Sorting a Vector with std::sort

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);
}
};

Post the actual code and the error messages generated by the compiler. — Sarfaraz Nawaz
– Sarfaraz Nawaz, Commented Jan 27, 2013 at 13:01
T2 is appareantly std::string so i guess its not important YET. Something else is the problem. — Adam Varhegyi
– Adam Varhegyi, Commented Jan 27, 2013 at 13:03
@AdamVarhegyi: Why don't you post the actual code? Your question has no clue to the problem. — Sarfaraz Nawaz
– Sarfaraz Nawaz, Commented Jan 27, 2013 at 13:04

Sarfaraz Nawaz · Accepted Answer · 2013-01-27 13:14:00Z

4

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.

edited Jan 27, 2013 at 13:14

answered Jan 27, 2013 at 13:08

Sarfaraz Nawaz

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1 Comment

Adam Varhegyi Over a year ago

I just wanted to tell the fact that you told me in one keyword "static", g++ told me in a 3 page error message :)

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