Practice using template

#include <iostream>
#include <functional>
#include <vector>
#include <algorithm>
#include <cstdio>

void f(int i)
{
    std::cout << "function f called.\n";
}

void g(int i)
{
    std::cout << "function g called.\n";
}

void h(int i)
{
    std::cout << "function h called.\n";
}

template <typename ResultType, typename... Args>
struct Delegate;

template <typename ResultType, typename... Args>
struct Delegate<ResultType(Args...)>
{
    typedef std::function<ResultType(Args...)> function_type;
    std::vector<function_type> fns;

    Delegate() = default;
    Delegate(std::initializer_list<function_type> l): fns(l) {}
    Delegate& operator +=(const function_type& fn)
    {
        fns.push_back(fn);
        return *this;
    }

    ResultType operator ()(Args... args)
    {
        std::for_each(fns.begin(), fns.end()-1, std::bind(&function_type::operator(), std::placeholders::_1, args...));
        return fns.back()(args...);
    }
};

int main()
{
    Delegate<void (int)> d({f, g});
    d+=h;
    d(1);
    Delegate<int (const char*, int, int, int)> pt({std::printf});
    pt("%d+%d=%d\n", 1, 1, 1+1);
}