Re: gpointer question

[Harring Figueiredo <harringf yahoo com>]

 Since you are dealing with a gpointer (void *) ,you really do not need to know
what the pointer is pointing to.

 If I understand what you are trying to do, you will need to create an id to
every possible struct that you have, that way you can identify what the
gpointer points to based on this id.

  There are 2 ways I can come up with:

/* each type. maps to a valid struct type */
typedef enum { TYPE_A, TYPE_B, TYPE_C } SomeType;

typedef struct {
        int  model_id;
        int  active;
        int  units;
        typGeometry  geodata;
        gpointer  modeldata;
        SomeType type;
         
 } typModel;

  Now you would create an interface to set the modeldata.. 

  set_modeldata(typModel * m, SomeType type; gpointer someValidStruct){
    m -> type = type;
    modeldata = someUnkownStruct.
  } 

  Now, you could cast to the approprite type, some palce else:

  switch(m->type){
    case  TYPE_A:
        struct Type_A_Struct* t = m->modelType;
         do_whaterver_with_it(t);
    ....
  }


  I hope you get the gist.

 The second way is a bit more envolving. It  requires a comom base object
(struct) that is the first member of all other structs and do it like GTK does
with Gobject/GtkWidget and so on, -- in onther words -- inheritance.

  Take a look at how GtkButton inherits from GtkWidget and you will see what I
mean.

  Hope this helps.

Harring


 
--- Jim Parker <hopeye cfl rr com> wrote:
> G'Day !
>
> I am writing a CAD application.  The 3-D modeler part is very generic, 
> so it can be used for any engineering displine, called modes.
>
> The system data is contained in list of models.  Each model structure 
> has pointers to parts, layers, geometric data etc.  But I also need to 
> add a struct containing mode specific data.  For example a ship would 
> have a length, beam, draft, etc., but this information has no meaning 
> for lets say a dam in a civil engineering model.
>
>  From what I have read, it looks like I can use a gpointer to add the 
> data to my struct.  Unfortunately, I really do not understand how (self 
> taught, and not covered in my C books).
>
> Here is an simplified example of what I would like to try:
>
> typedef struct {
>       int  model_id;
>       int  active;
>       int  units;
>       typGeometry  geodata;
>       gpointer  modeldata;
> } typModel;
>
> typdef struct {
>       float length;
>       float lwl;
>       float beam;
>       float draft;
>       float depth;
> } typShip;
>
> So I want to get the gpointer in the typModel struct to point to the 
> typShip struct.  The trick is that I do not
> know the size of the struct pointed to until the assignemt at runtime 
> (a designer may use the typMachine struct).
>
> Any help would be appreciated.
>
> Thanks.
>
> cheers,
> Jim Parker
>        
>
>   
> _______________________________________________
> gtk-app-devel-list mailing list
> gtk-app-devel-list gnome org
> http://mail.gnome.org/mailman/listinfo/gtk-app-devel-list


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