C++ std::transform vector of pairs->first to new vector

Question

Sorry for a little bit beginner question. There are vector and vector of pairs

typedef std::vector  TItems;
typedef std::vector < std::pair <         


        

Answer 1

First of all, you should use a back_inserter as the third argument to transform so that the transformed values are pushed to the back of the vector.

Second, you need some sort of functor which takes a pair of ints and returns the first one. This should do:

int firstElement( const std::pair &p ) {
    return p.first;
}

Now, to put the pieces together:

TPairs pairs;
pairs.push_back( std::make_pair( 1, 3 ) );
pairs.push_back( std::make_pair( 5, 7 ) );

TItems items;
std::transform( pairs.begin(), pairs.end(), std::back_inserter( items ),
                firstElement );

After this code, items contains 1 and 5.