions ([expr.arith.conv]) are performed.

Given the coefficients

x_{i}

and

y_{i}

of the base-2 representation ([basic.fundamental]) of the converted operands x and y, the coefficient

r_{i}

of the base-2 representation of the result r is 1 if at least one of

x_{i}

and

y_{i}

is 1, and 0 otherwise.

[Note 1:

The result is the bitwise inclusive or function of the operands.

— end note]

7.6.14 Logical AND operator [expr.log.and]

logical-and-expression:
inclusive-or-expression
logical-and-expression && inclusive-or-expression

The && operator groups left-to-right.

The operands are both contextually converted to bool ([conv]).

The result is true if both operands are true and false otherwise.

Unlike &, && guarantees left-to-right evaluation: the second operand is not evaluated if the first operand is false.

The result is a bool.

If the second expression is evaluated, the first expression is sequenced before the second expression ([intro.execution]).

7.6.15 Logical OR operator [expr.log.or]

logical-or-expression:
logical-and-expression
logical-or-expression || logical-and-expression

The || operator groups left-to-right.

The operands are both contextually converted to bool ([conv]).

The result is true if either of its operands is true, and false otherwise.

Unlike |, || guarantees left-to-right evaluation; moreover, the second operand is not evaluated if the first operand evaluates to true.

The result is a bool.

If the second expression is evaluated, the first expression is sequenced before the second expression ([intro.execution]).

7.6.16 Conditional operator [expr.cond]

conditional-expression:
logical-or-expression
logical-or-expression ? expression : assignment-expression

Conditional expressions group right-to-left.

The first expression is contextually converted to bool ([conv]).

It is evaluated and if it is true, the result of the conditional expression is the value of the second expression, otherwise that of the third expression.

Only one of the second and third expressions is evaluated.

The first expression is sequenced before the second or third expression ([intro.execution]).

If either the second or the third operand has type void, one of the following shall hold:

(2.1)
The second or the third operand (but not both) is a (possibly parenthesized) throw-expression ([expr.throw]); the result is of the type and value category of the other.

The conditional-expression is a bit-field if that operand is a bit-field.
(2.2)
Both the second and the third operands have type void; the result is of type void and is a prvalue.

[Note 1:
This includes the case where both operands are throw-expressions.
— end note]

Otherwise, if the second and third operand are glvalue bit-fields of the same value category and of types cv1 T and cv2 T, respectively, the operands are considered to be of type cv T for the remainder of this subclause, where cv is the union of cv1 and cv2.

Otherwise, if the second and third operand have different types and either has (possibly cv-qualified) class type, or if both are glvalues of the same value category and the same type except for cv-qualification, an attempt is made to form an implicit conversion sequence from each of those operands to the type of the other.

[Note 2:

Properties such as access, whether an operand is a bit-field, or whether a conversion function is deleted are ignored for that determination.

— end note]

Attempts are made to form an implicit conversion sequence from an operand expression E1 of type T1 to a target type related to the type T2 of the operand expression E2 as follows:

(4.1)
If E2 is an lvalue, the target type is “lvalue reference to T2”, but an implicit conversion sequence can only be formed if the reference would bind directly ([dcl.init.ref]) to a glvalue.
(4.2)
If E2 is an xvalue, the target type is “rvalue reference to T2”, but an implicit conversion sequence can only be formed if the reference would bind directly.
(4.3)
If E2 is a prvalue or if neither of the conversion sequences above can be formed and at least one of the operands has (possibly cv-qualified) class type:
- (4.3.1)
  if T1 and T2 are the same class type (ignoring cv-qualification):
  - (4.3.1.1)
    if T2 is at least as cv-qualified as T1, the target type is T2,
  - (4.3.1.2)
    otherwise, no conversion sequence is formed for this operand;
- (4.3.2)
  otherwise, if T2 is a base class of T1, the target type is cv1 T2, where cv1 denotes the cv-qualifiers of T1;
- (4.3.3)
  otherwise, the target type is the type that E2 would have after applying the lvalue-to-rvalue, array-to-pointer, and function-to-pointer standard conversions.

Using this process, it is determined whether an implicit conversion sequence can be formed from the second operand to the target type determined for the third operand, and vice versa, with the following outcome:

(4.4)
If both sequences can be formed, or one can be formed but it is the ambiguous conversion sequence, the program is ill-formed.
(4.5)
If no conversion sequence can be formed, the operands are left unchanged and further checking is performed as described below.
(4.6)
Otherwise, if exactly one conversion sequence can be formed, that conversion is applied to the chosen operand and the converted operand is used in place of the original operand for the remainder of this subclause.

