18 Concepts library [concepts]

18.7 Callable concepts [concepts.callable]

18.7.1 General [concepts.callable.general]

The concepts in [concepts.callable] describe the requirements on function objects ([function.objects]) and their arguments.

18.7.2 Concept invocable [concept.invocable]

The invocable concept specifies a relationship between a callable type ([func.def]) F and a set of argument types Args... which can be evaluated by the library function invoke ([func.invoke]).

template<class F, class... Args> concept [invocable](#concept:invocable "18.7.2 Concept invocable [concept.invocable]") = requires(F&& f, Args&&... args) { invoke(std::forward<F>(f), std::forward<Args>(args)...); // not required to be equality-preserving };

[Example 1:

A function that generates random numbers can model invocable, since the invoke function call expression is not required to be equality-preserving ([concepts.equality]).

â end example]

18.7.3 Concept regular_invocable [concept.regularinvocable]

template<class F, class... Args> concept [regular_invocable](#concept:regular_invocable "18.7.3 Concept regular_invocable [concept.regularinvocable]") = [invocable](#concept:invocable "18.7.2 Concept invocable [concept.invocable]")<F, Args...>;

The invoke function call expression shall be equality-preserving ([concepts.equality]) and shall not modify the function object or the arguments.

[Note 1:

This requirement supersedes the annotation in the definition ofinvocable .

â end note]

[Example 1:

A random number generator does not model regular_invocable .

â end example]

[Note 2:

The distinction between invocable and regular_invocable is purely semantic.

â end note]

18.7.4 Concept predicate [concept.predicate]

template<class F, class... Args> concept [predicate](#concept:predicate "18.7.4 Concept predicate [concept.predicate]") = [regular_invocable](#concept:regular_invocable "18.7.3 Concept regular_invocable [concept.regularinvocable]")<F, Args...> && [boolean-testable](concept.booleantestable#concept:boolean-testable "18.5.2 Boolean testability [concept.booleantestable]")<invoke_result_t<F, Args...>>;

18.7.5 Concept relation [concept.relation]

template<class R, class T, class U> concept [relation](#concept:relation "18.7.5 Concept relation [concept.relation]") = [predicate](#concept:predicate "18.7.4 Concept predicate [concept.predicate]")<R, T, T> && [predicate](#concept:predicate "18.7.4 Concept predicate [concept.predicate]")<R, U, U> && [predicate](#concept:predicate "18.7.4 Concept predicate [concept.predicate]")<R, T, U> && [predicate](#concept:predicate "18.7.4 Concept predicate [concept.predicate]")<R, U, T>;

18.7.6 Concept equivalence_relation [concept.equiv]

template<class R, class T, class U> concept [equivalence_relation](#concept:equivalence_relation "18.7.6 Concept equivalence_relation [concept.equiv]") = [relation](#concept:relation "18.7.5 Concept relation [concept.relation]")<R, T, U>;

A relation models equivalence_relation only if it imposes an equivalence relation on its arguments.

18.7.7 Concept strict_weak_order [concept.strictweakorder]

template<class R, class T, class U> concept [strict_weak_order](#concept:strict_weak_order "18.7.7 Concept strict_weak_order [concept.strictweakorder]") = [relation](#concept:relation "18.7.5 Concept relation [concept.relation]")<R, T, U>;

A relation models strict_weak_order only if it imposes a strict weak ordering on its arguments.

The termstrict refers to the requirement of an irreflexive relation (!comp(x, x) for all x), and the termweak to requirements that are not as strong as those for a total ordering, but stronger than those for a partial ordering.

If we defineequiv(a, b) as!comp(a, b) && !comp(b, a), then the requirements are thatcomp andequiv both be transitive relations: