pair <> question

[Miky1]

Hi there,
Let's say we have the following:

#include <map>
#include <string>
#include <vector>
#include <algorithm>

using namespace std;


typedef pair<string, int> Data;

class Compare
{
public:
    bool operator()(const Data& el1, const Data& el2) const
    {
        return lessK(el1.first, el2.first);
    }
    bool operator() (const Data& el1, const Data::first_type& k) const
    {
        return lessK(el1.first, k);
    }
    bool operator ()(const Data::first_type& k, const Data& el2)
    {
        return lessK(k, el2.first);
    }

private:
    bool keyLess(const Data::first_type& k1, const Data::first_type& k2) const
    {
        return k1 < k2;
    }
};

int _tmain(int argc, _TCHAR* argv[])
{
    vector<Data> dataVector;
    // .. do some insertion
    sort(dataVector.begin(), dataVector.end(),Compare());
    // ... do something again
    return 0;
}

This will work as long as same pair is in use, if there is another pair defined, pair<int, string>, Compare class should be re-written.
My question is – is there better approach to come up with ‘general’ class which could cover different cases? One of the approaches was :

    template <typename T, typename T2> class DataCompareTemplate 
    {
        public:
        bool operator()(const T& el1, const T& el2) const 
        {
            return el1.first < el2.first;
        }
    };

As you could see only single case, where T passed as argument is covered and not where 'T::firts_type&' is passed

[PanGalactic]

I think you could define a global templated function operator<():

template <typename T>
bool operator<(const T& a, const T& b)
{
    return a.first < b.first;
}

That should work for any pair type.