[Note 3:
The conversion might be ill-formed even if an implicit conversion sequence could be formed.
— end note]

If the second and third operands are glvalues of the same value category and have the same type, the result is of that type and value category and it is a bit-field if the second or the third operand is a bit-field, or if both are bit-fields.

Otherwise, the result is a prvalue.

If the second and third operands do not have the same type, and either has (possibly cv-qualified) class type, overload resolution is used to determine the conversions (if any) to be applied to the operands ([over.match.oper], [over.built]).

If the overload resolution fails, the program is ill-formed.

Otherwise, the conversions thus determined are applied, and the converted operands are used in place of the original operands for the remainder of this subclause.

Array-to-pointer and function-to-pointer standard conversions are performed on the second and third operands.

After those conversions, one of the following shall hold:

(7.1)
The second and third operands have the same type; the result is of that type and the result is copy-initialized using the selected operand.
(7.2)
The second and third operands have arithmetic or enumeration type; the usual arithmetic conversions are performed to bring them to a common type, and the result is of that type.
(7.3)
One or both of the second and third operands have pointer type; lvalue-to-rvalue, pointer, function pointer, and qualification conversions are performed to bring them to their composite pointer type .

The result is of the composite pointer type.
(7.4)
One or both of the second and third operands have pointer-to-member type; lvalue-to-rvalue ([conv.lval]), pointer to member ([conv.mem]), function pointer ([conv.fctptr]), and qualification conversions ([conv.qual]) are performed to bring them to their composite pointer type ([expr.type]).

The result is of the composite pointer type.
(7.5)
Both the second and third operands have type std::nullptr_t or one has that type and the other is a null pointer constant.

The result is of type std::nullptr_t.

7.6.17 Yielding a value [expr.yield]

yield-expression:
co_yield assignment-expression
co_yield braced-init-list

A yield-expression shall appear only within a suspension context of a function ([expr.await]).

Let e be the operand of the yield-expression and p be an lvalue naming the promise object of the enclosing coroutine ([dcl.fct.def.coroutine]), then the yield-expression is equivalent to the expression co_await p.yield_value(e).

[Example 1: template <typename T> struct my_generator { struct promise_type { T current_value; /* ... */ auto yield_value(T v) { current_value = std::move(v); return std::suspend_always{}; } }; struct iterator { /* ... */ }; iterator begin(); iterator end(); }; my_generator<pair<int,int>> g1() { for (int i = 0; i < 10; ++i) co_yield {i,i}; } my_generator<pair<int,int>> g2() { for (int i = 0; i < 10; ++i) co_yield make_pair(i,i); } auto f(int x = co_yield 5); // error: yield-expression outside of function suspension context int a[] = { co_yield 1 }; // error: yield-expression outside of function suspension context int main() { auto r1 = g1(); auto r2 = g2(); assert(std::equal(r1.begin(), r1.end(), r2.begin(), r2.end())); } — end example]

7.6.18 Throwing an exception [expr.throw]

throw-expression:
throw assignment-expression

_{o p t}

A throw-expression is of type void.

A throw-expression with an operand throws an exception ([except.throw]).

The array-to-pointer ([conv.array]) and function-to-pointer ([conv.func]) standard conversions are performed on the operand.

The type of the exception object is determined by removing any top-level cv-qualifiers from the type of the (possibly converted) operand.

The exception object is copy-initialized ([dcl.init.general]) from the (possibly converted) operand.

A throw-expression with no operand rethrows the currently handled exception ([except.handle]).

If no exception is presently being handled, the function std::terminate is invoked ([except.terminate]).

Otherwise, the exception is reactivated with the existing exception object; no new exception object is created.

The exception is no longer considered to be caught.

[Example 1:

An exception handler that cannot completely handle the exception itself can be written like this: try { // ... } catch (...) { // catch all exceptions // respond (partially) to exception throw; // pass the exception to some other handler }

— end example]

7.6.19 Assignment and compound assignment operators [expr.ass]

The assignment operator (=) and the compound assignment operators all group right-to-left.

All require a modifiable lvalue as their left operand; their result is an lvalue of the type of the left operand, referring to the left operand.

The result in all cases is a bit-field if the left operand is a bit-field.

In all cases, the assignment is sequenced after the value computation of the right and left operands, and before the value computation of the assignment expression.

The right operand is sequenced before the left operand.

With respect to an indeterminately-sequenced function call, the operation of a compound assignment is a single evaluation.

[Note 1:

Therefore, a function call cannot intervene between the lvalue-to-rvalue conversion and the side effect associated with any single compound assignment operator.

— end note]

assignment-expression:
conditional-expression
yield-expression
throw-expression
logical-or-expression assignment-operator initializer-clause

assignment-operator: one of
= *= /= %= += -= >>= <<= &= ^= |=

In simple assignment (=), let V be the result of the right operand; the object referred to by the left operand is modified ([defns.access]) by replacing its value with V or, if the object is of integer type, with the value congruent ([basic.fundamental]) to V.

