Re: Best practise inheritance

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

On Tue, 2017-03-21 at 18:27 +0100, S. Jacobi wrote:
> On Mon, 20 Mar 2017 16:01:39 +0000
> Tristan Van Berkom <tristan vanberkom codethink co uk> wrote:
>
> > Use instance private data, this will not need any priv pointer and
> > can be done with th G_DEFINE_TYPE_WITH_PRIVATE() macro, and another
> > to lookup your private data inside your C file (under the hood,
> > this
> > uses negative instance offsets with power nter arithmatic, so
> > public
> > and private data are on the same allocated memory slice)
>
> This is still a bit unclear to me, because I found no good reference
> what instance private data really is, apart from a patch on the
> mailing
> list. So is it just GObject "magic" that puts private fields into
> special memory locations and can therefore retain ABI compatibility?
> Because from what I read, I define my structs the same way.
>
> struct _MyType
> {
>   GtkWidget   parent;
>   MyTypePriv *priv;
> }
> G_DEFINE_TYPE (MyType, my_type, GTK_TYPE_WIDGET)
>
> and
>
> struct _MyType
> {
>   GtkWidget   parent;
>   MyTypePriv *priv;
> }
> G_DEFINE_TYPE_WITH_PRIVATE (MyType, my_type, GTK_TYPE_WIDGET)
>
> would be first a struct with "normal" private data and second a
> struct
> with instance private data. Or am I completely on the wrong track
> here?

This describes a bit of what you're after:

   https://developer.gnome.org/gobject/stable/howto-gobject-code.html

The key points here are that:

  o MyType structure does not need any 'priv' pointer, as it can be
    looked up at any time.
  o G_DEFINE_TYPE_WITH_PRIVATE() expects that a MyTypePrivate structure
    is defined above
  o G_TYPE_INSTANCE_GET_PRIVATE() can be called in your C file to
    access the allocated MyTypePrivate structure for a given instance.

    See: https://developer.gnome.org/gobject/stable/gobject-Type-Inform
ation.html#G-TYPE-INSTANCE-GET-PRIVATE:CAPS

At the end of the day, the main memory slice for an object instance
will be created with space for your MyTypePrivate structure, but the
sizeof() that structure will be computed at runtime so it does not
effect ABI.

This means:
  o No need for allocating an extra pointer to point to private data
  o Less overall memory fragmentation, because private data is
    allocated on the object instance itself.
  o I believe the lookups with G_TYPE_INSTANCE_GET_PRIVATE() are just
    as cheap as the pointer dereference (as it will be implemented
    using simple pointer arithmetic).

Cheers,
    -Tristan