ATL / WTL / STL

// The Original Code: this works
        typedef std::pair<int, int>Data;
	class DataCompare
	{
	public:
		bool operator () (const Data& lhs, const Data& rhs) const
		{
			return keyLess (lhs.first, rhs.first);
		}
		
		bool operator () (const Data& lhs, const Data::first_type& k) const
		{
			return keyLess (lhs.first, k);
		}
		
		bool operator () (const Data::first_type& k, const Data& rhs) const
		{
			return keyLess (k, rhs.first);
		}
	private:
		bool keyLess (const Data::first_type& k1, const Data::first_type& k2) const
		{
			return k1 < k2;
		}
	};

//The template Code this doesn't work it says that no operator () is provided
template <typename T1, typename T2>
class _DataCompare
{
public:
	bool operator () (const std::pair<T1,T2>& lhs, const std::pair<T1,T2>& rhs) const
	{
		return keyLess (lhs.first, rhs.first);
	}
	
	bool operator () (const std::pair<T1,T2>& lhs, const std::pair<T1,T2>::first_type& k) const
	{
		return keyLess (lhs.first, k);
	}
	
	bool operator () (const std::pair<T1,T2>::first_type& k, const std::pair<T1,T2>& rhs) const
	{
		return keyLess (k, rhs.first);
	}
private:
	bool keyLess (const std::pair<T1,T2>::first_type& k1, const std::pair<T1,T2>::first_type& k2) const
	{
		return k1 < k2
	}
};

_DataCompare<int, int> DataCompare;