emplace unique_ptr in list

Question

I am trying to emplace an std::unique_ptr into a std::list, but it's failing with the following error:

<source>:29:23: error: no matching function for call to 'std::__cxx11::list<std::unique_ptr<test> >::emplace(std::remove_reference<std::unique_ptr<test>&>::type)'
   29 |     mapList[1].emplace(std::move(t));

Following is the code:

using namespace std;

class test {
public:
    int i;
};


typedef std::list<std::unique_ptr<test>> testList;
int main() {
    unordered_map<int, testList> mapList;
    mapList.try_emplace(1, testList());
    std::unique_ptr<test> t = make_unique<test>();
    mapList[1].emplace(std::move(t));

  return 0;
}

Try it here: https://godbolt.org/z/Wq4hPrzPj

Answer 1

The error is quite explicit: There is no matching call for std::list<>::emplace() with the arguments you provided.

Indeed, if we look carefully at the documentation, we notice that std::list<>::emplace() takes a position as first argument (given through a std::list<>::const_iterator).

In your case, since the list is empty, what you probably wanted was either std::emplace_back() or std::emplace_front().

When in doubt, always refer to a good documentation.

---

Sidenote: You should really get rid of that bad using namespace std; habit.
You can see more information on the topic here: Why is "using namespace std;" considered bad practice?.