18 Concepts library [concepts]

18.4 Language-related concepts [concepts.lang]

18.4.1 General [concepts.lang.general]

Subclause [concepts.lang] contains the definition of concepts corresponding to language features.

These concepts express relationships between types, type classifications, and fundamental type properties.

18.4.2 Concept same_as [concept.same]

template<class T, class U>
  concept same-as-impl = is_same_v<T, U>;       // exposition only

template<class T, class U>
  concept same_as = same-as-impl<T, U> && same-as-impl<U, T>;

[Note 1:

same_as<T, U> subsumes same_as<U, T> and vice versa.

— end note]

18.4.3 Concept derived_from [concept.derived]

🔗

template<class Derived, class Base>
  concept derived_from =
    is_base_of_v<Base, Derived> &&
    is_convertible_v<const volatile Derived*, const volatile Base*>;

[Note 1:

derived_from<Derived, Base> is satisfied if and only if Derived is publicly and unambiguously derived from Base, or Derived and Base are the same class type ignoring cv-qualifiers.

— end note]

18.4.4 Concept convertible_to [concept.convertible]

Given types From and To and an expression E whose type and value category are the same as those of declval<From>(), convertible_to<From, To> requires E to be both implicitly and explicitly convertible to type To.

The implicit and explicit conversions are required to produce equal results.

🔗

template<class From, class To>
  concept convertible_to =
    is_convertible_v<From, To> &&
    requires {
      static_cast<To>(declval<From>());
    };

Let FromR be add_rvalue_reference_t<From> and test be the invented function: To test(FromR (&f)()) { return f(); } and let f be a function with no arguments and return type FromR such that f() is equality-preserving.

Types From and To model convertible_to<From, To> only if:

(2.1)
To is not an object or reference-to-object type, or static_cast<To>(f()) is equal to test(f).
(2.2)
FromR is not a reference-to-object type, or
- (2.2.1)
  If FromR is an rvalue reference to a non const-qualified type, the resulting state of the object referenced by f() after either above expression is valid but unspecified ([lib.types.movedfrom]).
- (2.2.2)
  Otherwise, the object referred to by f() is not modified by either above expression.

18.4.5 Concept common_reference_with [concept.commonref]

For two types T and U, if common_reference_t<T, U> is well-formed and denotes a type C such that both convertible_to<T, C> and convertible_to<U, C> are modeled, then T and U share a common reference type, C.

[Note 1:

C can be the same as T or U, or can be a different type.

C can be a reference type.

— end note]

🔗

template<class T, class U>
  concept common_reference_with =
    same_as<common_reference_t<T, U>, common_reference_t<U, T>> &&
    convertible_to<T, common_reference_t<T, U>> &&
    convertible_to<U, common_reference_t<T, U>>;

Let C be common_reference_t<T, U>.

Let t1 and t2 be equality-preserving expressions ([concepts.equality]) such that decltype((t1)) and decltype((t2)) are each T, and let u1 and u2 be equality-preserving expressions such that decltype((u1)) and decltype((u2)) are each U.

T and U model common_reference_with<T, U> only if:

(2.1)
C(t1) equals C(t2) if and only if t1 equals t2, and
(2.2)
C(u1) equals C(u2) if and only if u1 equals u2.

[Note 2:

Users can customize the behavior of common_reference_with by specializing the basic_common_reference class template ([meta.trans.other]).

— end note]

18.4.6 Concept common_with [concept.common]

If T and U can both be explicitly converted to some third type, C, then T and U share a common type, C.

[Note 1:

C can be the same as T or U, or can be a different type.

C is not necessarily unique.

— end note]

🔗

template<class T, class U>
  concept common_with =
    same_as<common_type_t<T, U>, common_type_t<U, T>> &&
    requires {
      static_cast<common_type_t<T, U>>(declval<T>());
      static_cast<common_type_t<T, U>>(declval<U>());
    } &&
    common_reference_with<
      add_lvalue_reference_t<const T>,
      add_lvalue_reference_t<const U>> &&
    common_reference_with<
      add_lvalue_reference_t<common_type_t<T, U>>,
      common_reference_t<
        add_lvalue_reference_t<const T>,
        add_lvalue_reference_t<const U>>>;

Let C be common_type_t<T, U>.

T and U model common_with<T, U> only if:

(2.1)
C(t1) equals C(t2) if and only if t1 equals t2, and
(2.2)
C(u1) equals C(u2) if and only if u1 equals u2.

[Note 2:

Users can customize the behavior of common_with by specializing the common_type class template ([meta.trans.other]).

