std::invocable, std::regular_invocable

 

C++

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Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

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Technical specifications

Symbols index

External libraries

 

Concepts library

Core language concepts same_as derived_from convertible_to common_reference_with      common_with integral signed_integral unsigned_integral floating_point swappableswappable_with destructible constructible_from default_initializable move_constructible copy_constructible assignable_from

Comparison concepts equality_comparableequality_comparable_with totally_orderedtotally_ordered_with Object concepts movable copyable semiregular regular Callable concepts invocableregular_invocable predicate relation equivalence_relation strict_weak_order

Exposition-only concepts boolean-testable

 

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).

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 notable example of a random number generator that satisfies both invocable and regular_invocable.