On Sun, Jun 16, 2019 at 7:30 AM Zach Laine via Boost-users <boost-users@lists.boost.org> wrote:
>
> Dear Boost community,
>
> The formal review of JeanHeyd Meneide's out_ptr library starts Sunday, June 16th
> and ends on Wednesday, June 26th.

Not yet a review, more like initial thoughts and questions.

I've always used shared_ptr to wrap all kinds of "handles" to objects, including IUnknown and HANDLE on Windows, but even things like OpenGL unsigned int handles. It would be really nice to support such non-pointer handle types, so my initial reaction is that out_ptr doesn't go far enough.

To clarify what I mean by non-pointer handle types, consider that it makes sense to keep an OpenGL shader alive by a shared_ptr<unsigned const>, even though the OpenGL handle itself is an unsigned int, not a pointer. To this end I use the following helper function make_handle, which takes a non-pointer handle and a deleter function, and returns a shared_ptr:

namespace handle_detail
{
  template <class H, class D>
  struct deleter
  {
    deleter( H h, D d ):
      h_(h),
      d_(d)
    {
    }

    void operator()( H * h )
    {
      assert(!h);
      (void) d_(h_);
    }

    H h_;
    D d_;
  };
}

template <class H,class D>
std::shared_ptr<H const> make_handle( H h, D d )
{
  std::shared_ptr<H> p((H *)0,handle_detail::deleter<H,D>(h,d));
  return std::shared_ptr<H const>(
    p,
    &std::get_deleter<handle_detail::deleter<H,D> >(p)->h_ );
}

Then I write a wrapper function for glCreateShader:

std::shared_ptr<unsigned const> CreateShader( unsigned shaderType )
{
  assert(!glGetError());
  if( unsigned s=glCreateShader(shaderType) )
  {
    if( unsigned err=glGetError_() )
      throw ....;
    return make_handle(s,glDeleteShader);
  }
  else
    throw zero_handle_detected();
}

Which in the end is used like this:

std::shared_ptr<unsigned const> sh = CreateShader(GL_VERTEX_SHADER);