When do you use "if ( a = b )"? (was Re: Funny mistake)

[Checkpoint Technologies]

In article <65837 at eerie.acsu.Buffalo.EDU> chu at acsu.buffalo.edu (john c chu) writes:
>In article <775 at camco.Celestial.COM> bill at camco.Celestial.COM (Bill Campbell) writes:
>[concerning "if ( a = b )"
>>Certainly it
>>is a legal construction, but 90% of the time when I do this it
>>was my mistake!
>
>It's been my mistake everytime I've done it!! I realize that it is a
>legal construction and I know what it does, but I was wondering...
>Is there a good use for this?

No.  More specifically, as long as it's really a simple assignment with
no side effects, it's out.

You *can* do this, and some might say it's a "valid" use:

b = function(args);
if(a = b) { ...

Some programmer might have really meant it this way, to test the result
from the function call for zero.  But if I were writing it (and I
hazard to guess most everyone else reading this would agree, at least in
principle) it would appear this way:

a = b = function(args);
if(a) { ..  /* or else if(b) */

I have seen the following code examples frequently, and I consider them
valid:

if(code = setjmp(jmploc)) { ...

 - and -

if(pid = fork()) { ...

The reason they are valid is because the expression contains a
significant side effect.
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