Re: How to Emulate C++ Macro with Static Local Variable?

["Robert I. Eachus" <rieachus@attbi.com>]

Tad Ashlock wrote:

> I've been trying ideas along these lines for a few days, but I haven't
> been able to make them work.  I understand how the procedure would get
> created, but I don't understand where the data is uniquely cached for
> each procedure call. (The C++ implementation above does it in t_->data.)

>>   type Subprogram_Record is record
>>     SP: Subprogram_Pointer;
-- Put whatever else you want right here. Just like in the C++ there has 
  -- to be a local modifiable object of this type to make the call.
>>   end record;

> I'm not following this one at all.  Where is the cached data?

As I said, it goes in the record with the access value.  The caller has 
to declare a local copy to make a call, and that stores the data.  I 
much prefer the more Ada-like abstraction where the user instantiates a 
generic, and the generic has both local data and a (local) instance of 
the subroutine.

> Thank you so much, Robert, for taking the time to reply to my question.

It is an interesting question, and as a result I may actually advocate a 
language change.  If we had the equivalent of access discriminants for 
Ada subprograms, this would be much cleaner.  I think that the right 
level of implementation is an attribute.  So that 
Some_Subprogram'Called_From gets you the called from address.  It is not 
the best solution for this problem, but there may be a way to define it 
to be useful in doing stack tracebacks. (Which argues for the system 
programming annex.)

-- 
                                         Robert I. Eachus

Ryan gunned down the last of his third white wine and told himself it 
would all be over in a few minutes.  One thing he'd learned from 
Operation Beatrix: This field work wasn't for him. --from Red Rabbit by 
Tom Clancy.