Re: Problems with GLUT on NT and "deeper accessibility"

[Robert A Duff <bobduff@world.std.com>]

Rolf Wester <WESTER@ilt.fhg.de> writes:

> Could you or anyone else tell an Ada novice why the program does'nt work with
> "access" but with "Unchecked_Access"? When I define my own "my_ac_GLfloat" in the
> same way  "ac_GLfloat"  is defined in win32.gl it works with "access".

The accessibility rules are intended to prevent dangling pointers,
unless you use one of the Unchecked_... features.  Mostly, these rules
are checked at compile time, which means they are necessarily
conservative.

You were able to create a pointer of type my_ac_GLfloat using 'Access
because that access type is nested inside the procedure -- it has the
same lifetime as the object, so it's impossible to assign those pointers
into globals.  The type ac_GLfloat, OTOH, is more global -- if you could
use 'Access to make a pointer of type ac_GLfloat pointing to a local,
then when the procedure returns, that pointer could remain -- it's a
dangling pointer.

Note that if you tried to do a type conversion from my_ac_GLfloat to
ac_GLfloat, you would get the same error, for the same reason.

Does glLightfv save that pointer?  If so, you had better not use
'Unchecked_Access -- you better move your data to a global place, or put
it on the heap.  It not, you can use 'Unchecked_Access.

- Bob
-- 
Change robert to bob to get my real email address.  Sorry.