tiny-di.cpp

#include <cassert>
#include <iostream>
#include <functional>
#include <memory>
#include <type_traits>
#include <typeinfo>
#include <typeindex>
#include <utility>
#include <unordered_map>


// ***** IMPLEMENTATION ****

struct base_instance_container 
{
	virtual ~base_instance_container() = default;
};

template <class T>
struct instance_container : base_instance_container 
{
	instance_container(std::unique_ptr<T> p) 
		: p(std::move(p))
	{
	}

	std::unique_ptr<T> p;
};

struct dependency_injector;

template <class T>
struct generic_argument 
{
	template <
		class Type, 
		std::enable_if_t<
			!std::is_same<T, std::decay_t<Type>>::value, 
			int > = 0
	> 
	constexpr operator Type&() const noexcept {
		return injector_. template create<Type>();
	}

	dependency_injector& injector_;
};

template <class T, class... arguments>
constexpr auto create_impl(dependency_injector& di)
	-> std::enable_if_t<std::is_constructible<T, arguments...>::value, std::unique_ptr<instance_container<T>>>
{
	return std::make_unique<instance_container<T>>(std::make_unique<T>(arguments{di}...));
}

template <class T, class... arguments>
constexpr auto create_impl(dependency_injector& di)
	-> std::enable_if_t<!std::is_constructible<T, arguments...>::value, std::unique_ptr<instance_container<T>>> 
{
	return create_impl<T, arguments..., generic_argument<T>>(di);
}

struct dependency_injector
{
	template <class T>
	T& create() {
		// Check if there is already an instance of that type
		std::cout << "Querying a instance of type: " << typeid(T).name() << std::endl;

		auto typeindex = std::type_index{typeid(T)};

		auto it = instances_.find(typeindex);

		if (it != instances_.end()) {
			std::cout << "Instance already existing: " << typeid(T).name() << std::endl;
			return *static_cast<instance_container<T>*>(it->second.get())->p;
		} else {
			std::cout << "Instance does not exist, creating it for you: " << typeid(T).name() << std::endl;
			return do_create<T>();
		}
	}

	template <class Interface, class Concret>
	void implement() {
		creator_.emplace(std::type_index{typeid(Interface)}, [this]() {
			std::cout << "Creating " << typeid(Interface).name() << " with concret type: " << typeid(Concret).name() << std::endl;
			instances_.emplace(std::type_index{typeid(Interface)}, create_impl<Concret>(*this));
		});	
	}


private:
	template <class T, std::enable_if_t<!std::is_abstract<T>::value, int> = 0>
	T& do_create() {
		auto typeindex = std::type_index{typeid(T)};

		auto creator_it = creator_.find(std::type_index{typeid(T)});
		if (creator_it != creator_.end()) {
			std::cout << "Instance created using a creator: " << typeid(T).name() << std::endl;
			creator_it->second();
			return *static_cast<instance_container<T>*>(instances_[std::type_index{typeid(T)}].get())->p;
		} else {
			auto result = instances_.emplace(typeindex, create_impl<T>(*this));
			std::cout << "Instance created manually: " << typeid(T).name() << std::endl;
			return *static_cast<instance_container<T>*>(result.first->second.get())->p;
		}
	}

	// Abstract types need a special treatment to avoid making create_impl crazy and detecting interfaces without concret classes...
	template <class T, std::enable_if_t<std::is_abstract<T>::value, int> = 0>
	T& do_create() {
		auto typeindex = std::type_index{typeid(T)};

		auto creator_it = creator_.find(std::type_index{typeid(T)});
		if (creator_it != creator_.end()) {
			std::cout << "Instance created using a creator: " << typeid(T).name() << std::endl;
			creator_it->second();
			return *static_cast<instance_container<T>*>(instances_[std::type_index{typeid(T)}].get())->p;
		} else {
			assert(false && "No concret class provided");
		}
	}

	std::unordered_map<std::type_index, std::function<void()>> creator_;
	std::unordered_map<std::type_index, std::unique_ptr<base_instance_container>> instances_;
};

// ***** TEST SCENARIO ****

struct A
{
};

struct B
{
};

struct Interface 
{
	virtual ~Interface() = default;
	virtual void SayHi() = 0;
};


struct Concret : Interface 
{
	Concret(A& a) {}

	void SayHi() override {
		std::cout << "Hi" << std::endl;
	}	
};


struct C
{
	C(B& b, Interface& i, A& a) : i(i) {}

	void SayHi() {
		i.SayHi();
	}

	Interface& i;
};

int main()
{
	dependency_injector di;
	di.implement<Interface, Concret>();
	auto& c = di.create<C>(); 

	c.SayHi();
}