one_weird_trick.cpp

#include <iostream>
#include <string>
#include <typeindex>
#include <typeinfo>
#include <unordered_map>

// We want to put all registered types into a map:
std::unordered_map<std::type_index, std::string> registered_types;

template<class T>
struct register_me
{
	register_me()
	{
		// do whatever is needed here...
		registered_types[std::type_index(typeid(T))] = typeid(T).name();
	}
};



// Base type:
template<class T>
struct base
{
	const static register_me<T> on_init;

	base()
	{
		// make sure it's actually referenced somewhere so it gets instantiated
		// (but there's no runtime cost in the constructor)
		on_init;
	}
};

template<class T>
const register_me<T> base<T>::on_init;



// Derivees:
class foo : base<foo>
{};

class bar : base<bar>
{};


int main()
{
	foo f;

	std::cout << registered_types.size() << '\n';
	for (const auto& types : registered_types)
	{
		std::cout << types.second << '\n';
	}

	// bar b;
	// if unused, it won't show up, but it doesn't matter where it's used,
	// it will always be registered

    return 0;
}