Fastest way to convert from vector of pairs to two independent vectors in C++

Question

lets say I have a vector of pair<int,int>. Now I want to extract the pair.first and pair.second as independent vectors. I can iterate on the vector and do that but is there a better/faster way?

Answer 1

In C++11, if you don't need the old vector anymore, you could perhaps get a tiny bit of extra efficiency from move semantics:

for (auto it = std::make_move_iterator(v.begin()),
         end = std::make_move_iterator(v.end()); it != end; ++it)
{
    v1.push_back(std::move(it->first));
    v2.push_back(std::move(it->second));
}

Other than that, you certainly cannot do better than one loop. You will have to touch every element at least once, so this is as efficient as it gets.

Note that moving can only make a difference if the element types themselves have move semantics that are better than copying. This is not the case for ints, or any PODs. But it can't hurt to write your code generically so that you can take advantage of this in future situations.

If copying/moving is a problem, though, you should consider whether some view adapter for your original vector might be a better approach.

Answer 2

No there is not. The one thing to take care of is to use reserve on the two resulting vectors to prevent unnecessary reallocations.