std::invocable, std::regular_invocable

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 */
  };

template< class F, class... Args >
concept regular_invocable = std::invocable<F, Args...>;

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

External links