defaultable_map.hpp

template<typename Key, typename T, T value = T()> 
class defaultable_map : 
	public std::unordered_map<Key, T> 
{ 
public: 
	// inherit std::unordered_map constructors 
	using std::unordered_map<Key, T>::unordered_map; 
 
	T & operator[](const Key & key) 
	{ 
		auto it = find(key); 
 
		if (it == end()) 
		{ 
			// insert default value 
			auto result = insert(std::make_pair(key, value)); 
			it = result.first; 
		} 
 
		return it->second; 
	} 
}; 
 
// 
defaultable_map<char, int, -1> m;