If the right operand is an expression, it is implicitly converted to the cv-unqualified type of the left operand.

When the left operand of an assignment operator is a bit-field that cannot represent the value of the expression, the resulting value of the bit-field is implementation-defined.

An assignment whose left operand is of a volatile-qualified type is deprecated ([depr.volatile.type]) unless the (possibly parenthesized) assignment is a discarded-value expression or an unevaluated operand .

The behavior of an expression of the form E1 op= E2 is equivalent to E1 = E1 op E2 except that E1 is evaluated only once.

[Note 2:

The object designated by E1 is accessed twice.

— end note]

For += and -=, E1 shall either have arithmetic type or be a pointer to a possibly cv-qualified completely-defined object type.

In all other cases, E1 shall have arithmetic type.

If the value being stored in an object is read via another object that overlaps in any way the storage of the first object, then the overlap shall be exact and the two objects shall have the same type, otherwise the behavior is undefined.

[Note 3:

This restriction applies to the relationship between the left and right sides of the assignment operation; it is not a statement about how the target of the assignment can be aliased in general.

See [basic.lval].

— end note]

A braced-init-list B may appear on the right-hand side of

(8.1)
an assignment to a scalar of type T, in which case B shall have at most a single element.

The meaning of x = B is x = t, where t is an invented temporary variable declared and initialized as T t = B.
(8.2)
an assignment to an object of class type, in which case B is passed as the argument to the assignment operator function selected by overload resolution ([over.ass], [over.match]).

[Example 1: complex<double> z; z = { 1,2 }; // meaning z.operator=({1,2}) z += { 1, 2 }; // meaning z.operator+=({1,2}) int a, b; a = b = { 1 }; // meaning a=b=1; a = { 1 } = b; // syntax error — end example]

7.6.20 Comma operator [expr.comma]

The comma operator groups left-to-right.

expression:
assignment-expression
expression , assignment-expression

A pair of expressions separated by a comma is evaluated left-to-right; the left expression is a discarded-value expression .

The left expression is sequenced before the right expression ([intro.execution]).

The type and value of the result are the type and value of the right operand; the result is of the same value category as its right operand, and is a bit-field if its right operand is a bit-field.

[Note 1:

In contexts where the comma token is given special meaning (e.g., function calls ([expr.call]), subscript expressions ([expr.sub]), lists of initializers ([dcl.init]), or template-argument-lists ([temp.names])), the comma operator as described in this subclause can appear only in parentheses.

[Example 1:

f(a, (t=3, t+2), c); has three arguments, the second of which has the value 5.

— end example]

— end note]

7.7 Constant expressions [expr.const]

Certain contexts require expressions that satisfy additional requirements as detailed in this subclause; other contexts have different semantics depending on whether or not an expression satisfies these requirements.

Expressions that satisfy these requirements, assuming that copy elision is not performed, are called constant expressions.

[Note 1:

Constant expressions can be evaluated during translation.

— end note]

constant-expression:
conditional-expression

The constituent values of an object o are

(2.1)
if o has scalar type, the value of o;
(2.2)
otherwise, the constituent values of any direct subobjects of o other than inactive union members.

The constituent references of an object o are

(2.3)
any direct members of o that have reference type, and
(2.4)
the constituent references of any direct subobjects of o other than inactive union members.

The constituent values and constituent references of a variable x are defined as follows:

(3.1)
If x declares an object, the constituent values and references of that object are constituent values and references of x.
(3.2)
If x declares a reference, that reference is a constituent reference of x.

For any constituent reference r of a variable x, if r is bound to a temporary object or subobject thereof whose lifetime is extended to that of r, the constituent values and references of that temporary object are also constituent values and references of x, recursively.

An object o is constexpr-referenceable from a point P if

(4.1)
o has static storage duration, or
(4.2)
o has automatic storage duration, and, letting v denote
- (4.2.1)
  the variable corresponding to o's complete object or
- (4.2.2)
  the variable to whose lifetime that of o is extended,
the smallest scope enclosing v and the smallest scope enclosing P that are neither
- (4.2.3)
  block scopes nor
- (4.2.4)
  function parameter scopes associated with a requirement-parameter-list
are the same function parameter scope.

[Example 1: struct A { int m; const int& r; }; void f() { static int sx; thread_local int tx; // tx is never constexpr-referenceable int ax; A aa = {1, 2}; static A sa = {3, 4}; // The objects sx, ax, and aa.m, sa.m, and the temporaries to which aa.r and sa.r are bound, are constexpr-referenceable. auto lambda = [] { int ay; // The objects sx, sa.m, and ay (but not ax or aa), and the // temporary to which sa.r is bound, are constexpr-referenceable. }; } — end example]

An object or reference x is constexpr-representable at a point P if, for each constituent value of x that points to or past an object o, and for each constituent reference of x that refers to an object o, o is constexpr-referenceable from P.

