Why does a GTK_ARG_CONSTRUCT arg have to be readable?

[Havoc Pennington <rhp zirx pair com>]

Hi,

In gtkobject.c, the debug checks include:

if (arg_flags & GTK_ARG_CONSTRUCT)
  g_return_if_fail ((arg_flags & GTK_ARG_READWRITE) == GTK_ARG_READWRITE);
else
  g_return_if_fail ((arg_flags & GTK_ARG_READWRITE) != 0);
if (arg_flags & GTK_ARG_CONSTRUCT_ONLY)
  g_return_if_fail ((arg_flags & GTK_ARG_WRITABLE) == GTK_ARG_WRITABLE);

It makes sense to me that an argument has to be either readable or
writable, and that a construct argument has to be writable, but why 
does a construct argument have to be readable? And why does a
CONSTRUCT_ONLY not have to be readable? The only difference I see between
them is that you can't use CONSTRUCT_ONLY post-construction.

Curious,
Havoc