[indirectcallable.indirectinvocable]

# 24 Iterators library [[iterators]](./#iterators)

## 24.3 Iterator requirements [[iterator.requirements]](iterator.requirements#indirectcallable.indirectinvocable)

### 24.3.6 Indirect callable requirements [[indirectcallable]](indirectcallable#indirectinvocable)

#### 24.3.6.3 Indirect callables [indirectcallable.indirectinvocable]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L2451)

The indirect callable concepts are used to constrain those algorithms
that accept callable objects ([[func.def]](func.def "22.10.3 Definitions")) as arguments[.](#1.sentence-1)

namespace std {template<class F, class I>concept [indirectly_unary_invocable](#concept:indirectly_unary_invocable "24.3.6.3 Indirect callables [indirectcallable.indirectinvocable]") =[indirectly_readable](iterator.concept.readable#concept:indirectly_readable "24.3.4.2 Concept indirectly_­readable [iterator.concept.readable]")<I> &&[copy_constructible](concept.copyconstructible#concept:copy_constructible "18.4.14 Concept copy_­constructible [concept.copyconstructible]")<F> &&[invocable](concept.invocable#concept:invocable "18.7.2 Concept invocable [concept.invocable]")<F&, *indirect-value-t*<I>> &&[invocable](concept.invocable#concept:invocable "18.7.2 Concept invocable [concept.invocable]")<F&, iter_reference_t<I>> &&[common_reference_with](concept.commonref#concept:common_reference_with "18.4.5 Concept common_­reference_­with [concept.commonref]")< invoke_result_t<F&, *indirect-value-t*<I>>,
 invoke_result_t<F&, iter_reference_t<I>>>; template<class F, class I>concept [indirectly_regular_unary_invocable](#concept:indirectly_regular_unary_invocable "24.3.6.3 Indirect callables [indirectcallable.indirectinvocable]") =[indirectly_readable](iterator.concept.readable#concept:indirectly_readable "24.3.4.2 Concept indirectly_­readable [iterator.concept.readable]")<I> &&[copy_constructible](concept.copyconstructible#concept:copy_constructible "18.4.14 Concept copy_­constructible [concept.copyconstructible]")<F> &&[regular_invocable](concept.regularinvocable#concept:regular_invocable "18.7.3 Concept regular_­invocable [concept.regularinvocable]")<F&, *indirect-value-t*<I>> &&[regular_invocable](concept.regularinvocable#concept:regular_invocable "18.7.3 Concept regular_­invocable [concept.regularinvocable]")<F&, iter_reference_t<I>> &&[common_reference_with](concept.commonref#concept:common_reference_with "18.4.5 Concept common_­reference_­with [concept.commonref]")< invoke_result_t<F&, *indirect-value-t*<I>>,
 invoke_result_t<F&, iter_reference_t<I>>>; template<class F, class I>concept [indirect_unary_predicate](#concept:indirect_unary_predicate "24.3.6.3 Indirect callables [indirectcallable.indirectinvocable]") =[indirectly_readable](iterator.concept.readable#concept:indirectly_readable "24.3.4.2 Concept indirectly_­readable [iterator.concept.readable]")<I> &&[copy_constructible](concept.copyconstructible#concept:copy_constructible "18.4.14 Concept copy_­constructible [concept.copyconstructible]")<F> &&[predicate](concept.predicate#concept:predicate "18.7.4 Concept predicate [concept.predicate]")<F&, *indirect-value-t*<I>> &&[predicate](concept.predicate#concept:predicate "18.7.4 Concept predicate [concept.predicate]")<F&, iter_reference_t<I>>; template<class F, class I1, class I2>concept [indirect_binary_predicate](#concept:indirect_binary_predicate "24.3.6.3 Indirect callables [indirectcallable.indirectinvocable]") =[indirectly_readable](iterator.concept.readable#concept:indirectly_readable "24.3.4.2 Concept indirectly_­readable [iterator.concept.readable]")<I1> && [indirectly_readable](iterator.concept.readable#concept:indirectly_readable "24.3.4.2 Concept indirectly_­readable [iterator.concept.readable]")<I2> &&[copy_constructible](concept.copyconstructible#concept:copy_constructible "18.4.14 Concept copy_­constructible [concept.copyconstructible]")<F> &&[predicate](concept.predicate#concept:predicate "18.7.4 Concept predicate [concept.predicate]")<F&, *indirect-value-t*<I1>, *indirect-value-t*<I2>> &&[predicate](concept.predicate#concept:predicate "18.7.4 Concept predicate [concept.predicate]")<F&, *indirect-value-t*<I1>, iter_reference_t<I2>> &&[predicate](concept.predicate#concept:predicate "18.7.4 Concept predicate [concept.predicate]")<F&, iter_reference_t<I1>, *indirect-value-t*<I2>> &&[predicate](concept.predicate#concept:predicate "18.7.4 Concept predicate [concept.predicate]")<F&, iter_reference_t<I1>, iter_reference_t<I2>>; template<class F, class I1, class I2 = I1>concept [indirect_equivalence_relation](#concept:indirect_equivalence_relation "24.3.6.3 Indirect callables [indirectcallable.indirectinvocable]") =[indirectly_readable](iterator.concept.readable#concept:indirectly_readable "24.3.4.2 Concept indirectly_­readable [iterator.concept.readable]")<I1> && [indirectly_readable](iterator.concept.readable#concept:indirectly_readable "24.3.4.2 Concept indirectly_­readable [iterator.concept.readable]")<I2> &&[copy_constructible](concept.copyconstructible#concept:copy_constructible "18.4.14 Concept copy_­constructible [concept.copyconstructible]")<F> &&[equivalence_relation](concept.equiv#concept:equivalence_relation "18.7.6 Concept equivalence_­relation [concept.equiv]")<F&, *indirect-value-t*<I1>, *indirect-value-t*<I2>> &&[equivalence_relation](concept.equiv#concept:equivalence_relation "18.7.6 Concept equivalence_­relation [concept.equiv]")<F&, *indirect-value-t*<I1>, iter_reference_t<I2>> &&[equivalence_relation](concept.equiv#concept:equivalence_relation "18.7.6 Concept equivalence_­relation [concept.equiv]")<F&, iter_reference_t<I1>, *indirect-value-t*<I2>> &&[equivalence_relation](concept.equiv#concept:equivalence_relation "18.7.6 Concept equivalence_­relation [concept.equiv]")<F&, iter_reference_t<I1>, iter_reference_t<I2>>; template<class F, class I1, class I2 = I1>concept [indirect_strict_weak_order](#concept:indirect_strict_weak_order "24.3.6.3 Indirect callables [indirectcallable.indirectinvocable]") =[indirectly_readable](iterator.concept.readable#concept:indirectly_readable "24.3.4.2 Concept indirectly_­readable [iterator.concept.readable]")<I1> && [indirectly_readable](iterator.concept.readable#concept:indirectly_readable "24.3.4.2 Concept indirectly_­readable [iterator.concept.readable]")<I2> &&[copy_constructible](concept.copyconstructible#concept:copy_constructible "18.4.14 Concept copy_­constructible [concept.copyconstructible]")<F> &&[strict_weak_order](concept.strictweakorder#concept:strict_weak_order "18.7.7 Concept strict_­weak_­order [concept.strictweakorder]")<F&, *indirect-value-t*<I1>, *indirect-value-t*<I2>> &&[strict_weak_order](concept.strictweakorder#concept:strict_weak_order "18.7.7 Concept strict_­weak_­order [concept.strictweakorder]")<F&, *indirect-value-t*<I1>, iter_reference_t<I2>> &&[strict_weak_order](concept.strictweakorder#concept:strict_weak_order "18.7.7 Concept strict_­weak_­order [concept.strictweakorder]")<F&, iter_reference_t<I1>, *indirect-value-t*<I2>> &&[strict_weak_order](concept.strictweakorder#concept:strict_weak_order "18.7.7 Concept strict_­weak_­order [concept.strictweakorder]")<F&, iter_reference_t<I1>, iter_reference_t<I2>>;}