[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 2*: *end 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 ([expr.compound]).
- The destination type is bool.
- The destination type is integral.

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*: *end 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.

— 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 of the set of potential results of E, and names a variable x that is not odr-used by ([basic.def.odr]),

[*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:

- [
*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*] - 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]), the behavior is implementation-defined.
- Otherwise, the object indicated by the glvalue is read ([defns.access]), and the value contained in the object is the prvalue result.

45)45)

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].

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*: *end note*]

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

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

A *qualification-decomposition* of a type T
is a sequence of
and
such that T is
“ ⋯ U” for ,
where
each is a set of cv-qualifiers ([basic.type.qualifier]), and
each is
“pointer to” ([dcl.ptr]),
“pointer to member of class of type” ([dcl.mptr]),
“array of ”, or
“array of unknown bound of” ([dcl.array]).

If designates an array,
the cv-qualifiers on the element type are also taken as
the cv-qualifiers of the array.

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

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

- for every , is the union of and ,
- if either or is “array of unknown bound of”, is “array of unknown bound of”, otherwise it is , and
- if the resulting is different from or , or the resulting is different from or , then const is added to every for ,

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

[*Note 1*: *end 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
}

— [*Note 3*: *end 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”.

— A prvalue of 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 type char8_t, 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
char8_t, 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 enumeration 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.

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 ,
where N is the width of the destination type.

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.

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.

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.

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.

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.

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.

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.

48)48)

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.

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*]