A variable v is constant-initializable if

(6.1)
the full-expression of its initialization is a constant expression when interpreted as a constant-expression,
[Note 2:
Within this evaluation, std::is_constant_evaluated() ([meta.const.eval]) returns true.
— end note]
and
(6.2)
immediately after the initializing declaration of v, the object or reference x declared by v is constexpr-representable, and
(6.3)
if x has static or thread storage duration, x is constexpr-representable at the nearest point whose immediate scope is a namespace scope that follows the initializing declaration of v.

A constant-initializable variable is constant-initialized if either it has an initializer or its default-initialization results in some initialization being performed.

[Example 2: void f() { int ax = 0; // ax is constant-initialized thread_local int tx = 0; // tx is constant-initialized static int sx; // sx is not constant-initialized static int& rss = sx; // rss is constant-initialized static int& rst = tx; // rst is not constant-initialized static int& rsa = ax; // rsa is not constant-initialized thread_local int& rts = sx; // rts is constant-initialized thread_local int& rtt = tx; // rtt is not constant-initialized thread_local int& rta = ax; // rta is not constant-initialized int& ras = sx; // ras is constant-initialized int& rat = tx; // rat is not constant-initialized int& raa = ax; // raa is constant-initialized } — end example]

A variable is potentially-constant if it is constexpr or it has reference or non-volatile const-qualified integral or enumeration type.

A constant-initialized potentially-constant variable V is usable in constant expressions at a point P if V's initializing declaration D is reachable from P and

(9.1)
V is constexpr,
(9.2)
V is not initialized to a TU-local value, or
(9.3)
P is in the same translation unit as D.

An object or reference is potentially usable in constant expressions at point P if it is

(9.4)
the object or reference declared by a variable that is usable in constant expressions at P,
(9.5)
a temporary object of non-volatile const-qualified literal type whose lifetime is extended ([class.temporary]) to that of a variable that is usable in constant expressions at P,
(9.6)
a template parameter object,
(9.7)
a string literal object,
(9.8)
a non-mutable subobject of any of the above, or
(9.9)
a reference member of any of the above.

An object or reference is usable in constant expressions at point P if it is an object or reference that is potentially usable in constant expressions at P and is constexpr-representable at P.

[Example 3: struct A { int* const & r; }; void f(int x) { constexpr A a = {&x}; static_assert(a.r == &x); // OK [&] { static_assert(a.r != nullptr); // error: a.r is not usable in constant expressions at this point }(); } — end example]

An expression E is a core constant expression unless the evaluation of E, following the rules of the abstract machine ([intro.execution]), would evaluate one of the following:

(10.1)
this ([expr.prim.this]), except
- (10.1.1)
  in a constexpr function ([dcl.constexpr]) that is being evaluated as part of E or
- (10.1.2)
  when appearing as the postfix-expression of an implicit or explicit class member access expression ([expr.ref]);
(10.2)
a control flow that passes through a declaration of a block variable ([basic.scope.block]) with static ([basic.stc.static]) or thread ([basic.stc.thread]) storage duration, unless that variable is usable in constant expressions;
[Example 4: constexpr char test() { static const int x = 5; static constexpr char c[] = "Hello World"; return *(c + x); } static_assert(' ' == test()); — end example]
(10.3)
an invocation of a non-constexpr function;67
(10.4)
an invocation of an undefined constexpr function;
(10.5)
an invocation of an instantiated constexpr function that is not constexpr-suitable;
(10.6)
an invocation of a virtual function ([class.virtual]) for an object whose dynamic type is constexpr-unknown;
(10.7)
an expression that would exceed the implementation-defined limits (see [implimits]);
(10.8)
an operation that would have undefined or erroneous behavior as specified in [intro] through [cpp];68
(10.9)
an lvalue-to-rvalue conversion unless it is applied to
- (10.9.1)
  a glvalue of type cvstd::nullptr_t,
- (10.9.2)
  a non-volatile glvalue that refers to an object that is usable in constant expressions, or
- (10.9.3)
  a non-volatile glvalue of literal type that refers to a non-volatile object whose lifetime began within the evaluation of E;
(10.10)
an lvalue-to-rvalue conversion that is applied to a glvalue that refers to a non-active member of a union or a subobject thereof;
(10.11)
an lvalue-to-rvalue conversion that is applied to an object with an indeterminate value;
(10.12)
an invocation of an implicitly-defined copy/move constructor or copy/move assignment operator for a union whose active member (if any) is mutable, unless the lifetime of the union object began within the evaluation of E;
(10.13)
in a lambda-expression, a reference to this or to a variable with automatic storage duration defined outside that lambda-expression, where the reference would be an odr-use ([basic.def.odr], [expr.prim.lambda]);
[Example 5: void g() { const int n = 0; [=] { constexpr int i = n; // OK, n is not odr-used here constexpr int j = *&n; // error: &n would be an odr-use of n }; } — end example]

