how copy the first elements of pair in vector to another vector?

Question

I have a std::vector with element of the type std::pair. With some algorithm, I return two iterators (range) so I would like to pick up all elements within that range and copy t

Answer 1

It's because of the operator precedence. The select operator . has higher precedence than the dereference operator *.

So what the compiler thinks you're writing is

*(it1.first)

when you mean

(*it1).first

Answer 2

If you are after an algorithm to do this for you, you can use the four parameter overload of std::transform:

#include <algorithm> // for transform
#include <iterator>  // for back_inserted and distance

....
std::vector< pair<double, int> > data;
std::vector<double> data2;
data2.reserve(std::distance(it1, it2));
std::transform(it1, 
               it2, 
               std::back_inserter(data2), 
               [](const std::pair<double, int>& p){return p.first;});

If you don't have C++11 support, you can use a function instead of the lambda expression:

double foo(const std::pair<double, int>& p) { return p.first; }

std::transform(it1, 
               it2, 
               std::back_inserter(data2),
               foo);