Path: utzoo!attcan!uunet!cs.utexas.edu!sun-barr!newstop!exodus!morocco.Eng.Sun.COM!landauer
From: landauer@morocco.Eng.Sun.COM (Doug Landauer)
Newsgroups: comp.lang.c++
Subject: Re: FORever
Summary: Style suggestion:  Always put extra braces around "for"s.
Keywords: auto, break, case, catch, char, class, ... :-)
Message-ID: <1070@exodus.Eng.Sun.COM>
Date: 30 Sep 90 01:17:26 GMT
References: <1990Sep27.150948.9109@sco.COM> <212@dumbcat.sf.ca.us>
Sender: news@exodus.Eng.Sun.COM
Organization: Sun Microsystems, Mt. View, Ca.
Lines: 109

ron> BEWARE there is a major problem with C++'s "for" statement.
ron> 
ron> main() {
ron>   	for (int i=10; i<13;) {
ron> 		for (int i=0; i<3; ++i)
ron> 			; // stuff
ron> 		++i;
ron> 	}
ron> }

It looks even worse than this if you put in the braces that
the inner "for" should have had:

	main() {
		for (int i=10; i<13;) {
			for (int i=0; i<3; ++i) {
				; // stuff
			}
			++i;
		}
	}

That is, now the "++i;" is outside of the inner for's braces, and
looks (to the C programmer's eye) even less a part of it than in
your example, but it still refers to the inner for's "i".

ron> Let's change the definition of
ron> the language NOW before we inflict horrendous pain on C++ programmers.

Too late.  To change it now would indeed inflict too much pain on
programmers who have existing C++ code that depends on this decision.

My suggestion is to adhere to a C++-Style-Guide rule that says:
***Whenever*** you declare a variable within a "for" loop, surround the
for loop with an extra set of braces, starting before the "for"
keyword.  Thus, I would have written your example like so:

	main() {
		{for (int i=10; i<13;) {
			{for (int i=0; i<3; ++i) {
				; // stuff
			}}
			++i;
		}}
	}

This also improves the situation in those cases where you
have two non-nested loops, both using the same variable:

	main() {
		{for (int i=10; i<13; ++i) {
			; // Do some stuff to 3 somethings
		}}

		// Do other stuff

		{for (int i=10; i<13; ++i) {
			; // Do some stuff to 3 other somethings
		}}
	}

Now, you no longer get the "two declarations of i" error message when
you copy a for loop to another part of the same function, and you get a
visually distinct way of showing that the "i" in this for loop really
belongs only inside the loop.  When you really do want to use "i" after
the loop is over, *then* you can break the double-brace apart, and of
course you would then give the loop an extra indentation level.

marc> Do you really want to break all the existing code that does something like
marc> 
marc>     for (int i=10; i<13; ++i) {
marc> 	     if (x[i] == magicValue) {
marc> 	         break;
marc> 	     }
marc>     }
marc>     if (i >= 13) {
marc> 	     // we didn't find magicValue
marc>     }

Under the proposed rule, this becomes

	{ // contain the scope of "i"
	    for( int i=10; i < 13; ++i) {
		    if( x[i] == magicValue) {
			    break;
		    }
	    }
	    if( i >= 13 {
		// we didn't find magicValue
	    }
	} // end scope of "i"

marc> Lots of C code uses this.

No C code uses this; it's not C code.  The ANSI C standard won't let you
declare a variable within that () part of a "for" loop, and I've never seen
any C compiler which would allow it, either.   However, lots of C++ code
does use this.  [ You have to, if you write any member functions.  :-) ]

marc> I assume (which will probably get me into
marc> trouble) that C++ programmers will want to do the same thing.

No trouble here -- C++ programmers will want to do almost any damn
thing that cfront will allow them to do.  Worse, C++ programmers
still want to do any damn thing that cfront has ever allowed them
to do.  (... and people in hell want icewater ...)
-- 
Doug Landauer - Sun Microsystems, Inc. - Languages - landauer@eng.sun.com
    Matt Groening on C++:  "Our language is one great salad."