std::invocable (since C++20), std::regular_invocable (since C++20)

 

C++

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Technical specifications

Symbols index

External libraries

 

Concepts library

Core language concepts same_as (C++20) derived_from (C++20) convertible_to (C++20) common_reference_with (C++20) common_with (C++20) integral (C++20) signed_integral (C++20) unsigned_integral (C++20) floating_point (C++20) swappableswappable_with (C++20)(C++20) destructible (C++20) constructible_from (C++20) default_initializable (C++20)   move_constructible (C++20) copy_constructible (C++20) assignable_from (C++20)

Comparison concepts equality_comparableequality_comparable_with (C++20)(C++20) totally_orderedtotally_ordered_with (C++20)(C++20) Object concepts movable (C++20) copyable (C++20) semiregular (C++20) regular (C++20) Callable concepts invocableregular_invocable (C++20)(C++20) predicate (C++20) relation (C++20) equivalence_relation (C++20) strict_weak_order (C++20)

Exposition-only concepts boolean-testable  (C++20)

 

Defined in header <concepts>
template< class F, class... Args > concept invocable =     requires(F&& f, Args&&... args) {         std::invoke(std::forward<F>(f), std::forward<Args>(args)...);             /* not required to be equality-preserving */     };		(since C++20)
template< class F, class... Args > concept regular_invocable = std::invocable<F, Args...>;		(since C++20)

The invocable concept specifies that a callable type F can be called with a set of arguments Args... using the function template std::invoke.

The regular_invocable concept adds to the invocable concept by requiring the invoke expression to be equality-preserving and not modify either the function object or the arguments.

Equality preservation

Expressions declared in requires expressions of the standard library concepts are required to be equality-preserving (except where stated otherwise).

Notes

The distinction between invocable and regular_invocable is purely semantic.

A random number generator may satisfy invocable but cannot satisfy regular_invocable (comical ones excluded).

References

• C++23 standard (ISO/IEC 14882:2024):

• 18.7.2 Concept invocable [concept.invocable]

• 18.7.3 Concept regular_invocable [concept.regularinvocable]

• C++20 standard (ISO/IEC 14882:2020):

• 18.7.2 Concept invocable [concept.invocable]

• 18.7.3 Concept regular_invocable [concept.regularinvocable]

See also

is_invocableis_invocable_ris_nothrow_invocableis_nothrow_invocable_r (C++17)

checks if a type can be invoked (as if by std::invoke) with the given argument types (class template)

External links

1.	A joke example of a random number generator that satisfies both invocable and regular_invocable.