Accept any kind of callable and also know argument type

Question

I\'m not sure if it\'s possible, so that\'s what I want to find out.

I\'d like to create a function which accepts any kind of functor/callable object, but I want to

Answer 1

Here's an example that will work for most callables including functors and lambdas (although not for generic functors as @Yakk demonstrated in a comment on the question).

The code can also be useful when determining return type and multiple arguments.

template 
struct func_traits : public func_traits {};

template 
struct func_traits {
    using result_type =  Ret;

    template 
    struct arg {
        using type = typename std::tuple_element>::type;
    };
};

template 
void option(T&& t) {
    using traits = func_traits::type>;

    using return_t = typename traits::result_type;         // Return type.
    using arg0_t = typename traits::template arg<0>::type; // First arg type.

    // Output types.
    std::cout << "Return type: " << typeid(return_t).name() << std::endl;
    std::cout << "Argument type: " << typeid(arg0_t).name() << std::endl;
}

To add support for regular functions add a specialization e.g.

template 
struct func_traits { /* ... */ }

More useful info: Is it possible to figure out the parameter type and return type of a lambda?