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Why use #if 0 for block commenting out?

Asked 2010-09-02T19:40:45.947
31

Reverse engineering code and I'm kind of appalled at the style, but I wanted to make sure there's no good reason for doing these things....

Is it just me or is this a horrible coding style

if ( pwbuf ) sprintf(username,"%s",pwbuf->pw_name);
else sprintf(username,"%d",user_id);

And why wrap code not intended for compilation in an

#if 0
....
#endif

Instead of comments?

EDIT: So as some explained below, this is due to the possibility to flummox /* */ which I didn't realize.

But I still don't understand, why not just use your programming environment tools or favorite text editor's macro's to block comment it out using "//"

wouldn't this be MUCH more straightforward and easy to know to visually skip?

Am I just inexperienced in C and missing why these things might be a good idea -- or is there no excuse, and I'm justified in feeling irritated at how ugly this code is?

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2 Answers

38

#if 0 is used pretty frequently when the removed block contains block-comments

I won't say it's a good practice, but I see it rather often.

The single line flow-control+statement is easy enough to understand, although I personally avoid it (and most of the coding guidelines I've worked under forbid it)

BTW, I'd probably edit the title to be somewhat useful "Why use #if 0 instead of block comments"

If you have the following

#if 0
        silly();
        if(foo)
           bar();
        /* baz is a flumuxiation */
        baz = fib+3;
#endif

If you naively replace the #if 0/#endif with /* */, that will cause the comment to end right after flumuxiation, causing a syntax error when you hit the */ in the place of the #endif above..

EDIT: One final note, often the #if 0 syntax is just used while developing, particularly if you have to support multiple versions or dependencies or hardware platforms. It's not unusual for the code to be modified to 

#ifdef _COMPILED_WITHOUT_FEATURE_BAZ_
    much_code();
#endif

With a centralized header defining (or not) hundreds of those #define constants. It's not the prettiest thing in the world, but every time I've worked on a decent sized project, we've used some combination of runtime switches, compile-time constants (this), compile-time compilation decisions (just use different .cpp's depending on the version), and the occasional template solution. It all depends on the details.

While you're the developer just getting the thing working in the first place, though... #if 0 is pretty common if you're not sure if the old code still has value.

answered 2010-09-02T19:44:04.097
0
if ( pwbuf ) sprintf(username,"%s",pwbuf->pw_name);
else sprintf(username,"%d",user_id);

Idiomatic and concise. If it got touched more than 2 or 3 times, I would bracket and next-line it. It's not very maintainable if you add logging information or other conditions.

#if 0
....
#endif

Good to turn on blocks of debug code or not. Also, would avoid compilation errors related to trying to block comment this sort of thing out:

/* line comment */
...
/* line comment again */

Since C block comments don't nest.

answered 2010-09-02T20:04:15.710

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