[Note 3:
If the odr-use occurs in an invocation of a function call operator of a closure type, it no longer refers to this or to an enclosing variable with automatic storage duration due to the transformation ([expr.prim.lambda.capture]) of the id-expression into an access of the corresponding data member.

[Example 6: auto monad = [](auto v) { return [=] { return v; }; }; auto bind = [](auto m) { return [=](auto fvm) { return fvm(m()); }; }; // OK to capture objects with automatic storage duration created during constant expression evaluation. static_assert(bind(monad(2))(monad)() == monad(2)()); — end example]
— end note]
(10.14)
a conversion from a prvalue P of type “pointer to cv void” to a type “cv1 pointer to T”, where T is not cv2 void, unless P is a null pointer value or points to an object whose type is similar to T;
(10.15)
a reinterpret_cast ([expr.reinterpret.cast]);
(10.16)
a modification of an object ([expr.ass], [expr.post.incr], [expr.pre.incr]) unless it is applied to a non-volatile lvalue of literal type that refers to a non-volatile object whose lifetime began within the evaluation of E;
(10.17)
an invocation of a destructor ([class.dtor]) or a function call whose postfix-expression names a pseudo-destructor ([expr.call]), in either case for an object whose lifetime did not begin within the evaluation of E;
(10.18)
a new-expression ([expr.new]), unless either
- (10.18.1)
  the selected allocation function is a replaceable global allocation function ([new.delete.single], [new.delete.array]) and the allocated storage is deallocated within the evaluation of E, or
- (10.18.2)
  the selected allocation function is a non-allocating form ([new.delete.placement]) with an allocated type T, where
  - (10.18.2.1)
    the placement argument to the new-expression points to an object whose type is similar to T ([conv.qual]) or, if T is an array type, to the first element of an object of a type similar to T, and
  - (10.18.2.2)
    the placement argument points to storage whose duration began within the evaluation of E;
(10.19)
a delete-expression ([expr.delete]), unless it deallocates a region of storage allocated within the evaluation of E;
(10.20)
a call to an instance of std::allocator<T>::allocate ([allocator.members]), unless the allocated storage is deallocated within the evaluation of E;
(10.21)
a call to an instance of std::allocator<T>::deallocate ([allocator.members]), unless it deallocates a region of storage allocated within the evaluation of E;
(10.22)
a construction of an exception object, unless the exception object and all of its implicit copies created by invocations of std::current_exception or std::rethrow_exception ([propagation]) are destroyed within the evaluation of E;
(10.23)
an await-expression ([expr.await]);
(10.24)
a yield-expression ([expr.yield]);
(10.25)
a three-way comparison ([expr.spaceship]), relational ([expr.rel]), or equality ([expr.eq]) operator where the result is unspecified;
(10.26)
a dynamic_cast ([expr.dynamic.cast]) or typeid ([expr.typeid]) expression on a glvalue that refers to an object whose dynamic type is constexpr-unknown;
(10.27)
a dynamic_cast ([expr.dynamic.cast]) expression, typeid ([expr.typeid]) expression, or new-expression ([expr.new]) that would throw an exception where no definition of the exception type is reachable;
(10.28)
an asm-declaration ([dcl.asm]);
(10.29)
an invocation of the va_arg macro ([cstdarg.syn]);
(10.30)
a non-constant library call ([defns.nonconst.libcall]); or
(10.31)
a goto statement ([stmt.goto]).
[Note 4:
A goto statement introduced by equivalence ([stmt.stmt]) is not in scope.

For example, a while statement ([stmt.while]) can be executed during constant evaluation.
— end note]

It is implementation-defined whether E is a core constant expression if E satisfies the constraints of a core constant expression, but evaluation of E has runtime-undefined behavior.

It is unspecified whether E is a core constant expression if E satisfies the constraints of a core constant expression, but evaluation of E would evaluate

(12.1)
an operation that has undefined behavior as specified in [library] through [exec] or
(12.2)
an invocation of the va_start macro ([cstdarg.syn]).

