Re: How to get around "access type must not be outside generic unit"

[Jean-Pierre Rosen <rosen@adalog.fr>]

Brian May a �crit :
>>>>>> "Pascal" == Pascal Obry <pascal@obry.net> writes:
> 
>     Pascal> Solution 2) Move access to generic spec
> 
> ...
> 
>     Pascal> begin Register ("Doit", Install_Access); end Usage;
> 
> How is this any different from using Install'Access?
> 
> I would have thought since Install was defined within the generic
> unit, Install'Access would be OK.
> 
> Or does Install'Access imply a temporary local variable?
It's all about the contract model. When you instantiate a generic, you 
are supposed to "see" the specification, but not the body. Hence, 
declarations in the specification "assume the best", i.e. anything is 
allowed, assuming that if certain constructs are illegal due to the 
properties of the actual parameters, the instantiation will be rejected.

Since you are not supposed to see the body, this is not possible for 
declarations in the body, hence the body "assumes the worst": anything 
that could be illegal *is* illegal. Therefore, moving the offending 
declaration from body to spec can solve the problem.

> Put another way, how can you create a dangling pointer with
> Install'Access but not Install_Access?
> 
Just instantiate that generic inside a procedure.
Install becomes a local function of that procedure, but you would keep a 
'access on it in a global structure. Taking the 'access in the 
specification is illegal, and (as explained above), the instantiation 
would be illegal.
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            J-P. Rosen (rosen@adalog.fr)
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