Question

I got a vector containing pairs. My pairs have template parameters.

std::vector<std::pair<T1, T2> > myVector;

I would like to sort myVector by the pairs second data tag, so by the “value”(T2), not the “key”(T1).
I saw here that i can use this sweet method:

std::sort(myVector.begin(), myVector.end(), mySortingFunc);

and this is my sortFunc:

bool mySortingFunc (std::pair<T1, T2> pair1,   std::pair<T1, T2> pair2){ 
     return (pair1.second<pair2.second); 
}

and its not compiling, throwing me 10 kilometers long error. (i use g++)
Any advice how should i do this?

• E D I T:

Actual code:

template<typename T1, typename T2>
class OrderedMMap
{

std::vector<std::pair<T1, T2> > myVector;

public:

bool sortFunc (std::pair<T1, T2> pair1,   std::pair<T1, T2> pair2) { 

    return (pair1.second<pair2.second); 
}

void sortIt()
{
    std::sort(myVector.begin(), myVector.end(), sortFunc);
}
};

Answer 1

Your sortFunc is a non-static member function. That is the problem. A non-static member function can be invoked only on an object of the class; std::sort cannot do that for you.

The easy fix is to make the function static:

static bool sortFunc (std::pair<T1, T2> pair1,   std::pair<T1, T2> pair2) { 
    return (pair1.second<pair2.second); 
}

With the static keyword, now it became just like regular function, which can be invoked without class instance, which means std::sort will work now.

It would be good if the function accepts the arguments by const reference:

static bool sortFunc(std::pair<T1,T2> const& p1,std::pair<T1,T2> const& p2) 
{ 
    return p1.second < p2.second; 
}

Hope that helps.