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std::invocable, std::regular_invocable - cppreference.com

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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

External links

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