Subj : Re: A really stupid Question - how to build a library
To   : borland.public.cpp.borlandcpp
From : Ed Mulroy [TeamB]
Date : Thu Oct 07 2004 09:18 am

Note that I assumed from your message that you were creating code for 
use by students.  Upon re-reading the message I am not sure how I 
formed that opinion.  I have made more than a few bucks teaching 
professors C++ and C who were assigned to teach language classes but 
were only a few pages ahead (sometimes behind!) the students and 
wanted to express caution about some things.

> Machines are fast.  The interrupt rate is 16K ...

You appear to have that issue well in hand.

> ... I now have to go read about #pragma,
> post and pre increment/decrement,
> figure out how to use decrement given
> that my mind set is to make the
> variable change after the fact rather than before...., etc.

#pragma :

This is a method for giving the compiler a command, analagous to a 
command line option but in the code.

For example:
#pragma startup function1

will cause function1 to be added to the list of functions to be called 
in the startup/initialization process prior to when main is called. 
As shown it would be called after all the default initialization is 
completed.

#pragma exit function1

does the same except at the end, with function1 being called prior to 
any of the compiler's ending/cleanup functions.


Post and pre increment/decrement:

++var
Increment var.  The value used in the expression is the new value.

var++
Increment var and save the value, but use the original value of var in 
the expression.

Same comments and syntax with -- except that it decrements instead of 
incrementing.

Consider the differences between what the loops do in this code:

  int var;

  for (var = 6; var < 22; var++)
      function_A();

  for (var = 6; var < 22; ++var)
      function_A();

The logical operations caused by these loops differ in that the first 
loop requires that a copy of the original value of 'var' must be 
formed, saved somewhere and loaded again after the increment and store 
of the new value.

Restating:
There is some additional overhead associated with the post increment 
(the var++).  Compiler writers have long since caught on to this 
common bad habit of programmers and optimize this specific usage for 
integers, but the syntax is the same if 'var' is an object other than 
an integral type.

When 'var' is a class or structure, not a uncommon situation in C++, 
post-increment forces construction of a temporary instance of the 
class to capture the original state, save of the temporary somewhere 
(possibly causing an allocation call), call the post increment 
function, restore the temporary test and test, save the test results, 
possibly a de-allocation call for the temporary - BIG overhead.  Using 
var++ instead of ++var is not a good habit.

..  Ed

> dkat wrote in message
> news:4164b548@newsgroups.borland.com...
>
> ... we are a science research lab that knows just enough
> about programming to get by (no one is going to see our
> code other than us).   The all caps is from Unix days -
> maybe fortran - can't remember... where all caps were used
> for #define which is how we used to do constants (aren't
> you proud of me that I now use const rather than #define
> for my Constants :)   )....  But I do want to learn what I can
> and very much appreciate your effort to educate.
> I now have to go read about #pragma, post and pre
> increment/decrement, figure out how to use decrement given
> that my mind set is to make the variable change after the
> fact rather than before...., etc.
>
> Thank you again.

.