[concepts.callable]

# 18 Concepts library [[concepts]](./#concepts)

## 18.7 Callable concepts [concepts.callable]

### [18.7.1](#general) General [[concepts.callable.general]](concepts.callable.general)

[1](#general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/concepts.tex#L1209)

The concepts in [concepts.callable] describe the requirements on function
objects ([[function.objects]](function.objects "22.10 Function objects")) and their arguments[.](#general-1.sentence-1)

### [18.7.2](#concept.invocable) Concept invocable [[concept.invocable]](concept.invocable)

[1](#concept.invocable-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/concepts.tex#L1215)

The [invocable](#concept:invocable "18.7.2 Concept invocable [concept.invocable]") concept specifies a relationship between a callable
type ([[func.def]](func.def "22.10.3 Definitions")) F and a set of argument types Args... which
can be evaluated by the library function invoke ([[func.invoke]](func.invoke "22.10.5 invoke functions"))[.](#concept.invocable-1.sentence-1)

[🔗](#concept:invocable)

`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
  };
`

[2](#concept.invocable-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/concepts.tex#L1228)

[*Example [1](#concept.invocable-example-1)*:

A function that generates random numbers can model [invocable](#concept:invocable "18.7.2 Concept invocable [concept.invocable]"),
since the invoke function call expression is not required to be
equality-preserving ([[concepts.equality]](concepts.equality "18.2 Equality preservation"))[.](#concept.invocable-2.sentence-1)

— *end example*]

### [18.7.3](#concept.regularinvocable) Concept regular_invocable [[concept.regularinvocable]](concept.regularinvocable)

[🔗](#concept:regular_invocable)

`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...>;
`

[1](#concept.regularinvocable-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/concepts.tex#L1244)

The invoke function call expression shall be
equality-preserving ([[concepts.equality]](concepts.equality "18.2 Equality preservation")) and
shall not modify the function object or the arguments[.](#concept.regularinvocable-1.sentence-1)

[*Note [1](#concept.regularinvocable-note-1)*:

This requirement supersedes the annotation in the definition of[invocable](#concept:invocable "18.7.2 Concept invocable [concept.invocable]")[.](#concept.regularinvocable-1.sentence-2)

— *end note*]

[2](#concept.regularinvocable-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/concepts.tex#L1253)

[*Example [1](#concept.regularinvocable-example-1)*:

A random number generator does not model [regular_invocable](#concept:regular_invocable "18.7.3 Concept regular_­invocable [concept.regularinvocable]")[.](#concept.regularinvocable-2.sentence-1)

— *end example*]

[3](#concept.regularinvocable-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/concepts.tex#L1258)

[*Note [2](#concept.regularinvocable-note-2)*:

The distinction between [invocable](#concept:invocable "18.7.2 Concept invocable [concept.invocable]") and [regular_invocable](#concept:regular_invocable "18.7.3 Concept regular_­invocable [concept.regularinvocable]") is purely semantic[.](#concept.regularinvocable-3.sentence-1)

— *end note*]

### [18.7.4](#concept.predicate) Concept predicate [[concept.predicate]](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 [[concept.relation]](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.equiv) Concept equivalence_relation [[concept.equiv]](concept.equiv)

[🔗](#concept:equivalence_relation)

`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>;
`

[1](#concept.equiv-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/concepts.tex#L1290)

A [relation](#concept:relation "18.7.5 Concept relation [concept.relation]") models [equivalence_relation](#concept:equivalence_relation "18.7.6 Concept equivalence_­relation [concept.equiv]") only if
it imposes an equivalence relation on its arguments[.](#concept.equiv-1.sentence-1)

### [18.7.7](#concept.strictweakorder) Concept strict_weak_order [[concept.strictweakorder]](concept.strictweakorder)

[🔗](#concept:strict_weak_order)

`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>;
`

[1](#concept.strictweakorder-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/concepts.tex#L1303)

A [relation](#concept:relation "18.7.5 Concept relation [concept.relation]") models [strict_weak_order](#concept:strict_weak_order "18.7.7 Concept strict_­weak_­order [concept.strictweakorder]") only if
it imposes a [*strict weak ordering*](#def:strict_weak_ordering) on its arguments[.](#concept.strictweakorder-1.sentence-1)

[2](#concept.strictweakorder-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/concepts.tex#L1307)

The term[*strict*](#def:strict) refers to the
requirement of an irreflexive relation (!comp(x, x) for all x),
and the term[*weak*](#def:weak) to requirements that are not as strong as
those for a total ordering,
but stronger than those for a partial
ordering[.](#concept.strictweakorder-2.sentence-1)

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

- [(2.1)](#concept.strictweakorder-2.1)

comp(a, b) && comp(b, c) impliescomp(a, c)

- [(2.2)](#concept.strictweakorder-2.2)

equiv(a, b) && equiv(b, c) impliesequiv(a, c)

[3](#concept.strictweakorder-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/concepts.tex#L1338)

[*Note [1](#concept.strictweakorder-note-1)*:

Under these conditions, it can be shown that

- [(3.1)](#concept.strictweakorder-3.1)

equiv is an equivalence relation,

- [(3.2)](#concept.strictweakorder-3.2)

comp induces a well-defined relation on the equivalence
classes determined byequiv, and

- [(3.3)](#concept.strictweakorder-3.3)

the induced relation is a strict total ordering[.](#concept.strictweakorder-3.sentence-1)

— *end note*]