naive bind c++17

naive_bind.cc

// https://godbolt.org/z/7hefs586j
#include <functional>
#include <tuple>
#include <iostream> 
template <size_t n> using index_constant = std::integral_constant<size_t, n>;
template<class ... Args>
class callee_list
{
public:
	template<class ... TArgs>
	constexpr callee_list(TArgs&&... args) noexcept
	: boundedArgs_{std::forward<TArgs>(args)...}
	{}

	template<class T>
  constexpr decltype(auto) operator[](T&& t) noexcept
	{
    if constexpr (!std::is_placeholder_v<std::decay_t<T>>)
    {
        return std::forward<T>(t);
    }
    else
    {
      constexpr size_t Index = std::is_placeholder<std::decay_t<T>>::value - 1;
      return std::get<Index>(std::move(boundedArgs_));
    }
	}

  // Bind以值类型存储所有变量，会擦除int& 到int
	std::tuple<typename std::decay_t<Args>...> boundedArgs_;	
};

template<class Fn, class ... Args>
class binder
{
public:
	template<class TFn, class ... TArgs>
	constexpr binder(TFn&& f, TArgs&&... args) noexcept 
	: f_{std::forward<TFn>(f)},
	  UnresolvedArgsList_{std::forward<TArgs>(args)...}
	{}

	template<class ... CallArgs>
	constexpr decltype(auto) operator()(CallArgs&&... args) 
	{
		return call(std::make_index_sequence<sizeof...(Args)>{}, std::forward<CallArgs>(args)...);
	}
	
private:
	template<class ... CallArgs, size_t ... Seq>
	constexpr decltype(auto) call(std::index_sequence<Seq...>, CallArgs&&... args) 
	{
		// 创建callee_List保存调用Operator()时候传入的参数,用于补齐占位符
		auto calleeList = callee_list<CallArgs...>{std::forward<CallArgs>(args)...};
		// 调用callee_list.operator[]

		// Redchards原版:
		// 参数折叠展开 f_(calleeList[argumentList_[index_constant<0>{}]],calleeList[argumentList_[index_constant<1>{}]],...);

		return f_(calleeList[std::get<index_constant<Seq>{}>(UnresolvedArgsList_)]...);
	}
private:
	std::function<std::remove_reference_t<std::remove_pointer_t<Fn>>> f_;
	// 将占位符以及原有的参数保存在UnResolvedArgsLists_里，在call的时候填充占位符
	std::tuple<typename std::decay_t<Args>...> UnresolvedArgsList_;
};

template<class Fn, class ... Args>
// The code is from RedChards https://gist.github.com/Redchards/c5be14c2998f1ca1d757
binder<Fn, Args...> Redchards_Bind(Fn&& f, Args&&... args)
{
	return binder<Fn, Args...>{std::forward<Fn>(f), std::forward<Args>(args)...};
}

int foobar(int a,int& b)
{
    std::cout << "a: " << a << std::endl;
    return ++b;
}

int main()
{
    std::cout << "naive bind " << std::endl;
    int n = 0;
    int& n_ref = n;
    auto fn = Redchards_Bind(foobar,1,n_ref);
    // bind的引用捕获不能修改外部变量，内部复制了一个值
    std::cout << fn() << " " << n << std::endl;
    std::cout << fn() << " " << n << std::endl;

}

code is modified from https://gist.github.com/Redchards/c5be14c2998f1ca1d757 by Redchards