Re: problems using classwide types

[mheaney@ni.net (Matthew Heaney)]

In article <33B10EF3.72B4@dcs.gla.NOSPAM.ac.uk>, andy billimore
<billimad@dcs.gla.NOSPAM.ac.uk> wrote:


>procedure Main is
>   L_Check : Check_Type;
>   TP : Transaction_Ptr_Type;
>begin
>   TP := new Check_Type;
>   TP := L_Check; -- Error (see below)
>end Main;

>So I can allocate enough memory to hold an object of Check_Type, but I
>can't assign anything to it which I don't understand.

You don't have a "classwide types" problem, you have an "access types" problem.

The object TP is an access object.  It points to an object of type
Check_Type, but it not itself a Check_Type.

So when you do the assignment 

TP := L_Check;

you are trying to assign an apple (Check_Type) to an orange (a pointer to
Check_Type).

Just do this

TP.all := L_Check;

and you should be fine.

Of course, you can combine steps:

TP : Transaction_Pointer_Type := new Check_Type'(L_Check);

which does heap allocation and initialization ("assignment") all in one
step.  We wouldn't want to create any uninitialized objects, now would we,
hmmmm?

Hope that helps,
Matt

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Matthew Heaney
Software Development Consultant
<mailto:matthew_heaney@acm.org>
(818) 985-1271