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From: Georg Bauhaus <sb463ba@l1-hrz.uni-duisburg.de>
Newsgroups: comp.lang.ada
Subject: Re: In-Out Parameters for functions
Date: Tue, 27 Jan 2004 12:45:40 +0000 (UTC)
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Dmitry A. Kazakov <mailbox@dmitry-kazakov.de> wrote:
: On Tue, 27 Jan 2004 02:26:17 GMT, Hyman Rosen <hyrosen@mail.com>
: wrote:
: 
:>Robert A Duff wrote:
:>> It does seem of some value to attempt to make sure "order doesn't matter".
:>
:>But since this will never be the case in Ada, the next best thing is to
:>define the order.
: 
: There could be also an opposite point of view. Fixing the evaluation
: order you strengthen the "precondition" of the code.

And how does it help, anyway? If functions have side effects,
you have to consider all values affected and affecting the function,
recursively. Is that practial (or, the next best thing)? Example:

#include <iostream>

class Weird {
  int const modulus;
public:
  Weird(int step) : modulus(step) {}
  int state() const;
};

int Weird::state() const {
  extern int global_state;
  return ++global_state % modulus;
}

int global_state = 2;

bool operator==(const Weird& a, const Weird& b) {
  return a.state() == b.state();
}

int main(void)
{
  Weird x(3), y(4);

  const Weird& w1 = x, w2 = y;

  while (w1 == w2) {
    std::cout << "we have reached the predecessors of " << w1.state()
	      << " and " << w2.state() << std::endl;
  }
  return 0;
}