Getting the preprocessor to process a block of code

Question

I have a system where I specify on the command line the verbosity level. In my functions I check against what was specified to determine if I enter a code block or not:

#ifdef DEBUG
if (verbose_get_bit(verbose_level_1)) {
    // arbitrary debugging/printing style code generally goes in here, e.g.:
    printf("I am printing this because it was specified and I am compiling debug build\n");
}
#endif

I'd like to make this less tedious to set up, so here's what I have so far:

// from "Verbose.h"
bool verbose_get_bit(verbose_group_name name); // verbose_group_name is an enum
#ifdef DEBUG
#define IF_VERBOSE_BIT_D(x) if (verbose_get_bit(x))
#else // not debug: desired behavior is the entire block that follows gets optimized out
#define IF_VERBOSE_BIT_D(x) if (0)
#endif // not debug

Now, I can do this:

IF_VERBOSE_BIT_D(verbose_GL_debug) {
    printf("I don't want the release build to execute any of this code");
    glBegin(GL_LINES);
    // ... and so on
}

I like this because it looks like an if-statement, it functions as an if-statement, it's clear that it's a macro, and it does not get run in the release build.

I'd be reasonably sure that the code will get optimized out since it will be wrapped in a if(false) block but I would prefer it if there was some way I can get the preprocessor to actually throw the entire block away. Can it be done?

Please make it clear what (if anything) may vary in the statement beyond the argument to verbose_get_bit. — outis
– outis, Commented Dec 30, 2011 at 0:52

Mysticial · Accepted Answer · 2011-12-30 01:00:16Z

2

I can't think of a way to do it without wrapping the entire block in a macro.

But this might work for your purposes:

#if DEBUG
#define IF_VERBOSE_BIT_D(x) {x}
#else
#define IF_VERBOSE_BIT_D(x)
#endif

IF_VERBOSE_BIT_D(
    cout << "this is" << endl;
    cout << "in verbose" << endl;
    printf("Code = %d\n", 1);
)

Indeed the compiler should be able to optimize out an if (0), but I often do something similar to this when the code inside the block will not compile at all when not in debug mode.

edited Dec 30, 2011 at 1:00

answered Dec 30, 2011 at 0:47

Mysticial

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10 Comments

Steven Lu Over a year ago

I was not aware that entire blocks may be used as macro arguments. What if a macro with multiple arguments is defined, and there are commas in the middle of a block sent to that macro? Regardless, this is definitely an option though quite strange looking.

Mysticial Over a year ago

It will actually still work even with commas somewhere in the block. Either way, you'll need to wrap it with {} if you want more than one statement.

Steven Lu Over a year ago

So the preprocessor will keep track of scope according to {}? good to know.

Voo Over a year ago

Well it makes sense thinking about it: It's just strings for the preprocessor after all, so why would it distinguish between statements and whole blocks of code? I'll put that under "interesting to know" :)

Steven Lu Over a year ago

This is definitely a good answer to my original question. I hadn't even considered the case of stuff in the block which would fail to compile which this does address. I'm sure that would come in handy.

|

Dan · Accepted Answer · 2011-12-30 00:46:18Z

1

Not anywhere near as neatly as you just did it. Don't worry, your compiler will fully optimize out any if(0) block.

You can if you so desire check this by writing a program in which you have it as you described and compiling it. If you then remove the if(false) blocks, it should compile to the exact same binary, as shown by an MD5 hash. But that's not necessary, I promise your compiler can figure it out!

answered Dec 30, 2011 at 0:46

Dan

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3 Comments

ildjarn Over a year ago

Many compilers (including VC++) do not produce byte-for-byte identical binaries from identical source upon repeated compilations.

Dan Over a year ago

True, but if you make the program as simple as possible it is more likely to.

ildjarn Over a year ago

At least in VC++'s case, even an empty int main(){} program is guaranteed not to.

Dietmar Kühl · Accepted Answer · 2011-12-30 00:48:08Z

-1

Just create a condition in the if-statement which is starts with "false &&" if you want to have it disabled entirely. Unless you compile without any optimization the compiler typically removes dead code.

answered Dec 30, 2011 at 0:48

Dietmar Kühl

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1 Comment

Steven Lu Over a year ago

I have definitely done this on many many occasions as a quick way to "comment out" some code.

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