Simple implementation of a function like std::bind.

Bind.cxx

#include <iostream>
#include <functional>
#include <tuple>
#include <type_traits>

// Actually, a subtle bug may arise when the function is using index_constant as parameter.
// For this reason, a more correct implementation should define a specialized class, hidden in a namespace,
// and forbid the use in other places.
// An easier, more correct, but more verbose way would be to just define two different classes for the argument list
// and the bounded arguments, to avoid the confusion with the index_constant bracket operator overload.
// However, as this is only an academic example, it should strive to stay extremly simple and straightforward, so this
// improvment will not be applied. I just wanted anyone wanting to use this somewhere to be aware of this caveheat.
// Moreover, you should use std::bind anyway.

template<size_t n>
using index_constant = std::integral_constant<size_t, n>;

template<class ... Args>
class binder_list
{
public:
	template<class ... TArgs>
	constexpr binder_list(TArgs&&... args) noexcept
	: boundedArgs_{std::forward<TArgs>(args)...}
	{}
	
	template<size_t n>
	constexpr decltype(auto) operator[](index_constant<n>) noexcept
	{
		return std::get<n>(boundedArgs_);
	}
	
private:
	std::tuple<Args...> boundedArgs_;	
};

template<class ... Args>
class callee_list
{
public:
	template<class ... TArgs>
	constexpr callee_list(TArgs&&... args) noexcept
	: boundedArgs_{std::forward<TArgs>(args)...}
	{}
	
	template<class T, std::enable_if_t<(std::is_placeholder<std::remove_reference_t<T>>::value == 0)>* = nullptr>
	constexpr decltype(auto) operator[](T&& t) noexcept
	{
		return std::forward<T>(t);
	}
	
	template<class T, std::enable_if_t<(std::is_placeholder<T>::value != 0)>* = nullptr>
	constexpr decltype(auto) operator[](T) noexcept
	{
		return std::get<std::is_placeholder<T>::value - 1>(std::move(boundedArgs_));
	}
	
private:
	std::tuple<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)},
	  argumentList_{std::forward<TArgs>(args)...}
	{}
	
// Please C++, give me a way of detecting noexcept :'(
	template<class ... CallArgs>
	constexpr decltype(auto) operator()(CallArgs&&... args) 
	//noexcept(noexcept(call(std::make_index_sequence<sizeof...(Args)>{}, std::declval<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) 
	//noexcept(noexcept(f_(this->binder_list<CallArgs...>{std::declval<CallArgs>()...}[this->argumentList_[index_constant<Seq>{}]]...)))
	{
		return f_((callee_list<CallArgs...>{std::forward<CallArgs>(args)...}[argumentList_[index_constant<Seq>{}]])...);
	}
private:
	std::function<std::remove_reference_t<std::remove_pointer_t<Fn>>> f_;
	binder_list<Args...> argumentList_;
};

namespace me
{
template<class Fn, class ... Args>
binder<Fn, Args...> bind(Fn&& f, Args&&... args)
{
	return binder<Fn, Args...>{std::forward<Fn>(f), std::forward<Args>(args)...};
}
}

Thanks a lot for this sample!
Helped me once with my problem, much better than reading standard or boost sources.

By the way. AFAIK, std::bind stores copies of the given parameters. Something like std::tuple<typename std::decay_t<Args>...> boundedArgs_; could improve it.

Same response as above with the little added bonus that yeah, the "binder_list" class is useless, no flipping idea why I wrote it this way. Gonna update it with proper standard compliance and taking your remark into account. Thanks.

you can detect noexcept, like this:

noexcept(run(args...))
it returns a bool, plug that back into noexcept

noexcept(noexcept(run(args...)))

you can detect noexcept, like this:

noexcept(run(args...))
it returns a bool, plug that back into noexcept

noexcept(noexcept(run(args...)))

Yes, as you can see in the code comments, I have indeed tried to do that but it must've been acting out back then. Bear in mind that I have written this code a few years ago.