[Example 7: int x; // not constant struct A { constexpr A(bool b) : m(b?42:x) { } int m; }; constexpr int v = A(true).m; // OK, constructor call initializes m with the value 42 constexpr int w = A(false).m; // error: initializer for m is x, which is non-constant constexpr int f1(int k) { constexpr int x = k; // error: x is not initialized by a constant expression // because lifetime of k began outside the initializer of x return x; } constexpr int f2(int k) { int x = k; // OK, not required to be a constant expression // because x is not constexpr return x; } constexpr int incr(int &n) { return ++n; } constexpr int g(int k) { constexpr int x = incr(k); // error: incr(k) is not a core constant expression // because lifetime of k began outside the expression incr(k) return x; } constexpr int h(int k) { int x = incr(k); // OK, incr(k) is not required to be a core constant expression return x; } constexpr int y = h(1); // OK, initializes y with the value 2 // h(1) is a core constant expression because // the lifetime of k begins inside h(1) — end example]

For the purposes of determining whether an expression E is a core constant expression, the evaluation of the body of a member function of std::allocator<T> as defined in [allocator.members], where T is a literal type, is ignored.

For the purposes of determining whether E is a core constant expression, the evaluation of a call to a trivial copy/move constructor or copy/move assignment operator of a union is considered to copy/move the active member of the union, if any.

[Note 5:

The copy/move of the active member is trivial.

— end note]

For the purposes of determining whether E is a core constant expression, the evaluation of an id-expression that names a structured binding v ([dcl.struct.bind]) has the following semantics:

(16.1)
If v is an lvalue referring to the object bound to an invented reference r, the behavior is as if r were nominated.
(16.2)
Otherwise, if v names an array element or class member, the behavior is that of evaluating e[i] or e.m, respectively, where e is the name of the variable initialized from the initializer of the structured binding declaration, and i is the index of the element referred to by v or m is the name of the member referred to by v, respectively.

[Example 8: #include <tuple> struct S { mutable int m; constexpr S(int m): m(m) {} virtual int g() const; }; void f(std::tuple<S&> t) { auto [r] = t; static_assert(r.g() >= 0); // error: dynamic type is constexpr-unknown constexpr auto [m] = S(1); static_assert(m == 1); // error: lvalue-to-rvalue conversion on mutable // subobject e.m, where e is a constexpr object of type S using A = int[2]; constexpr auto [v0, v1] = A{2, 3}; static_assert(v0 + v1 == 5); // OK, equivalent to e[0] + e[1] where e is a constexpr array } — end example]

During the evaluation of an expression E as a core constant expression, all id-expressions and uses of *this that refer to an object or reference whose lifetime did not begin with the evaluation of E are treated as referring to a specific instance of that object or reference whose lifetime and that of all subobjects (including all union members) includes the entire constant evaluation.

For such an object that is not usable in constant expressions, the dynamic type of the object is constexpr-unknown.

For such a reference that is not usable in constant expressions, the reference is treated as binding to an unspecified object of the referenced type whose lifetime and that of all subobjects includes the entire constant evaluation and whose dynamic type is constexpr-unknown.

[Example 9: template <typename T, size_t N> constexpr size_t array_size(T (&)[N]) { return N; } void use_array(int const (&gold_medal_mel)[2]) { constexpr auto gold = array_size(gold_medal_mel); // OK } constexpr auto olympic_mile() { const int ledecky = 1500; return []{ return ledecky; }; } static_assert(olympic_mile()() == 1500); // OK struct Swim { constexpr int phelps() { return 28; } virtual constexpr int lochte() { return 12; } int coughlin = 12; }; constexpr int how_many(Swim& swam) { Swim* p = &swam; return (p + 1 - 1)->phelps(); } void splash(Swim& swam) { static_assert(swam.phelps() == 28); // OK static_assert((&swam)->phelps() == 28); // OK Swim* pswam = &swam; static_assert(pswam->phelps() == 28); // error: lvalue-to-rvalue conversion on a pointer // not usable in constant expressions static_assert(how_many(swam) == 28); // OK static_assert(Swim().lochte() == 12); // OK static_assert(swam.lochte() == 12); // error: invoking virtual function on reference // with constexpr-unknown dynamic type static_assert(swam.coughlin == 12); // error: lvalue-to-rvalue conversion on an object // not usable in constant expressions } extern Swim dc; extern Swim& trident; constexpr auto& sandeno = typeid(dc); // OK, can only be typeid(Swim) constexpr auto& gallagher = typeid(trident); // error: constexpr-unknown dynamic type — end example]

An object a is said to have constant destruction if

(18.1)
it is not of class type nor (possibly multidimensional) array thereof, or
(18.2)
it is of class type or (possibly multidimensional) array thereof, that class type has a constexpr destructor, and for a hypothetical expression E whose only effect is to destroy a, E would be a core constant expression if the lifetime of a and its non-mutable subobjects (but not its mutable subobjects) were considered to start within E.

