std::invocable, std::regular_invocable

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

Equality preservation

Expressions declared in requires expressions of the standard library concepts are required to be equality-preserving

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

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

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