Question

I got a list and a vector:

list<int> l;
for(int i=0; i<10; i++)
    l.push_back(i);

vector<int> v;

I want to initialize v using the first 5 elements in l, that is in this case {0,1,2,3,4}.

Besides for loop coupled with v.push_back, any other way? memcpy or copy?

PS: I am not using C++0x/C++11 at present.

Answer 1

You can use std::copy and std::advance (instead of +, for the non-random-access iterator):

#include <list>
#include <vector>
#include <iterator>
#include <algorithm>

int main() {
   std::list<int> l;
   for (int i = 0; i < 10; i++)
       l.push_back(i);

   std::vector<int> v;
   v.reserve(5);

   std::list<int>::iterator start = l.begin(), end = start;
   std::advance(end, 5);
   std::copy(start, end, std::back_inserter(v));

   std::cout << v.size();   // 5
}

The advance is “slow”, though, because of the nature of std::list<>; that’s why op+ is not available for the std::list<>::iterator.