Re: c++ question

[Shai Ayal]

On Mon, Oct 12, 2009 at 8:37 AM, Jaroslav Hajek <address@hidden> wrote:
> On Sun, Oct 11, 2009 at 10:55 PM, Shai Ayal <address@hidden> wrote:
>> Hi all,
>>
>> This is a question I came across while trying to work on opengl
>> backend. At first I thought it was too trivial to ask on the list, so
>> I asked Michael privately, but since he agrees that it is strange, I
>> gained some self confidence and so I'm asking on the list :)
>> I am using gcc version 4.3.4 (Debian 4.3.4-5).
>>
>> Any help would be apriciated. The definitions of the opengl_renderer
>> class are in src/gl-render.h
>>
>> Shai
>>
>> On Sat, Oct 10, 2009 at 8:55 PM, Shai Ayal <address@hidden> wrote:
>>> On Sat, Oct 10, 2009 at 10:35 AM, Shai Ayal <address@hidden> wrote:
>>>> Hi Michael,
>>>>
>>>> I'm trying to implement the gl2ps code in the gl-renderer, and I've
>>>> come across a c++ problem:
>>>> As a first step I tried to defined a subclass of opengl_render which
>>>> should override the draw method for a handle. In the new method gl2ps
>>>> will be initialized. This is the definition:
>>>> #include "gl-render.h"
>>>>
>>>> class
>>>> OCTINTERP_API
>>>> glps_renderer : public opengl_renderer
>>>> {
>>>>  public:
>>>>  glps_renderer (const std::string& _filename)
>>>>    : opengl_renderer () , filename (_filename) { }
>>>>
>>>>  ~glps_renderer (void) { }
>>>>
>>>>  void draw (const graphics_object& go);
>>>>
>>>>  private:
>>>>  std::string filename;
>>>>
>>>> };
>>>>
>>>> However, when I try to use this in the code:
>>>>
>>>>        glps_renderer gl2ps (print_filename);
>>>>        gl2ps.draw (gh_manager::lookup (number));
>>>>
>>>> The compiler complains that there is no matching function for call to
>>>> ‘glps_renderer::draw(graphics_handle).
>>>>
>>>> I thought it would automatically use the base class
>>>> opengl_render::draw(graphics_handle) which would then call
>>>> glps_renderer::draw (const graphics_object& go);
>>>>
>>> OK, I think I figured it out :
>>> opengl_renderer *rend = new glps_renderer (print_filename);
>>> rend->draw (gh_manager::lookup (number));
>>>
>>> so it appears the compiler is able to do run time dynamic functions,
>>> but not compile time. Strange.
>>>
>
> No, it would work the same if the methods were non-virtual, it plays
> no role. What you see is a bit unknown C++ feature called "name
> hiding". In short, methods from base class are not automatically
> imported into the subclass namespace if there's an overloaded version
> declared in the subclass. (Trust me, there are good reasons for this).
>
> You can either import it explicitly:
>
> class
> OCTINTERP_API
> glps_renderer : public opengl_renderer
> {
>  public:
>   using opengl_renderer::draw;
>  glps_renderer (const std::string& _filename)
>   : opengl_renderer () , filename (_filename) { }
>
>  ~glps_renderer (void) { }
>
>  void draw (const graphics_object& go);
>
>  private:
>  std::string filename;
>
> };
>
> to the public section of gpls_renderer,
>
> or make the call like this:
>
>  glps_renderer gl2ps (print_filename);
>  gl2ps.opengl_renderer::draw (gh_manager::lookup (number));
>
>
>>> Now for the next problem:
>>> the opengl_renderer::draw function is actually a dispatcher based upon
>>> the object type. The way I implemented the gl2ps renderer, I override
>>> the draw method, do the gl2ps initializations and then call
>>> opengl_renderer::draw.
>>> The problem is I would like to override some specific draw function
>>> (e.g.  virtual void draw (const text::properties& props);), but
>>> instead of the glps_renderer version being called, the base
>>> opengl_renderer version is called. So when calling with a text object
>>> the sequence is:
>>>
>>> opengl_renderer::draw (const graphics_handle& h)
>>> glps_renderer::draw (const graphics_object& go);
>>> opengl_renderer::draw (const graphics_object& go);
>>> opengl_renderer::draw (const text::properties& props)
>>>
>>> although there is a glps_renderer::draw (const text::properties& props)
>>>
>>> Any thoughts?
>>
>
> Please post your code, or a (not working) changeset. I'd like to help,
> but I need more info.
>
Thanks Jaroslav. I'll try to post some code tonight

Shai