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Sender: "Team Ada: Ada Advocacy Issues (83 & 95)" <[log in to unmask]>
Date: Mon, 30 Nov 1998 12:29:35 -0500
Reply-To: "Robert I. Eachus" <[log in to unmask]>
From: "Robert I. Eachus" <[log in to unmask]>
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At 12:44 PM 11/26/98 +0100, Mats Weber wrote:
>Could you please give an example of that (determination of the generic
>actual subprogram cannot be done at compile time) ?

   As I said, at compile time you can determine which text will be
designated, but that textual declaration can result in many instances of
the subprogram which are callable from that point at run-time.  If you
worked with some PL/I implementations, think of it in terms of fat
pointers.  (A fat pointer has a pointer to the executable code, and also to
the context--stack frame plus static pointer--for a particular instance of
that subprogram.)  You can determine the code pointer at compile time, but
the stack pointer part must be filled in at run-time.  Only it is a little
more complex than that.  In general it is the context pointer for the stack
frame containing the instance that must be determined, during the call you
create a new instance which uses that context.  (Most compilers today don't
use static pointers, they store the code and static pointers at a
designated (often negative) offset from the stack pointer.)

    If you want to make up a test program and try it out, what you want to
do is to have a generic formal subprogram with fewer parameters than the
actual it matches.  (This is not the only case, just the easiest to

    Now instantiate the generic in places where the visible subprogram or
subprograms matching the generic formal have different default values for the
"missing" parameters.  As I said, the compiler can figure out at compile
time which subprogram will be called, but the default values have to be
picked up at run-time.  They could even be entered from the terminal if you

    More complex examples involve--again staying within Ada 83--things like
having address values in the context of one of the subprograms, and having
an address clause inside the actual subprogram, which uses that value.
(I've used this in Ada 83 to get run-time polymorphism.)

                                        Robert I. Eachus

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