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From: zhenglonggen <zhenglonggen@163.net>
Newsgroups: comp.lang.ada
Subject: Re: Access type conversions, how?
Date: Tue, 13 Apr 2004 22:57:12 +0800
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Date: 2004-04-13T22:57:12+08:00

Luke A. Guest wrote:

 >Hi,
 >
 >We have the following in our GL package:
 >
 >package C renames Interfaces.C;
 >...
 >type GLfloat      is new C.C_float;
 >...
 >type GLfloatPtr   is access all GLfloat;
 >...
 >procedure glMultMatrixf(M : in GLfloatPtr);
 >
 >Now, in my code, I've created a Matrix package which has as it's type:
 >
 >type Object is array(Integer range 0 .. 15) of aliased Float;
 >Now, I've made it aliased because I want to pass this to glMultMatrixf,
 >but for some reason I cannot work out exactly how to do it.
 >
 >I've tried all sorts, like:
 >
 >GL.glMultMatrixf(M(M'First)'Unchecked_Access); -- Wrong type
 >
The following code works:


with Ada.Text_Io;
use Ada.Text_Io;
with System;
use System;
with Interfaces.C;
use Interfaces.C;
procedure Test_access2address is
    type GLFloat is new C_Float;
    type GLFloat_Ptr is access all GLFloat;
    type Object is array(0..15) of aliased GLFloat;
    type Object_Ptr is access all Object;
    function Access2address(An_Object_Ptr:Object_Ptr) return GLFloat_Ptr
    is
       A_GLfloat:aliased GLFloat;
       for A_GLFloat'Address use An_Object_Ptr(1)'Address;
    begin
       return A_GLfloat'Unchecked_Access;
    end;
    procedure Test(A_Float:GLFloat_Ptr) is
    begin
       Put_Line("the first element is "&GLfloat'Image(A_Float.all));
    end;
    My_Object:aliased Object;
begin
    My_Object(1):=0.001;
    Test(Access2address(My_Object'Access));
end;