An integral constant expression is an expression of integral or unscoped enumeration type, implicitly converted to a prvalue, where the converted expression is a core constant expression.

[Note 6:

Such expressions can be used as bit-field lengths ([class.bit]), as enumerator initializers if the underlying type is not fixed ([dcl.enum]), and as alignments .

— end note]

If an expression of literal class type is used in a context where an integral constant expression is required, then that expression is contextually implicitly converted ([conv]) to an integral or unscoped enumeration type and the selected conversion function shall be constexpr.

[Example 10: struct A { constexpr A(int i) : val(i) { } constexpr operator int() const { return val; } constexpr operator long() const { return 42; } private: int val; }; constexpr A a = alignof(int); alignas(a) int n; // error: ambiguous conversion struct B { int n : a; }; // error: ambiguous conversion — end example]

A converted constant expression of type T is an expression, implicitly converted to type T, where the converted expression is a constant expression and the implicit conversion sequence contains only

(21.1)
user-defined conversions,
(21.2)
lvalue-to-rvalue conversions ([conv.lval]),
(21.3)
array-to-pointer conversions ([conv.array]),
(21.4)
function-to-pointer conversions ([conv.func]),
(21.5)
qualification conversions ([conv.qual]),
(21.6)
integral promotions ([conv.prom]),
(21.7)
integral conversions ([conv.integral]) other than narrowing conversions ([dcl.init.list]),
(21.8)
floating-point promotions ([conv.fpprom]),
(21.9)
floating-point conversions ([conv.double]) where the source value can be represented exactly in the destination type,
(21.10)
null pointer conversions ([conv.ptr]) from std::nullptr_t,
(21.11)
null member pointer conversions ([conv.mem]) from std::nullptr_t, and
(21.12)
function pointer conversions ([conv.fctptr]),

and where the reference binding (if any) binds directly.

[Note 7:

Such expressions can be used in new expressions ([expr.new]), as case expressions ([stmt.switch]), as enumerator initializers if the underlying type is fixed, as array bounds, and as non-type template arguments .

— end note]

A contextually converted constant expression of type bool is an expression, contextually converted to bool ([conv]), where the converted expression is a constant expression and the conversion sequence contains only the conversions above.

A constant expression is either a glvalue core constant expression that refers to an object or a non-immediate function, or a prvalue core constant expression whose value satisfies the following constraints:

(22.1)
each constituent reference refers to an object or a non-immediate function,
(22.2)
no constituent value of scalar type is an indeterminate value ([basic.indet]),
(22.3)
no constituent value of pointer type is a pointer to an immediate function or an invalid pointer value ([basic.compound]), and
(22.4)
no constituent value of pointer-to-member type designates an immediate function.

[Note 8:

A glvalue core constant expression that either refers to or points to an unspecified object is not a constant expression.

— end note]

[Example 11: consteval int f() { return 42; } consteval auto g() { return f; } consteval int h(int (*p)() = g()) { return p(); } constexpr int r = h(); // OK constexpr auto e = g(); // error: a pointer to an immediate function is // not a permitted result of a constant expression struct S { int x; constexpr S() {} }; int i() { constexpr S s; // error: s.x has erroneous value } — end example]

Recommended practice: Implementations should provide consistent results of floating-point evaluations, irrespective of whether the evaluation is performed during translation or during program execution.

[Note 9:

Since this document imposes no restrictions on the accuracy of floating-point operations, it is unspecified whether the evaluation of a floating-point expression during translation yields the same result as the evaluation of the same expression (or the same operations on the same values) during program execution.

[Example 12: bool f() { char array[1 + int(1 + 0.2 - 0.1 - 0.1)]; // Must be evaluated during translation int size = 1 + int(1 + 0.2 - 0.1 - 0.1); // May be evaluated at runtime return sizeof(array) == size; }

It is unspecified whether the value of f() will be true or false.

— end example]

— end note]

An expression or conversion is in an immediate function context if it is potentially evaluated and either:

(24.1)
its innermost enclosing non-block scope is a function parameter scope of an immediate function,
(24.2)
it is a subexpression of a manifestly constant-evaluated expression or conversion, or
(24.3)
its enclosing statement is enclosed ([stmt.pre]) by the compound-statement of a consteval if statement ([stmt.if]).

An invocation is an immediate invocation if it is a potentially-evaluated explicit or implicit invocation of an immediate function and is not in an immediate function context.

An aggregate initialization is an immediate invocation if it evaluates a default member initializer that has a subexpression that is an immediate-escalating expression.

An expression or conversion is immediate-escalating if it is not initially in an immediate function context and it is either

(25.1)
a potentially-evaluated id-expression that denotes an immediate function that is not a subexpression of an immediate invocation, or
(25.2)
an immediate invocation that is not a constant expression and is not a subexpression of an immediate invocation.