— end note]

18.4.7 Arithmetic concepts [concepts.arithmetic]

🔗

template<class T>
  concept integral = is_integral_v<T>;
template<class T>
  concept signed_integral = integral<T> && is_signed_v<T>;
template<class T>
  concept unsigned_integral = integral<T> && !signed_integral<T>;
template<class T>
  concept floating_point = is_floating_point_v<T>;

[Note 1:

signed_integral can be modeled even by types that are not signed integer types ([basic.fundamental]); for example, char.

— end note]

[Note 2:

unsigned_integral can be modeled even by types that are not unsigned integer types ([basic.fundamental]); for example, bool.

— end note]

18.4.8 Concept assignable_from [concept.assignable]

🔗

template<class LHS, class RHS>
  concept assignable_from =
    is_lvalue_reference_v<LHS> &&
    common_reference_with<const remove_reference_t<LHS>&, const remove_reference_t<RHS>&> &&
    requires(LHS lhs, RHS&& rhs) {
      { lhs = std::forward<RHS>(rhs) } -> same_as<LHS>;
    };

Let:

(1.1)
lhs be an lvalue that refers to an object lcopy such that decltype((lhs)) is LHS,
(1.2)
rhs be an expression such that decltype((rhs)) is RHS, and
(1.3)
rcopy be a distinct object that is equal to rhs.

LHS and RHS model assignable_from<LHS, RHS> only if

(1.4)
addressof(lhs = rhs) == addressof(lcopy).
(1.5)
After evaluating lhs = rhs:
- (1.5.1)
  lhs is equal to rcopy, unless rhs is a non-const xvalue that refers to lcopy.
- (1.5.2)
  If rhs is a non-const xvalue, the resulting state of the object to which it refers is valid but unspecified ([lib.types.movedfrom]).
- (1.5.3)
  Otherwise, if rhs is a glvalue, the object to which it refers is not modified.

[Note 1:

Assignment need not be a total function ([structure.requirements]); in particular, if assignment to an object x can result in a modification of some other object y, then x = y is likely not in the domain of =.

— end note]

18.4.9 Concept swappable [concept.swappable]

Let t1 and t2 be equality-preserving expressions that denote distinct equal objects of type T, and let u1 and u2 similarly denote distinct equal objects of type U.

[Note 1:

t1 and u1 can denote distinct objects, or the same object.

— end note]

An operation exchanges the values denoted by t1 and u1 if and only if the operation modifies neither t2 nor u2 and:

(1.1)
If T and U are the same type, the result of the operation is that t1 equals u2 and u1 equals t2.
(1.2)
If T and U are different types and common_reference_with<decltype((t1)), decltype((u1))> is modeled, the result of the operation is that C(t1) equals C(u2) and C(u1) equals C(t2) where C is common_reference_t<decltype((t1)), decltype((u1))>.

The name ranges::swap denotes a customization point object ([customization.point.object]).

The expression ranges::swap(E1, E2) for subexpressions E1 and E2 is expression-equivalent to an expression S determined as follows:

(2.1)
S is (void)swap(E1, E2)201 if E1 or E2 has class or enumeration type ([basic.compound]) and that expression is valid, with overload resolution performed in a context that includes the declaration template<class T> void swap(T&, T&) = delete; and does not include a declaration of ranges::swap.

If the function selected by overload resolution does not exchange the values denoted by E1 and E2, the program is ill-formed, no diagnostic required.

[Note 2:
This precludes calling unconstrained program-defined overloads of swap.

When the deleted overload is viable, program-defined overloads need to be more specialized ([temp.func.order]) to be selected.
— end note]
(2.2)
Otherwise, if E1 and E2 are lvalues of array types ([basic.compound]) with equal extent and ranges::swap(*E1, *E2) is a valid expression, S is (void)ranges::swap_ranges(E1, E2), except that noexcept(S) is equal to noexcept(ranges::swap(*E1, *E2)).
(2.3)
Otherwise, if E1 and E2 are lvalues of the same type T that models move_constructible<T> and assignable_from<T&, T>, S is an expression that exchanges the denoted values.
S is a constant expression if
- (2.3.1)
  T is a literal type ([basic.types.general]),
- (2.3.2)
  both E1 = std::move(E2) and E2 = std::move(E1) are constant subexpressions ([defns.const.subexpr]), and
- (2.3.3)
  the full-expressions of the initializers in the declarations T t1(std::move(E1)); T t2(std::move(E2)); are constant subexpressions.
noexcept(S) is equal to is_nothrow_move_constructible_v<T> && is_nothrow_move_assignable_v<T>.
(2.4)
Otherwise, ranges::swap(E1, E2) is ill-formed.

