7 Standard conversions [conv]

Standard conversions are implicit conversions with built-in meaning.
Clause [conv] enumerates the full set of such conversions.
A standard conversion sequence is a sequence of standard conversions in the following order:
  • Zero or one conversion from the following set: lvalue-to-rvalue conversion, array-to-pointer conversion, and function-to-pointer conversion.
  • 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.
  • Zero or one function pointer conversion.
  • Zero or one qualification conversion.
[Note
:
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 a required destination type.
[Note
:
Expressions with a given type will be implicitly converted to other types in several contexts:
  • When used as operands of operators.
    The operator's requirements for its operands dictate the destination type (Clause [expr]).
  • When used in the condition of an if statement or iteration statement ([stmt.select], [stmt.iter]).
    The destination type is bool.
  • When used in the expression of a switch statement.
    The destination type is integral ([stmt.select]).
  • 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.
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 E 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: E 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
:
For class types, user-defined conversions are considered as well; see [class.conv].
In general, an implicit conversion sequence consists of a standard conversion sequence followed by a user-defined conversion followed by another standard conversion sequence.
end note
]
[Note
:
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
]

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

A glvalue of a non-function, non-array type T can be converted to a prvalue.57
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.58
When an lvalue-to-rvalue conversion is applied to an expression e, and either
  • e is not potentially evaluated, or
  • the evaluation of e results in the evaluation of a member ex of the set of potential results of e, and ex names a variable x that is not odr-used by ex,
the value contained in the referenced object is not accessed.
[Example
:
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 due to 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:
  • If T is cv std​::​nullptr_­t, the result is a null pointer constant.
    [Note
    :
    Since no value is fetched from memory, there is no side effect for a volatile access ([intro.execution]), and an inactive member of a union may be accessed.
    end note
    ]
  • Otherwise, if T has a class type, the conversion copy-initializes the result object from the glvalue.
  • Otherwise, if the object to which the glvalue refers contains an invalid pointer value ([basic.stc.dynamic.deallocation], [basic.stc.dynamic.safety]), the behavior is implementation-defined.
  • Otherwise, the value contained in the object indicated by the glvalue is the prvalue result.
[Note
:
See also [basic.lval].
end note
]
For historical reasons, this conversion is called the “lvalue-to-rvalue” conversion, even though that name does not accurately reflect the taxonomy of expressions described in [basic.lval].
In C++ class and array prvalues can have cv-qualified types.
This differs from ISO C, in which non-lvalues never have cv-qualified types.

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

7.3 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.59
[Note
:
See [over.over] for additional rules for the case where the function is overloaded.
end note
]
This conversion never applies to non-static member functions because an lvalue that refers to a non-static member function cannot be obtained.

7.4 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 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
:
If T is a class type (or array thereof), it must have an accessible and non-deleted destructor; see [class.dtor].
end note
]
[Example
:
struct X { int n; };
int k = X().n;      // OK, X() prvalue is converted to xvalue
end example
]

7.5 Qualification conversions [conv.qual]

A cv-decomposition of a type T is a sequence of and such that T is U” for , where each is a set of cv-qualifiers, and each is “pointer to”, “pointer to member of class of type”, “array of ”, or “array of unknown bound of”.
If designates an array, the cv-qualifiers on the element type are also taken as the cv-qualifiers of the array.
[Example
:
The type denoted by the type-id const int ** has two cv-decompositions, taking U as “int” and as “pointer to const int.
end example
]
The n-tuple of cv-qualifiers after the first one in the longest cv-decomposition of T, that is, , is called the cv-qualification signature of T.
Two types and are similar if they have cv-decompositions with the same n such that corresponding components are the same and the types denoted by U are the same.
A prvalue expression of type can be converted to type if the following conditions are satisfied, where denotes the cv-qualifiers in the cv-qualification signature of :60
  • and are similar.
  • For every , if const is in then const is in , and similarly for volatile.
  • If the and are different, then const is in every for .
[Note
:
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
:
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
:
Function types (including those used in pointer to member function types) are never cv-qualified ([dcl.fct]).
end note
]
These rules ensure that const-safety is preserved by the conversion.

