Using std::unique_ptr inside a map as a key

问题:

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

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

I'm trying to insert data like this:

std::unique_ptr<sf::SoundBuffer> soundBuffer(new sf::SoundBuffer());
if (soundBuffer->loadFromFile("assets/sound/" + _fileName) != false)
{
    std::unique_ptr<sf::Sound> sound(new sf::Sound(*soundBuffer));
    typedef std::map<std::unique_ptr<sf::Sound>, std::unique_ptr<sf::SoundBuffer>> innerMap;
    listSoundContainer[_fileName].insert(innerMap::value_type(std::move(sound), std::move(soundBuffer)));               
}

and im getting the following error at compile time:

microsoft visual studio 11.0\vc\include\utility(182): error C2248: 'std::unique_ptr<_Ty>::unique_ptr' : cannot access private member declared in class 'std::unique_ptr<_Ty>' 1> with 1>
[ 1> _Ty=sf::Sound 1> ] 1> c:\program files (x86)\microsoft visual studio 11.0\vc\include\memory(1447) : see declaration of 'std::unique_ptr<_Ty>::unique_ptr' 1> with 1> [ 1> _Ty=sf::Sound 1> ] 1> c:\program files (x86)\microsoft visual studio 11.0\vc\include\xmemory0(617) : see reference to function template instantiation 'std::pair<_Ty1,_Ty2>::pair(std::pair<_Ty1,_Ty2> &&,void **)' being compiled 1> with 1> [ 1>
_Ty1=const std::unique_ptr, 1> _Ty2=std::unique_ptr, 1> _Kty=std::unique_ptr, 1> _Ty=std::unique_ptr 1> ]

I have also tried to insert data using make_pair with the same problem. What am I missing? Ive been trying to solve this problem for 2 hours now and can't get my head around it.

I can actually solve this problem by NOT using smart pointers:

sf::SoundBuffer* soundbuffer = new sf::SoundBuffer();
soundbuffer->loadFromFile(_file);
sf::Sound* sound = new sf::Sound(*soundbuffer);
typedef std::map<sf::SoundBuffer*, sf::Sound*> mapType;
listSound[_file].insert(mapType::value_type(soundbuffer, sound));

回答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

回答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.