[Note 3:
This case can result in substitution failure when ranges::swap(E1, E2) appears in the immediate context of a template instantiation.
— end note]

[Note 4:

Whenever ranges::swap(E1, E2) is a valid expression, it exchanges the values denoted by E1 and E2 and has type void.

— end note]

🔗

template<class T>
  concept swappable = requires(T& a, T& b) { ranges::swap(a, b); };

🔗

template<class T, class U>
  concept swappable_with =
    common_reference_with<T, U> &&
    requires(T&& t, U&& u) {
      ranges::swap(std::forward<T>(t), std::forward<T>(t));
      ranges::swap(std::forward<U>(u), std::forward<U>(u));
      ranges::swap(std::forward<T>(t), std::forward<U>(u));
      ranges::swap(std::forward<U>(u), std::forward<T>(t));
    };

[Note 5:

The semantics of the swappable and swappable_with concepts are fully defined by the ranges::swap customization point object.

— end note]

[Example 1:

User code can ensure that the evaluation of swap calls is performed in an appropriate context under the various conditions as follows: #include <cassert> #include <concepts> #include <utility> namespace ranges = std::ranges; template<class T, std::swappable_with<T> U> void value_swap(T&& t, U&& u) { ranges::swap(std::forward<T>(t), std::forward<U>(u)); } template<std::swappable T> void lv_swap(T& t1, T& t2) { ranges::swap(t1, t2); } namespace N { struct A { int m; }; struct Proxy { A* a; Proxy(A& a) : a{&a} {} friend void swap(Proxy x, Proxy y) { ranges::swap(*x.a, *y.a); } }; Proxy proxy(A& a) { return Proxy{ a }; } } int main() { int i = 1, j = 2; lv_swap(i, j); assert(i == 2 && j == 1); N::A a1 = { 5 }, a2 = { -5 }; value_swap(a1, proxy(a2)); assert(a1.m == -5 && a2.m == 5); }

— end example]

201)201)

The name swap is used here unqualified.

18.4.10 Concept destructible [concept.destructible]

The destructible concept specifies properties of all types, instances of which can be destroyed at the end of their lifetime, or reference types.

🔗

template<class T>
  concept destructible = is_nothrow_destructible_v<T>;

[Note 1:

Unlike the Cpp17Destructible requirements (Table 35), this concept forbids destructors that are potentially throwing, even if a particular invocation of the destructor does not actually throw.

— end note]

18.4.11 Concept constructible_from [concept.constructible]

The constructible_from concept constrains the initialization of a variable of a given type with a particular set of argument types.

🔗

template<class T, class... Args>
  concept constructible_from = destructible<T> && is_constructible_v<T, Args...>;

18.4.12 Concept default_initializable [concept.default.init]

🔗

template<class T>
  constexpr bool is-default-initializable = see below;         // exposition only

template<class T>
  concept default_initializable = constructible_from<T> &&
                                  requires { T{}; } &&
                                  is-default-initializable<T>;

For a type T, is-default-initializable<T> is true if and only if the variable definition T t; is well-formed for some invented variable t; otherwise it is false.

Access checking is performed as if in a context unrelated to T.

Only the validity of the immediate context of the variable initialization is considered.

18.4.13 Concept move_constructible [concept.moveconstructible]

🔗

template<class T>
  concept move_constructible = constructible_from<T, T> && convertible_to<T, T>;

If T is an object type, then let rv be an rvalue of type T and u2 a distinct object of type T equal to rv.

T models move_constructible only if

(1.1)
After the definition T u = rv;, u is equal to u2.
(1.2)
T(rv) is equal to u2.
(1.3)
If T is not const, rv's resulting state is valid but unspecified ([lib.types.movedfrom]); otherwise, it is unchanged.

18.4.14 Concept copy_constructible [concept.copyconstructible]

🔗

template<class T>
  concept copy_constructible =
    move_constructible<T> &&
    constructible_from<T, T&> && convertible_to<T&, T> &&
    constructible_from<T, const T&> && convertible_to<const T&, T> &&
    constructible_from<T, const T> && convertible_to<const T, T>;

If T is an object type, then let v be an lvalue of type T or const T or an rvalue of type const T.

T models copy_constructible only if

(1.1)
After the definition T u = v;, u is equal to v ([concepts.equality]) and v is not modified.
(1.2)
T(v) is equal to v and does not modify v.