18 Concepts library [concepts]

18.7 Callable concepts [concepts.callable]

18.7.3 Concept regular_­invocable [concept.regularinvocable]

template<class F, class... Args> concept regular_invocable = invocable<F, Args...>;
The invoke function call expression shall be equality-preserving and shall not modify the function object or the arguments ([concepts.equality]).
Note
:
This requirement supersedes the annotation in the definition of invocable.
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 ]
Example
:
A random number generator does not model regular_­invocable.
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 ]
Note
:
The distinction between invocable and regular_­invocable is purely semantic.
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 ]