Creating a default deleter for shared_ptr for a specific type

Question

Is there something similar to Well, how does the custom deleter of std::unique_ptr work? for shared_ptrs?

When I try creating a default deleter for smart pointers to ALLEGRO_BITMAP pointers

namespace std {
    template<>
    class default_delete < ALLEGRO_BITMAP > {
    public:
        void operator()(ALLEGRO_BITMAP* ptr) {
            al_destroy_bitmap(ptr);
        }
    };
}

The compiler spits out

1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\memory(696): warning C4150: deletion of pointer to incomplete type 'ALLEGRO_BITMAP'; no destructor called
1>          c:\users\john\allegro\allegro-5.0.10-msvc-11.0\include\allegro5\bitmap.h(12) : see declaration of 'ALLEGRO_BITMAP'
1>          c:\program files (x86)\microsoft visual studio 12.0\vc\include\memory(482) : see reference to function template instantiation 'void std::shared_ptr<ALLEGRO_BITMAP>::_Resetp<_Ux>(_Ux *)' being compiled
1>          with
1>          [
1>              _Ux=ALLEGRO_BITMAP
1>          ]
1>          c:\program files (x86)\microsoft visual studio 12.0\vc\include\memory(482) : see reference to function template instantiation 'void std::shared_ptr<ALLEGRO_BITMAP>::_Resetp<_Ux>(_Ux *)' being compiled
1>          with
1>          [
1>              _Ux=ALLEGRO_BITMAP
1>          ]
1>          c:\users\john\documents\visual studio 2013\projects\blockstacking\blockstacking\systems.cpp(190) : see reference to function template instantiation 'std::shared_ptr<ALLEGRO_BITMAP>::shared_ptr<ALLEGRO_BITMAP>(_Ux *)' being compiled
1>          with
1>          [
1>              _Ux=ALLEGRO_BITMAP
1>          ]
1>          c:\users\john\documents\visual studio 2013\projects\blockstacking\blockstacking\systems.cpp(190) : see reference to function template instantiation 'std::shared_ptr<ALLEGRO_BITMAP>::shared_ptr<ALLEGRO_BITMAP>(_Ux *)' being compiled
1>          with
1>          [
1>              _Ux=ALLEGRO_BITMAP
1>          ]
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\memory(159): warning C4150: deletion of pointer to incomplete type 'ALLEGRO_BITMAP'; no destructor called
1>          c:\users\john\allegro\allegro-5.0.10-msvc-11.0\include\allegro5\bitmap.h(12) : see declaration of 'ALLEGRO_BITMAP'
1>          c:\program files (x86)\microsoft visual studio 12.0\vc\include\memory(158) : while compiling class template member function 'void std::_Ref_count<_Ux>::_Destroy(void)'
1>          with
1>          [
1>              _Ux=ALLEGRO_BITMAP
1>          ]
1>          c:\program files (x86)\microsoft visual studio 12.0\vc\include\memory(694) : see reference to class template instantiation 'std::_Ref_count<_Ux>' being compiled
1>          with
1>          [
1>              _Ux=ALLEGRO_BITMAP
1>          ]
1>          c:\program files (x86)\microsoft visual studio 12.0\vc\include\memory(482) : see reference to function template instantiation 'void std::shared_ptr<ALLEGRO_BITMAP>::_Resetp<_Ux>(_Ux *)' being compiled
1>          with
1>          [
1>              _Ux=ALLEGRO_BITMAP
1>          ]
1>          c:\program files (x86)\microsoft visual studio 12.0\vc\include\memory(482) : see reference to function template instantiation 'void std::shared_ptr<ALLEGRO_BITMAP>::_Resetp<_Ux>(_Ux *)' being compiled
1>          with
1>          [
1>              _Ux=ALLEGRO_BITMAP
1>          ]

on the line

shared_ptr<ALLEGRO_BITMAP> black(al_create_bitmap(groundWidth, TILESIZE));

I realize I can pass the deleter to the shared_ptr, but I was wondering if there was a way to create a default deleter so I don't have to keep passing in the same function every time I create a new bitmap.

Thanks.

Answer 1

If you can enforce a convention to always allocate it via std::make_unique then this will work:

std::shared_ptr<ALLEGRO_BITMAP> bmp;
...
bmp = std::make_unique<ALLEGRO_BITMAP>();

The deleter will move with the object, and therefore default_delete will be called upon object destruction. Unfortunately, the compiler can't enforce this convention; not without help from some custom types.