Re: Best practise inheritance

[Tristan Van Berkom <tristan vanberkom codethink co uk>]

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