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From: tmoran@acm.org
Newsgroups: comp.lang.ada
Subject: Re: How to Emulate C++ Macro with Static Local Variable?
References: <3F73107A.1060502@attbi.com>
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Date: Thu, 25 Sep 2003 19:09:48 GMT
Xref: archiver1.google.com comp.lang.ada:42920
Date: 2003-09-25T19:09:48+00:00
List-Id: <comp.lang.ada>

I remain confused.  I thought the OP wanted something that used storage
that invisibly depended on the static *location* of a call, and that's
why he used separate "static" variables.
> > cache information for each unique call to bar() such that subsequent
> > calls from that location can be processed *much* faster.
          ^^^^^^^^^^^^^^^^^^
I gather that's not the correct interpretation.  If he is willing to
manually specify the particular cache by
  BAR(10);  -- cache number 10
or
  package P10 is new P(10);  -- cache number 10
  package P20 is new P(20);  -- cache number 20
  package P30 is new P(30);  -- cache number 30
or
  Local_Pointer.SP.all(3, Local_Pointer);   -- cache number 3

then why not just:

  type s is record ...
  t : array(1 .. 100) of s;  -- array of caches
  ...
  for i in 0 .. 54 loop
    case wozit is
      when 1 => bar(cache_to_use=>t( 3));
      when 2 => bar(cache_to_use=>t(20));
      when 3 => bar(cache_to_use=>t(14));
    end case;
  end loop;

To somewhat change the example, suppose we want to keep track of how
often different sections of code are run.  The usual method is to
do something like
  type sections is (initialization, processing_input, ...
and sprinkle around statements like
  log(initialization);
  ...
  log(process input);
Suppose that for some reason we don't just want to set a boolean, or
a counter for each section, but we want to raise an exception if any
section is run more than 100 times (maybe we're worried its RAM area
will die of overuse or something ;)  Then in C we could

void log(int *t){if(*t++ > 100)raise_the_exception;}
#define LOG if (1) { \
   static int t_; \
   log(&t_);\
 } else ((void)0)

and then we could sprinkle the statement
  LOG;
around in the program.  How could one implement a similar thing in Ada,
where the "log" call does not need a parameter, but implicitly has one
by virtue of its static location in the code?