CppSpec::Util::is_functional Concept Reference

Concept definition

template<typename C>
concept is_functional =
    // The C++ reference says that all "functions" (not FunctionObjects) respond to
    // std::is_function<T>
    std::is_function_v<C> ||
 
    // An alternative from Stack Overflow question 18107368. Anything that could
    // possibly be "functional" should be able to be cast to std::function<void()>
    std::is_convertible_v<C, std::function<void()>> ||
 
    // The C++ reference also says that something qualifies as a FunctionObject iff
    // std::is_object_v<T> is true and the object responds to the call operator()
    requires(C func) {
      requires std::is_object_v<C>;
      { func() } -> std::invocable<>;
    }