30 KiB
[concepts.lang]
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.
â end note]
18.4.3 Concept derived_from [concept.derived]
template<class Derived, class Base> concept [derived_from](#concept:derived_from "18.4.3 Concept derived_from [concept.derived]") = 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 ifDerived is publicly and unambiguously derived from Base, orDerived and Base are the same class type ignoring cv-qualifiers.
â 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(),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](#concept:convertible_to "18.4.4 Concept convertible_to [concept.convertible]") = is_convertible_v<From, To> && requires { static_cast<To>(declval<From>()); };
Let FromR be add_rvalue_reference_t andtest 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:
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To is not an object or reference-to-object type, orstatic_cast(f()) is equal to test(f).
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FromR is not a reference-to-object type, or
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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]).
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Otherwise, the object referred to by f() is not modified by either above expression.
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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 bothconvertible_to<T, C> andconvertible_to<U, C> are modeled, then T and U share acommon 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.
â end note]
template<class T, class U> concept [common_reference_with](#concept:common_reference_with "18.4.5 Concept common_reference_with [concept.commonref]") = [same_as](#concept:same_as "18.4.2 Concept same_as [concept.same]")<common_reference_t<T, U>, common_reference_t<U, T>> && [convertible_to](#concept:convertible_to "18.4.4 Concept convertible_to [concept.convertible]")<T, common_reference_t<T, U>> && [convertible_to](#concept:convertible_to "18.4.4 Concept convertible_to [concept.convertible]")<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 thatdecltype((t1)) and decltype((t2)) are each T, and let u1 and u2 be equality-preserving expressions such thatdecltype((u1)) and decltype((u2)) are each U.
T and U model common_reference_with<T, U> only if
C(t1) equals C(t2) if and only if t1 equals t2, and
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]).
â 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.
â end note]
template<class T, class U> concept [common_with](#concept:common_with "18.4.6 Concept common_with [concept.common]") = [same_as](#concept:same_as "18.4.2 Concept same_as [concept.same]")<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](#concept:common_reference_with "18.4.5 Concept common_reference_with [concept.commonref]")< add_lvalue_reference_t<const T>, add_lvalue_reference_t<const U>> && [common_reference_with](#concept:common_reference_with "18.4.5 Concept common_reference_with [concept.commonref]")< 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>.
Let t1 and t2 be equality-preserving expressions ([concepts.equality]) such thatdecltype((t1)) and decltype((t2)) are each T, and let u1 and u2 be equality-preserving expressions such thatdecltype((u1)) and decltype((u2)) are each U.
T and U model common_with<T, U> only if
C(t1) equals C(t2) if and only if t1 equals t2, and
C(u1) equals C(u2) if and only if u1 equals u2.
[Note 2:
Users can customize the behavior of common_with by specializing thecommon_type class template ([meta.trans.other]).
â end note]
18.4.7 Arithmetic concepts [concepts.arithmetic]
template<class T> concept [integral](#concept:integral "18.4.7 Arithmetic concepts [concepts.arithmetic]") = [is_integral_v](meta.type.synop#lib:is_integral_v "21.3.3 Header <type_traits> synopsis [meta.type.synop]")<T>; template<class T> concept [signed_integral](#concept:signed_integral "18.4.7 Arithmetic concepts [concepts.arithmetic]") = [integral](#concept:integral "18.4.7 Arithmetic concepts [concepts.arithmetic]")<T> && [is_signed_v](meta.type.synop#lib:is_signed_v "21.3.3 Header <type_traits> synopsis [meta.type.synop]")<T>; template<class T> concept [unsigned_integral](#concept:unsigned_integral "18.4.7 Arithmetic concepts [concepts.arithmetic]") = [integral](#concept:integral "18.4.7 Arithmetic concepts [concepts.arithmetic]")<T> && <T>; template<class T> concept [floating_point](#concept:floating_point "18.4.7 Arithmetic concepts [concepts.arithmetic]") = 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.
â end note]
[Note 2:
unsigned_integral can be modeled even by types that are not unsigned integer types ([basic.fundamental]); for example, bool.
â end note]
18.4.8 Concept assignable_from [concept.assignable]
template<class LHS, class RHS> concept [assignable_from](#concept:assignable_from "18.4.8 Concept assignable_from [concept.assignable]") = is_lvalue_reference_v<LHS> && [common_reference_with](#concept:common_reference_with "18.4.5 Concept common_reference_with [concept.commonref]")<const remove_reference_t<LHS>&, const remove_reference_t<RHS>&> && requires(LHS lhs, RHS&& rhs) { { lhs = std::forward<RHS>(rhs) } -> [same_as](#concept:same_as "18.4.2 Concept same_as [concept.same]")<LHS>; };
Let:
lhs be an lvalue that refers to an object lcopy such that decltype((lhs)) is LHS,
rhs be an expression such that decltype((rhs)) is RHS, and
rcopy be a distinct object that is equal to rhs.
