NAME

std::common_with - std::common_with

Synopsis

Defined in header <concepts>
template <class T, class U>

concept common_with =
std::same_as<std::common_type_t<T, U>, std::common_type_t<U, T>> &&
requires {
static_cast<std::common_type_t<T, U>>(std::declval<T>());
static_cast<std::common_type_t<T, U>>(std::declval<U>());
} &&
std::common_reference_with< (since C++20)
std::add_lvalue_reference_t<const T>,
std::add_lvalue_reference_t<const U>> &&
std::common_reference_with<
std::add_lvalue_reference_t<std::common_type_t<T, U>>,
std::common_reference_t<
std::add_lvalue_reference_t<const T>,

std::add_lvalue_reference_t<const U>>>;

The concept common_with<T, U> specifies that two types T and U share a common type
(as computed by std::common_type_t) to which both can be converted.

Semantic requirements

T and U model common_with<T, U> only if, given equality-preserving expressions t1,
t2, u1 and u2 such that decltype((t1)) and decltype((t2)) are both T and
decltype((u1)) and decltype((u2)) are both U,

* std::common_type_t<T, U>(t1) equals std::common_type_t<T, U>(t2) if and only if
t1 equals t2; and
* std::common_type_t<T, U>(u1) equals std::common_type_t<T, U>(u2) if and only if
u1 equals u2.

In other words, the conversion to the common type must preserve equality.

Equality preservation

An expression is equality preserving if it results in equal outputs given equal
inputs.

* The inputs to an expression consist of its operands.
* The outputs of an expression consist of its result and all operands modified by
the expression (if any).

In specification of standard concepts, operands are defined as the largest
subexpressions that include only:

* an id-expression, and
* invocations of std::move, std::forward, and std::declval.

The cv-qualification and value category of each operand is determined by assuming
that each template type parameter denotes a cv-unqualified complete non-array object
type.

Every expression required to be equality preserving is further required to be
stable: two evaluations of such an expression with the same input objects must have
equal outputs absent any explicit intervening modification of those input objects.

See also

common_type determines the common type of a group of types
(C++11) (class template)
common_reference determines the common reference type of a group of types
basic_common_reference (class template)
(C++20)
common_reference_with specifies that two types share a common reference type
(C++20) (concept)