Move vector<unique_ptr<Derived>> to a vector<unique_ptr<Base>>

Question

I have two classes, say Base and Derived: public Base and two std::vectors, one holding elements of type unique_ptr<Base> and other one holds elements of type unique_ptr<Derived> How can I transfer ownership of all elements from second vector into first one? I already tried:

vector<unique_ptr<Base>> v1;
vector<unique_ptr<Derived>> v2;
// do something
std::move(v2.begin(),v2.end(),v1.begin()); // This line crashed
v1 = std::move(v2); // And this gives a compiler error

Answer 1

You need to make sure v1 is the proper size before moving into it. For example:

#include <algorithm>
#include <vector>
#include <memory>

using std::vector;
using std::unique_ptr;

struct Base {
};

struct Derived : Base {
};

int main()
{
  vector<unique_ptr<Base>> v1;
  vector<unique_ptr<Derived>> v2;
  v2.push_back(unique_ptr<Derived>(new Derived));
  v2.push_back(unique_ptr<Derived>(new Derived));
  v2.push_back(unique_ptr<Derived>(new Derived));
  v1.resize(v2.size());
  std::move(v2.begin(),v2.end(),v1.begin());
}

This is because the move algorithm doesn't change the size of the container itself. It is implemented like this (taken from http://en.cppreference.com/w/cpp/algorithm/move):

template<class InputIt, class OutputIt>
OutputIt move(InputIt first, InputIt last, OutputIt d_first)
{
    while (first != last) {
        *d_first++ = std::move(*first++);
    }
    return d_first;
}