Deleting objects pointed to by smart pointers

Question

In my code I have a SoundManager class, which contains and operates on all the sounds for my game. This class needs to be instantiated and it's methods called by multiple other classes. However I wish for there only to be one set of sounds occupying memory, so in the interest of efficiency all of the assets are declared as static shared_ptrs.

#include "SoundManager.h"

static shared_ptr<ISoundEngine> sEngine;

static shared_ptr<ISoundSource> hoverSound;
static shared_ptr<ISoundSource> confirmSound;
static shared_ptr<ISoundSource> mainBGM;
static shared_ptr<ISound> bgmInterface;

SoundManager::SoundManager(void)
{


//first we need to create the irrKlang sound engine instance
    if(!sEngine)
    {
        sEngine.reset(createIrrKlangDevice());
    }


    if(!hoverSound)hoverSound.reset(sEngine->addSoundSourceFromFile("Sounds/ButtonHover.mp3"));
    if(!confirmSound)confirmSound.reset(sEngine->addSoundSourceFromFile("Sounds/ButtonConfirm.mp3"));
    if(!mainBGM)mainBGM.reset(sEngine->addSoundSourceFromFile("Sounds/mainBGM.mp3"));


    //set some default volumes
    hoverSound->setDefaultVolume(1.0f);
    confirmSound->setDefaultVolume(0.4f);
    mainBGM->setDefaultVolume(0.5f);


}


SoundManager::~SoundManager(void)
{   
}

This SoundManager is instantiated in my main() function and every time i need to load the title-screen (SoundManager is instantiated in this titlescreen class too). Initialising and destroying the title-screen over and over do not cause a problem. The static shared_ptrs objects are not destroyed as they are still in use by the main functions instance of SoundManager.

Now this all works fine in practice for running my game. However when it comes to exiting cleanly, when the static objects above are dismantled, unhandled runtime exceptions (access violations) are thrown at me. With VS2012's debugger pointing me to a line in memory.h.

private:
    virtual void _Destroy()
        {   // destroy managed resource
        delete _Ptr;       <<<<<<<<<The debugger points to this line
        }

I was to understand that similar to obj-c, c++ shared_ptrs use reference counters to ensure that the objects aren't deleted until no object exists that require their use any more. I don't understand what could be causing these errors.

Maybe an important part I shouldn't omit: My game is exited via a call to exit(0); as close to the main() function as possible. I haven't taken any sort of action to cleanup the SoundManagers members before doing this, as I thought the shared_ptr's handled this.

Does anybody know what could be causing my cleaning up problem?

Answer 1

If you want to manually free a resource used by a shared_ptr you need to call reset . As for using static shared_ptr I don't think I get the reasoning. The whole point is that they do not copy of the resource around, so you will only have one resource.

Answer 2

Your are using the IRRKLang library. This library comes as a precompiled binary (dll if you are on windows). This library makes itself binary compatible by using pure virtual bases. This works, but you cannot delete an object for a library like that, because your program new/delete are different from the new/delete of the library. Those type of libraries provide a way to deallocate the memory, this this case drop.

To use shared_ptr etc you have to use a custom deleter. Take a look at Using custom deleter with std::shared_ptr to see how to do it and modify it to your own needs.

In your case since you are using Visual Studio 2012 you can probably do something like this

template<class T>
struct IrrDeleter{
   void operator()(T* t){
       t->drop();
   }
};

then change all your reset lines to include the deleter, for example

sEngine.reset(createIrrKlangDevice(),IrrDeleter<ISoundEngine>());