On Mon, 2017-03-20 at 16:36 +0100, S. Jacobi wrote:
> First of all, inheritance may be the wrong word here in plain c, but
> I
> don't know how else to name it.
Sorry replied to this from my phone and missed some things...
> In different projects I see different approaches how to derive custom
> widgets from existing ones. I can roughly group them into 2 to 3.
>
> 1) The header only has a typedef to make the struct opaque. All
> variables needed are put into the struct in the .c file.
>
> myType.h
> typedef struct _MyType MyType;
>
> myType.c
> struct _MyType
> {
> GtkWidget *parent;
> /* additions */
> guint i;
> ...
> };
Note also that this approach imposes that absolutely nothing can derive
from MyType later, because MyType becomes anonymous.
>
> 2a) The header defines a private struct, and all variables needed are
> put into this private struct.
>
> myType.h
> typedef struct _MyTypePriv MyTypePriv;
> typedef struct _MyType MyType;
>
> myType.c
> struct _MyTypePriv
> {
> GtkWidget *parent;
> /* additions */
> guint i;
> };
>
> struct _MyType
> {
> MyTypePriv *priv;
> };
Actually this approach (2a) I've never seen before, not a good idea,
but possibly works (except for the other detail below)...
> 2b) Similar to 2a, but the parent is put in the "main" struct, not
> the
> private part.
>
> myType.h
> typedef struct _MyTypePriv MyTypePriv;
> typedef struct _MyType MyType;
>
> myType.c
> struct _MyTypePriv
> {
> /* additions */
> guint i;
> };
>
> struct _MyType
> {
> GtkWidget *parent
> MyTypePriv *priv;
> };
So in all of these cases, you have missed an important detail:
struct _MyType {
GtkWidget *parent_instance;
... anything else...
}
Is wrong, as it will only reserve one pointer size for the parent
structure, what you have been seeing is always in fact:
struct _MyType {
GtkWidget parent_instance
... anything else ...
}
The parent instance must be known and not anonymous, you need to
include the struct itself as the first member of your derived type, not
merely a pointer to that.
Cheers,
-Tristan
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