moving into unique_ptr container

Question

I have something I want to do that is extremely similar to the code snippet at cppreference.com for unique_ptr. The snippet is produced below. It compiles fine.

#include <iostream>
#include <list>
#include <vector>
#include <string>
#include <iterator>

int main()
{
    std::list<std::string> s{"one", "two", "three"};

    std::vector<std::string> v1(s.begin(), s.end()); // copy

    std::vector<std::string> v2(std::make_move_iterator(s.begin()),
                                std::make_move_iterator(s.end())); // move

    std::cout << "v1 now holds: ";
    for (auto str : v1)
            std::cout << "\"" << str << "\" ";
    std::cout << "\nv2 now holds: ";
    for (auto str : v2)
            std::cout << "\"" << str << "\" ";
    std::cout << "\noriginal list now holds: ";
    for (auto str : s)
            std::cout << "\"" << str << "\" ";
    std::cout << '\n';
}

What I really want is move the strings from s into a vector of unique_ptr .

so something like std::vector<std::unique_ptr<std::string>> v2(&std::make_move_iterator(s.begin()), &std::make_move_iterator(s.end()));

but this of course does not work.

I can only get it to do what I want with this bit of code:

int main()
{
    std::list<std::string> s{"one", "two", "three"};

    std::vector<std::string> v1(s.begin(), s.end()); // copy

    std::vector<std::unique_ptr<std::string>> v2;
    for(auto& o : s)
    {
        std::unique_ptr<std::string> p ( new std::string(move(o)));
        v2.push_back(move(p));
    }

    std::cout << "\nv2 now holds: ";
    for (auto& pstr : v2)
            std::cout << "\"" << *pstr << "\" ";
    std::cout << "\noriginal list now holds: ";
    for (auto str : s)
            std::cout << "\"" << str << "\" ";
    std::cout << '\n';
} 

Is there a way to move resources into a container of unique_ptrs in one line?

Answer 1

Yes if you use a make_unique function as Herb Sutter recommends you can do:

template<typename T, typename ...Args>
std::unique_ptr<T> make_unique( Args&& ...args )
{
    return std::unique_ptr<T> ( new T( std::forward<Args>(args)... ) );
}

int main()
{
    std::list<std::string> s{"one", "two", "three"};

    std::vector<std::unique_ptr<std::string>> v2;
    std::transform(begin(s), end(s), std::back_inserter(v2),
            &make_unique<std::string, std::string&>
    );
}

I have lifted make_unique from Herbs page on the matter, it is included with C++14 or just use this version.

http://herbsutter.com/gotw/_102/

Unfortunately we can't use type deduction so we have to provide the types manually.