Re: Constructing an object

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Fri, 23 Sep 2005 10:23:02 +0200, Maciej Sobczak wrote:

> Is it really enough? Consider this:
> 
> -- file shapes.ads
> package Shapes is
>     type Color is (Black, White);
>     type Shape is private;   -- or tagged private

Replace it with:

type Shape (<>) is private;

BTW, private vs. tagged private is a huge difference.

>     function New_Shape(C : in Color) return Shape;
> private
>     type Shape is record     -- or tagged
>        C : Color;
>     end record;
> end Shapes;
> 
> -- file shapes.adb
> package body Shapes is
>     function New_Shape(C : in Color) return Shape is
>     begin
>        return Shape'(C => C);  -- I like this syntax :)
>     end New_Shape;
> end Shapes;
> 
> -- file hello.adb
> with Shapes;
> procedure Hello is
>     S : Shapes.Shape;  -- oops, uninitialized Shape
> begin
>     null;
> end Hello;
> 
> It compiles fine (GNAT) and it allows me to declare an uninitialized 
> object of private type Shape. The only thing that private type gives me 
> is that I cannot tinker with its internals directly. This is good in 
> itself, but not enough to save me from having uninitialized objects.

It is enough if the public view is made up so that objects look
unconstrained. This is what (<>) does.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de