Cannot add a unique_ptr to a std::array

Question

After searching through books and online until the onset of a big pain between my ears, I cannot figure out how to add a std::unique_ptr to a std::array.

The following is a class member:

std::array<std::unique_ptr<Media>, MAX_ELMS_NUM> m_collection;

In the .cpp file I am trying to add a new media pointer stuffed in a std::unique_ptr to the array:

Media* newMedia = CreateNewMedia(Media info stuff);
unique_ptr<Media> np(newMedia);
m_collection[0] = np;

Everything compiles except for the last line.

Answer 1

The last line is an attempt to do a copy assignment operation, which is deleted for std::unique_ptr. You need to use the move-assignment operator:

m_collection[0] = std::move(np);

Answer 2

You can't copy a unique_ptr, full stop. That's what "unique" means - there is only one unique_ptr pointing to the same object.

Now, you can assign a unique_ptr to a different one, but that clears the original one. Since it would be really confusing if a = b modified b, you have to specifically indicate that you want this, with std::move:

m_collection[0] = std::move(np);

After this line, m_collection[0] will point to the Media object, and np will be cleared.