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Date: Fri, 02 May 2003 11:52:05 +0200
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Subject: Re: Generic formal access types
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Date: 2003-05-02T11:52:05+02:00
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tmoran@acm.org wrote:
>>Ok, it is not very useful to have an access type if you do not know its
>>content, but the package implements a list and I am just storing them. I
>>"need" that because I want to return the value "null" from a function in
>>the generic package that returns "Access_Type".
> 
>   What's wrong with
> generic
>     type Element is (<>);

First of all, I wrote this too fast and it is erroneous. It should read:

generic
    type Element (<>) is private;

> package Lists is
>     type Node_Content_Ptr is access Element;

If I declare the access type in the package, I will have a problem with 
the return type of the function:

function Whatever return Node_Content_Ptr;

Because I already have access types defined where I do the instantiation.

declare
    type Ptr is access Integer;
    package Instance is new Generic_Package (Integer);
    P : Ptr;
begin
    P := Instance.Whatever;
end;

Here, P is of type "Ptr" and the function returns "Node_Content_Ptr". 
For the compiler to accept this, I will have to declare Ptr as "access 
all Integer" and do an explicit conversion:

P := Ptr (Instance.Whatever);

Since I cannot change the definition of Ptr, this solution is not valid 
for me... Nice try, though!  :^)

Rodrigo