7 Expressions [expr]

Postfix expressions group left-to-right.

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.49

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.

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.

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.

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 ${\text{T}}_{\text{call}}$ is call-compatible with a function type ${\text{T}}_{\text{func}}$ if ${\text{T}}_{\text{call}}$ is the same type as ${\text{T}}_{\text{func}}$ or if the type “pointer to ${\text{T}}_{\text{func}}$” can be converted to type “pointer to ${\text{T}}_{\text{call}}$” 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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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]).

If the initializer is a parenthesized single expression, the type conversion expression is equivalent to the corresponding cast expression.

Otherwise, if the type is cv void and the initializer is () or {} (after pack expansion, if any), the expression is a prvalue of type void that performs no initialization.

Otherwise, the expression is a prvalue of the specified type whose result object is direct-initialized with the initializer.

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

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.

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).50

The postfix expression before the dot is evaluated;51 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”.

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.

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.

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.

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.

The value of a postfix ++ expression is the value of its operand.

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.

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.

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.52

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

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.

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].53

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]).

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.

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.

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.

The result of the expression static_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.

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.

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]).

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.

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.

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.

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.

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.

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.

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.

No other conversion can be performed using static_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.

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.

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.

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.

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.54

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)).

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.

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.55

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.

A glvalue of type T1, designating an object or function x, 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”.

No temporary is created, no copy is made, and no constructors ([class.ctor]) or conversion functions ([class.conv]) are called.56

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.

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

No other conversion shall be performed explicitly using const_cast.

For two similar types T1 and T2, a prvalue of type T1 may 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.

The result of a const_cast refers to the original entity.

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 type, a prvalue of type T1 can be explicitly converted to an xvalue of type T2 using the cast const_cast<T2&&>.

The result of a reference const_cast refers to the original object if the operand is a glvalue and to the result of applying the temporary materialization conversion otherwise.

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

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

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 $cv{}_0^2$ $P_0^2$ $cv{}_1^2$ $P_1^2$ ⋯ $cv{}_{n-1}^2$ $P_{n-1}^2$ $cv{}_n^2$ $U_2$, and there is no qualification conversion that converts T1 to $cv{}_0^2$ $P_0^1$ $cv{}_1^2$ $P_1^1$ ⋯ $cv{}_{n-1}^2$ $P_{n-1}^1$ $cv{}_n^2$ $U_1$.

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.