Using std::unique_ptr inside a map as a key

Question

I am using Visual Studio 2012. I have a map that looks like this:

std::map,std::unique_ptr

        

Answer 1

Look at the template definition for std::map:

template<
    class Key,
    class T,
    class Compare = std::less<Key>,
    class Allocator = std::allocator<std::pair<const Key, T> >
> class map;

And now lets look at how you try to instantiate it:

std::map<
    std::string, 
    std::map<
        std::unique_ptr<sf::Sound>, 
        std::unique_ptr<sf::SoundBuffer>
    >
> 
listSoundContainer

The problem here is that a std::unique_ptr<sf::Sound> cannot act as a key.

What you seem trying to do is to make some kind of list of std::pair<std::unique_ptr<sf::Sound>, std::unique_ptr<sf::SoundBuffer>>

I would suggest using this instead:

std::map<
    std::string, 
    std::list<
        std::pair<
            std::unique_ptr<sf::Sound>, 
            std::unique_ptr<sf::SoundBuffer>
        >
    >
> 
listSoundContainer

Answer 2

Smart pointers should not be used in combination with STL containers.

The background is that smart pointers do not behave as expected by the STL-containers. For instance, the STL expects source objects of a copy operation to remain unchanged. This is not the case with smart pointers. This can lead to strange effects as you are experiencing...

EDIT: My answer was not fully correct. Since C++11 it is possible to use smart pointers, e.g. unique_ptr, with STL-containers.