libasync/include/async/basic.hpp

#ifndef LIBASYNC_BASIC_HPP
#define LIBASYNC_BASIC_HPP

namespace async {

template<typename S>
struct callback;

template<typename R, typename... Args>
struct callback<R(Args...)> {
private:
	using storage = std::aligned_storage_t<sizeof(void *), alignof(void *)>;

	template<typename F>
	static R invoke(storage object, Args... args) {
		return (*reinterpret_cast<F *>(&object))(std::move(args)...);
	}

public:
	callback()
	: _function(nullptr) { }

	template<typename F, typename = std::enable_if_t<
			sizeof(F) == sizeof(void *) && alignof(F) == alignof(void *)
			&& std::is_trivially_copy_constructible<F>::value
			&& std::is_trivially_destructible<F>::value>>
	callback(F functor)
	: _function(&invoke<F>) {
		new (&_object) F{std::move(functor)};
	}

	explicit operator bool () {
		return static_cast<bool>(_function);
	}

	R operator() (Args... args) {
		return _function(_object, std::move(args)...);
	}

private:
	R (*_function)(storage, Args...);
	std::aligned_storage_t<sizeof(void *), alignof(void *)> _object;
};

template<typename T>
struct awaitable {
	virtual void then(callback<void(T)> awaiter) = 0;
	virtual void detach() = 0;
};

template<>
struct awaitable<void> {
	virtual void then(callback<void()> cb) = 0;
	virtual void detach() = 0;
};

} // namespace async

#endif // LIBASYNC_BASIC_HPP