7.6 Integral promotions [conv.prom]

A prvalue of an integer type other than bool, char16_­t, char32_­t, or wchar_­t whose integer conversion 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 type char16_­t, char32_­t, or wchar_­t ([basic.fundamental]) 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, or unsigned long long int.
If none of the types in that list can represent all the values of its underlying type, a prvalue of type char16_­t, char32_­t, or wchar_­t can be converted to a prvalue of its underlying type.
A prvalue of an unscoped enumeration type whose underlying type is not fixed ([dcl.enum]) can be converted to a prvalue of the first of the following types that can represent all the values of the enumeration (i.e., the values in the range to as described in [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 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.
A prvalue for an integral bit-field 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.
If the bit-field is larger yet, no integral promotion applies to it.
If the bit-field has an enumerated type, it is treated as any other value of that type for promotion purposes.
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.

7.7 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.

7.8 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 unsigned, the resulting value is the least unsigned integer congruent to the source integer (modulo where n is the number of bits used to represent the unsigned type).
[Note
:
In a two's complement representation, this conversion is conceptual and there is no change in the bit pattern (if there is no truncation).
end note
]
If the destination type is signed, the value is unchanged if it can be represented in the destination type; otherwise, the value is implementation-defined.
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.
The conversions allowed as integral promotions are excluded from the set of integral conversions.

7.9 Floating-point conversions [conv.double]

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

7.10 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
:
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
:
Loss of precision occurs if the integral value cannot be represented exactly as a value of the floating 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.

7.11 Pointer conversions [conv.ptr]

A null pointer constant is an integer literal 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 and is distinguishable from every other value of object pointer or function pointer type.
Such a conversion is called a null pointer conversion.
Two null pointer values of the same type shall compare equal.
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.
A null pointer constant of integral type can be converted to a prvalue of type std​::​nullptr_­t.
[Note
:
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 is unchanged by this conversion.
A prvalue of type “pointer to cv D”, where D is a class type, can be converted to a prvalue of type “pointer to cv B”, where B is a base class of D.
If B is an inaccessible or ambiguous base class of D, a program that necessitates this conversion is ill-formed.
The result of the conversion is a pointer to the base class subobject of the derived class object.
The null pointer value is converted to the null pointer value of the destination type.

7.12 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.
Two null member pointer values of the same type shall compare equal.
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.
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 derived class of B.
If B is an inaccessible, ambiguous, or virtual base class of D, or a base class of a virtual base class of D, a program that necessitates this conversion is ill-formed.
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.61
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], Clause [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*.

7.13 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 points to the member function.
[Example
:
  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
]

7.14 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.
For direct-initialization, a prvalue of type std​::​nullptr_­t can be converted to a prvalue of type bool; the resulting value is false.

7.15 Integer conversion rank [conv.rank]

Every integer type has an integer conversion rank defined as follows:
  • No two signed integer types other than char and signed char (if char is signed) shall have the same rank, even if they have the same representation.
  • The rank of a signed integer type shall be greater than the rank of any signed integer type with a smaller size.
  • The rank of long long int shall be greater than the rank of long int, which shall be greater than the rank of int, which shall be greater than the rank of short int, which shall be greater than the rank of signed char.
  • The rank of any unsigned integer type shall equal the rank of the corresponding signed integer type.
  • The rank of any standard integer type shall be greater than the rank of any extended integer type with the same size.
  • The rank of char shall equal the rank of signed char and unsigned char.
  • The rank of bool shall be less than the rank of all other standard integer types.
  • The ranks of char16_­t, char32_­t, and wchar_­t shall equal the ranks of their underlying types.
  • The rank of any extended signed integer type relative to another extended signed integer type with the same size is implementation-defined, but still subject to the other rules for determining the integer conversion rank.
  • For all integer types T1, T2, and T3, if T1 has greater rank than T2 and T2 has greater rank than T3, then T1 shall have greater rank than T3.
[Note
:
The integer conversion rank is used in the definition of the integral promotions and the usual arithmetic conversions.
end note
]