An immediate-escalating function is

(26.1)
the call operator of a lambda that is not declared with the consteval specifier,
(26.2)
a defaulted special member function that is not declared with the consteval specifier, or
(26.3)
a function that results from the instantiation of a templated entity defined with the constexpr specifier.

An immediate-escalating expression shall appear only in an immediate-escalating function.

An immediate function is a function or constructor that is

(27.1)
declared with the consteval specifier, or
(27.2)
an immediate-escalating function F whose function body contains an immediate-escalating expression E such that E's innermost enclosing non-block scope is F's function parameter scope.
[Note 10:
Default member initializers used to initialize a base or member subobject ([class.base.init]) are considered to be part of the function body ([dcl.fct.def.general]).
— end note]

[Example 13: consteval int id(int i) { return i; } constexpr char id(char c) { return c; } template<class T> constexpr int f(T t) { return t + id(t); } auto a = &f<char>; // OK, f<char> is not an immediate function auto b = &f<int>; // error: f<int> is an immediate function static_assert(f(3) == 6); // OK template<class T> constexpr int g(T t) { // g<int> is not an immediate function return t + id(42); // because id(42) is already a constant } template<class T, class F> constexpr bool is_not(T t, F f) { return not f(t); } consteval bool is_even(int i) { return i % 2 == 0; } static_assert(is_not(5, is_even)); // OK int x = 0; template<class T> constexpr T h(T t = id(x)) { // h<int> is not an immediate function // id(x) is not evaluated when parsing the default argument ([dcl.fct.default], [temp.inst]) return t; } template<class T> constexpr T hh() { // hh<int> is an immediate function because of the invocation return h<T>(); // of the immediate function id in the default argument of h<int> } int i = hh<int>(); // error: hh<int>() is an immediate-escalating expression // outside of an immediate-escalating function struct A { int x; int y = id(x); }; template<class T> constexpr int k(int) { // k<int> is not an immediate function because A(42) is a return A(42).y; // constant expression and thus not immediate-escalating } — end example]

An expression or conversion is manifestly constant-evaluated if it is:

(28.1)
a constant-expression, or
(28.2)
the condition of a constexpr if statement ([stmt.if]), or
(28.3)
an immediate invocation, or
(28.4)
the result of substitution into an atomic constraint expression to determine whether it is satisfied ([temp.constr.atomic]), or
(28.5)
the initializer of a variable that is usable in constant expressions or has constant initialization ([basic.start.static]).69
[Example 14: template<bool> struct X {}; X<std::is_constant_evaluated()> x; // type X<true> int y; const int a = std::is_constant_evaluated() ? y : 1; // dynamic initialization to 1 double z[a]; // error: a is not usable // in constant expressions const int b = std::is_constant_evaluated() ? 2 : y; // static initialization to 2 int c = y + (std::is_constant_evaluated() ? 2 : y); // dynamic initialization to y+y constexpr int f() { const int n = std::is_constant_evaluated() ? 13 : 17; // n is 13 int m = std::is_constant_evaluated() ? 13 : 17; // m can be 13 or 17 (see below) char arr[n] = {}; // char[13] return m + sizeof(arr); } int p = f(); // m is 13; initialized to 26 int q = p + f(); // m is 17 for this call; initialized to 56 — end example]

[Note 11:

Except for a static_assert-message, a manifestly constant-evaluated expression is evaluated even in an unevaluated operand ([expr.context]).

— end note]

An expression or conversion is potentially constant evaluated if it is:

(29.1)
a manifestly constant-evaluated expression,
(29.2)
a potentially-evaluated expression,
(29.3)
an immediate subexpression of a braced-init-list,70
(29.4)
an expression of the form & cast-expression that occurs within a templated entity,71 or
(29.5)
a potentially-evaluated subexpression ([intro.execution]) of one of the above.

A function or variable is needed for constant evaluation if it is:

(29.6)
a constexpr function that is named by an expression that is potentially constant evaluated, or
(29.7)
a potentially-constant variable named by a potentially constant evaluated expression.

67)67)

Overload resolution ([over.match]) is applied as usual.

68)68)

This includes, for example, signed integer overflow ([expr.pre]), certain pointer arithmetic ([expr.add]), division by zero ([expr.mul]), or certain shift operations ([expr.shift]).

69)69)

Testing this condition can involve a trial evaluation of its initializer as described above.

70)70)

In some cases, constant evaluation is needed to determine whether a narrowing conversion is performed ([dcl.init.list]).

71)71)

In some cases, constant evaluation is needed to determine whether such an expression is value-dependent ([temp.dep.constexpr]).