C++ overloading template on assigning value or callback function

Question

Trying to do something like...

template <class T>
struct Wrapper
{
    template <class U>
    void set(const U& u) { myT = u; }

    template <class F>
    void set(F f) { myT = f(); }

    T myT;
};

I know I need to use SFINAE here but how do I distinguish a callback parameter from a value parameter? It's safe to assume that a value can't be used as a callback.

I've tried enable_if with is_function, result_of, invoke_result, is_invocable, and others, but none of it works right. Is it even possible?

Answer 1

You can do it without SFINAE:

template<class U>
void set(const U& u) { 
    if constexpr (std::is_invocable_v<U>)
        myT = u();
    else
        myT = u;
}

or in a more generic way:

template<class U>
void set(U&& u) { 
    if constexpr (std::is_invocable_v<U>)
        myT = std::forward<U>(u)();
    else
        myT = std::forward<U>(u);
}

Answer 2

Yes you can apply SFINAE with the help of std::is_invocable (since C++17).

template <class U>
std::enable_if_t<!std::is_invocable_v<U>> set(const U& u) { myT = u; }

template <class F>
std::enable_if_t<std::is_invocable_v<F>> set(F f) { myT = f(); }

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