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 argument 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

An expression is equality preserving if it results in equal outputs given equal inputs.

• The inputs to an expression consist of its operands.
• The outputs of an expression consist of its result and all operands modified by the expression (if any).

In specification of standard concepts, operands are defined as the largest subexpressions that include only:

• an id-expression, and
• invocations of std::move, std::forward, and std::declval.

The cv-qualification and value category of each operand is determined by assuming that each template type parameter denotes a cv-unqualified complete non-array object type.

Every expression required to be equality preserving is further required to be stable: two evaluations of such an expression with the same input objects must have equal outputs absent any explicit intervening modification of those input objects.

Unless noted otherwise, every expression used in a requires-expression is required to be equality preserving and stable, and the evaluation of the expression may modify only its non-constant operands. Operands that are constant must not be modified.

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