Xref: news2.mv.net comp.os.msdos.djgpp:4962
From: Mark Habersack <grendel AT ananke DOT amu DOT edu DOT pl>
Newsgroups: comp.os.msdos.djgpp
Subject: Is post-increment a pre-increment?
Date: Wed, 12 Jun 1996 21:39:52 +0200
Organization: Adam Mickiewicz University
Lines: 54
Message-ID: <Pine.NEB.3.93.960612212436.3438B-100000@ananke.amu.edu.pl>
NNTP-Posting-Host: ananke.amu.edu.pl
Mime-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp


Hi,
  Today, debugging some code, I have found strange behaviour of the
post-increment operator. Maybe it is a bug (or "hidden feature" how Microsoft
would describe it) and maybe just a side effect which I don't understand.
Anyway, here's what was going on:

I was writing the code using TWS library and wanted to calculate the width
in pixels of the string with embedded formatting codes (skipping them of
course). To achieve this I had to add the width of every individual character 
to the total value. The algorithm presented here is not entirely what I used
in my code, but illustrates the problem. I used the following approach (the
code was in C++ module):

int     len = 0;
char*   walker = text; // Text is a char* argument passed to the func

while(*walker)
  switch(*walker)
  {
    case '%':
      switch(*(++walker))
      {
        .
        . // Do the processing of formatting codes
        .
      }
 
    default:
      len += CharWidth(*walker++, curFont); // Here, the walker variable has
                                              // been everytime incremented
                                              // TWICE, not ONCE as it was
                                              // meant to be - what is wrong?
   }

 Later on I changed the offending line to
   len += CharWidth(*walker, curFont);
   walker++;
 and everything worked just fine. Where lies the error - is it my ignorance or
a bug or side effect?

N.B. Composing this message a thought has crossed my mind: maybe it is a case
of operator precedence - I always keep forgetting about this stuff? If so I'm
just dumb - forgive me!

  Greetings, Mark

/*******************************/
/** So here I am once more... **************************************/
/** When you grown up and leave your playground, where you kissed **/
/** your Prince and found your Frog - remember the Jester that    **/
/** showed you tears, a Script for Tears... ************************/
/*********************************************/