7 Expressions [expr]

7.3 Standard conversions [conv]

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

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:

(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

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

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

]

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

A glvalue of a non-function, non-array type T can be converted to a prvalue.52

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.53

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

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
:
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], [basic.stc.dynamic.safety]), the behavior is implementation-defined.
(3.4)
Otherwise, the object indicated by the glvalue is read ([defns.access]), and the value contained in the object is the prvalue result.

[ Note

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

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

7.3.3 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.54

54)

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

⮥

7.3.4 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 ([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

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

]

7.3.5 Qualification conversions [conv.qual]

A cv-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 \geq 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

The type denoted by the type-id const int ** has three cv-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 cv-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 cv-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 cv-combined type of two types T1 and T2 is the type T3 similar to T1 whose cv-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 $P_{i}^{1}$ ;
(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 cv-decomposition of Tj.

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

[ 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

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

— 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

]

7.3.6 Integral promotions [conv.prom]

A prvalue of an integer type other than bool, char16_t, char32_t, or wchar_t whose integer conversion rank ([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 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 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.

A prvalue for an integral bit-field ([class.bit]) 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.

7.3.7 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.

7.3.8 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 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.

7.3.9 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.

7.3.10 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-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.

7.3.11 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.

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 ([conv.qual]).

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

A prvalue 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.

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.

7.3.12 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 ([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.

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.55

55)

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*.

⮥

7.3.13 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 designates 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

]

7.3.14 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.