I understand the basic concept of using the recursive nature of variadic template parameters and a specific template instantiation to sort of "eat" my way through the parameter list, one by one.
I understand that lambdas can be written to take certain types and then return certain types. Keep in mind that I'm still learning C++14 and C++11, so I haven't mastered one or the other.
Here was my attempt after at looking at other Stack Overflow questions:
// For std::string
#include <string>
// For std::cout
#include <iostream>
//Create a generalized list instantiation
template <typename ... F>
struct overload : public F... {
overload(F... f) : F(f)... {}
};
//Create an specific end-case, where we directly
//inherit the () operator in order to inherit
//multiple () overloads
template <typename F>
struct overload : F {
using F::operator();
};
//template function to create an overload
template <class... F>
auto make_overload(F... f) {
return (f...);
}
int main() {
auto f = [](int x,int y) -> int {
return x+y;
};
auto g = [](double x,double y) -> int {
return std::ftoi(x+y);
};
auto h = [](std::string x,std::string y) -> int {
return std::stoi(x+y);
};
//Ah, but this is a function.
auto fgh = make_overload(f,g,h);
std::cout << (fgh(1,2)) << std::endl;
std::cout << (fgh(1.5,2.5)) << std::endl;
std::cout << (fgh("bob","larry")) << std::endl;
}
Coliru: http://coliru.stacked-crooked.com/a/5df2919ccf9e99a6
What am I conceptually missing here? Other answers might succinctly answer this problem at face value, but I'm looking for an explanation why the answer eludes my thinking. If I understand that I need to do using F::operator()
to inherit the operators and I correctly state that the return and parameter types are different, what else do I need to do to make this work?
Here's my train of thought:
operator()
.You didn't actually recurse.
// primary template; not defined.
template <class... F> struct overload;
// recursive case; inherit from the first and overload<rest...>
template<class F1, class... F>
struct overload<F1, F...> : F1, overload<F...> {
overload(F1 f1, F... f) : F1(f1), overload<F...>(f...) {}
// bring all operator()s from the bases into the derived class
using F1::operator();
using overload<F...>::operator();
};
// Base case of recursion
template <class F>
struct overload<F> : F {
overload(F f) : F(f) {}
using F::operator();
};