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 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

See also

is_invocable is_invocable_r is_nothrow_invocable is_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 .