Using custom deleter with std::shared_ptr

Question

I'm trying to work out how to use std::shared_ptr with a custom deleter. Specifically, I'm using it with SDL_Surface as:

std::shared_ptr<SDL_Surface>(SDL_LoadBMP(....),SDL_FreeSurface);

which compiles and runs fine. However, I would like to try out my own deleter and cannot work out how to do so. The documentation for SDL_FreeSurface is found here:

http://sdl.beuc.net/sdl.wiki/SDL_FreeSurface

in which I find the SDL_FreeSurface is declared as:

void SDL_FreeSurface(SDL_Surface* surface);

As a test, and going by that information, I tried the following function:

void DeleteSurface(SDL_Surface* surface)
{
    std::cout << "Deleting surface\n";
    SDL_FreeSurface(surface);
}

However, compiling with g++ gives me the following error:

error: no matching function for call to 'std::shared_ptr<SDL_Surface>::shared_ptr(SDL_Surface*, <unresolved overloaded function type>)'

I have looked at the gnu documentation for the gcc std::shared_ptr implementation but cannot make much sense of it. What am I doing wrong?

EDIT: I've since narrowed down the problem, but will leave the original question above. What I had was a Game class which, if I strip it down to a basic implementation, was something like:

class Game {
    public:
        /* various functions */
    private:
        void DeleteSurface(SDL_Surface* surface);
        bool CacheImages();
        std::vector<std::shared_ptr<SDL_Surface> > mCachedImages;

        /* various member variables and other functions */
}

with the implementation of DeleteSurface as above, and the implementation of CacheImages() as:

bool CacheImages()
{
    mCachedImages.push_back(std::shared_ptr<SDL_Surface>(SDL_LoadBMP(...),DeleteSurface);
    return true;
}

which game me the error I listed above. However, if I move the DeleteSurface() function outside the Game class without otherwise altering it, the code compiles. What is it about including the DeleteSurface function in the Game class that is causing problems?

Answer 1

std::shared_ptr<SDL_Surface>(SDL_LoadBMP(....), [=](SDL_Surface* surface)
{
    std::cout << "Deleting surface\n";
    SDL_FreeSurface(surface);
});

or

void DeleteSurface(SDL_Surface* surface)
{
    std::cout << "Deleting surface\n";
    SDL_FreeSurface(surface);
}

std::shared_ptr<SDL_Surface>(SDL_LoadBMP(....), DeleteSurface);

EDIT:

Seeing your updated question, DeleteSurface should be a non-member function, otherwise you need to use std::bind or std::mem_fn or some other member function pointer adapter.

Answer 2

This code provides an example of a shared pointer construction with the deleter as an object method. It display the std::bind instruction to use.

The example is a simple object recycler. When the last reference to the object is destroyed, the object is returned to the free object pool inside the recycler.

The recyler can be easily changed into an object cache by adding a key to the get() and add() methods and by storing the objects in a std::map.

class ObjRecycler
{
private:
    std::vector<Obj*> freeObjPool;
public:
    ~ObjRecycler()
    {
        for (auto o: freeObjPool)
            delete o;
    }

    void add(Obj *o)
    {
        if (o)
            freeObjPool.push_back(o);
    }

    std::shared_ptr<Obj> get()
    {
        Obj* o;
        if (freeObjPool.empty())
            o = new Obj();
        else
        {
            o = freeObjPool.back();
            freeObjPool.pop_back();
        }
        return std::shared_ptr<Obj>(o, 
             std::bind(&ObjRecycler::add, this, std::placeholders::_1));
    }
}