[gtk/jjardon/no_G_TYPE_INSTANCE_GET_PRIVATE] gtk/gtktoolbutton.c: Document why we need to still use G_TYPE_INSTANCE_GET_PRIVATE

[Javier Jardón Cabezas <jjardon src gnome org>]

commit ea7128907b18b66b1ddeb3e5c7547792f636cf6d
Author: Javier Jardón <jjardon gnome org>
Date:   Wed Nov 7 17:27:00 2018 +0000

    gtk/gtktoolbutton.c: Document why we need to still use G_TYPE_INSTANCE_GET_PRIVATE
    
    This is the excellent explanation from Emmanuele at
    https://gitlab.gnome.org/GNOME/gtk/merge_requests/402#note_361210:
    "
    Every time you instantiate a type, the instance_init() function is called for each
    parent type T_p of your type T; to preserve invariants, the class pointer inside
    the instance data is set to the parent type before each invocation, until you hit
    your type T. This means that calling GET_CLASS() inside an instance_init() function
    will give you a pointer to the class vtable for the parent type T_p while you're
    iterating over parent types. What if you want to access the actual class vtable of
    the type T, though? Well, you can because the actual signature of instance_init() is:
    
      void (* GInstanceInitFunc) (GTypeInstance *instance, gpointer g_class);
    
    i.e. all instance_init() functions get passed the instance they are initialising
    and the class vtable of the real type you're instantiating.
    
    This is how GtkToolButton works: it "peeks ahead" at instance initialisation time,
    to use the button_type class field of the actual type you're instantiating,
    and calls g_object_new() with it to store the resulting object in its own private
    data structure.
    
    This whole contrived mechanism is needed to allow out-of-tree tool buttons to just
    set the button type on their class init, and have their parent class create the
    button they want, instead of asking all tool buttons to do this themselves and have
    a virtual function called get_button() for GtkToolButton to call whenever it needs
    to operate on the button instance.
    
    Now we're coming to a close: we cannot use the G_DEFINE_TYPE macro because the
    instance_init() function it creates internally will not pass the class pointer
    to your custom instance_init(). Since we cannot use G_DEFINE_TYPE, we also cannot use
    G_ADD_PRIVATE either.
    
    This is the reason why, when I ported GTK 3 to the new private instance data structure
    macros, I left GtkToolButton alone. I should have left a comment there, because @matthiasc
    tried doing that as well, and then had to revert it in commit 1c4a7bd5. So: my bad,
    sorry about that.
    
    If we want to drop the G_TYPE_INSTANCE_GET_PRIVATE and the g_type_class_add_private() calls,
    we cannot use G_DEFINE_TYPE, but what we can do is unrolling what the macros do themselves:
    
    - add a global GtkToolButton_private_offset variable
    - add a static inline gtk_tool_button_get_instance_private() that does return
    (G_STRUCT_MEMBER_P (self, GtkToolButton_private_offset));
    - call g_type_add_instance_private (g_define_type_id, sizeof (GtkToolButtonPrivate)) inside
    gtk_tool_button_get_type() and store the result in GtkToolButton_private_offset
    - replace g_type_class_add_private() inside gtk_tool_button_class_init() with
    g_type_class_adjust_private_offset (klass, &GtkToolButton_private_offset)
    "

 gtk/gtktoolbutton.c | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/gtk/gtktoolbutton.c b/gtk/gtktoolbutton.c
index e7571b88c2..350a91e72e 100644
--- a/gtk/gtktoolbutton.c
+++ b/gtk/gtktoolbutton.c
@@ -279,6 +279,10 @@ gtk_tool_button_init (GtkToolButton      *button,
 {
   GtkToolItem *toolitem = GTK_TOOL_ITEM (button);
 
+  # We still need to use G_TYPE_INSTANCE_GET_PRIVATE() because GtkToolButton
+  # need to access the class pointer inside instance_init
+  # See an detailed explanation of this at
+  # https://gitlab.gnome.org/GNOME/gtk/merge_requests/402#note_361210
   button->priv = G_TYPE_INSTANCE_GET_PRIVATE (button,
                                               GTK_TYPE_TOOL_BUTTON,
                                               GtkToolButtonPrivate);