LHS and RHS modelassignable_from<LHS, RHS> only if
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addressof(lhs = rhs) == addressof(lcopy).
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After evaluating lhs = rhs:
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lhs is equal to rcopy, unless rhs is a non-const xvalue that refers to lcopy.
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If rhs is a non-const xvalue, the resulting state of the object to which it refers is valid but unspecified ([lib.types.movedfrom]).
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Otherwise, if rhs is a glvalue, the object to which it refers is not modified.
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[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 =.
â 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.
â end note]
An operationexchanges the values denoted by t1 and u1 if and only if the operation modifies neither t2 nor u2 and:
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If T and U are the same type, the result of the operation is that t1 equals u2 and u1 equals t2.
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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 expressionranges::swap(E1, E2) for subexpressions E1 and E2 is expression-equivalent to an expressionS determined as follows:
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S is (void)swap(E1, E2)192 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 declarationtemplatevoid 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. â end note]
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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)).
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Otherwise, if E1 and E2 are lvalues of the same type T that models move_constructible and assignable_from<T&, T>, S is an expression that exchanges the denoted values. S is a constant expression if
T is a literal type ([basic.types.general]),
both E1 = std::move(E2) and E2 = std::move(E1) are constant subexpressions ([defns.const.subexpr]), and
the full-expressions of the initializers in the declarationsT t1(std::move(E1)); T t2(std::move(E2)); are constant subexpressions.
noexcept(S) is equal to is_nothrow_move_constructible_v && is_nothrow_move_assignable_v.
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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. â end note]
[Note 4:
Whenever ranges::swap(E1, E2) is a valid expression, it exchanges the values denoted byE1 and E2 and has type void.
â end note]
template<class T> concept [swappable](#concept:swappable "18.4.9 Concept swappable [concept.swappable]") = requires(T& a, T& b) { ranges::swap(a, b); };
template<class T, class U> concept [swappable_with](#concept:swappable_with "18.4.9 Concept swappable [concept.swappable]") = [common_reference_with](#concept:common_reference_with "18.4.5 Concept common_reference_with [concept.commonref]")<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.
â 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 #include #include namespace ranges = std::ranges;
template<class T, std::swappable_with U>void value_swap(T&& t, U&& u) { ranges::swap(std::forward(t), std::forward(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);}
â end example]
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](#concept:destructible "18.4.10 Concept destructible [concept.destructible]") = [is_nothrow_destructible_v](meta.type.synop#lib:is_nothrow_destructible_v "21.3.3 Header <type_traits> synopsis [meta.type.synop]")<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.
â 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](#concept:constructible_from "18.4.11 Concept constructible_from [concept.constructible]") = [destructible](#concept:destructible "18.4.10 Concept destructible [concept.destructible]")<T> && [is_constructible_v](meta.type.synop#lib:is_constructible_v "21.3.3 Header <type_traits> synopsis [meta.type.synop]")<T, Args...>;
18.4.12 Concept default_initializable [concept.default.init]
`template constexpr bool is-default-initializable = see below; // exposition only
template concept default_initializable = constructible_from && requires { T{}; } && is-default-initializable; `
For a type T, is-default-initializable is true if and only if the variable definitionT 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](#concept:move_constructible "18.4.13 Concept move_constructible [concept.moveconstructible]") = [constructible_from](#concept:constructible_from "18.4.11 Concept constructible_from [concept.constructible]")<T, T> && [convertible_to](#concept:convertible_to "18.4.4 Concept convertible_to [concept.convertible]")<T, T>;
If T is an object type, then let rv be an rvalue of typeT and u2 a distinct object of type T equal torv.
T models move_constructible only if
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After the definition T u = rv;, u is equal to u2.
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T(rv) is equal to u2.
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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](#concept:copy_constructible "18.4.14 Concept copy_constructible [concept.copyconstructible]") = [move_constructible](#concept:move_constructible "18.4.13 Concept move_constructible [concept.moveconstructible]")<T> && [constructible_from](#concept:constructible_from "18.4.11 Concept constructible_from [concept.constructible]")<T, T&> && [convertible_to](#concept:convertible_to "18.4.4 Concept convertible_to [concept.convertible]")<T&, T> && [constructible_from](#concept:constructible_from "18.4.11 Concept constructible_from [concept.constructible]")<T, const T&> && [convertible_to](#concept:convertible_to "18.4.4 Concept convertible_to [concept.convertible]")<const T&, T> && [constructible_from](#concept:constructible_from "18.4.11 Concept constructible_from [concept.constructible]")<T, const T> && [convertible_to](#concept:convertible_to "18.4.4 Concept convertible_to [concept.convertible]")<const T, T>;
If T is an object type, then let v be an lvalue of typeT or const T or an rvalue of type const T.
T models copy_constructible only if
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After the definition T u = v;,u is equal to v ([concepts.equality]) andv is not modified.
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T(v) is equal to v and does not modify v.