preprocessor question??

Red83 · #1 21-Feb-2006, 06:16

I was looking at a piece of code and trying to figure out how the preprocessor would expand it. I really can't seem to figure it out correctly.
Could someone help me out? I just need a little explanation. I know it's not formatted very well, but I wanted to give exactly what I'm seeing. My guess is that it will just be expanded to just int N, but I'm not sure.
Thanks a bunch.

CPP / C++ / C Code:

#define USEFLOAT 0
int N;
#if USEFLOAT
float X;
#else
double X;
#endif

#2 21-Feb-2006, 07:24

Quote:

Originally Posted by Red83

I was looking at a piece of code and trying to figure out how the preprocessor would expand it. I really can't seem to figure it out correctly.
Could someone help me out? I just need a little explanation. I know it's not formatted very well, but I wanted to give exactly what I'm seeing. My guess is that it will just be expanded to just int N, but I'm not sure.
Thanks a bunch.

CPP / C++ / C Code:

#define USEFLOAT 0
int N;
#if USEFLOAT
float X;
#else
double X;
#endif

...you don't need to ask these kinds of questions. Just drop it into a piece of code and build it (or try to), you'll see what happens.

CPP / C++ / C Code:

#include <stdio.h>

#define USEFLOAT 0
int N;
#if USEFLOAT
float X;
#else
double X;
#endif

int main(void)
{
    N = 1;
    X = 1.05;

    printf( "N = %d\n", N );
    printf( "sizeof( X ) = %d\n", sizeof( X ) );

    return 0;
}

// output:

N = 1
sizeof( X ) = 8

Here we see that N is being defined, otherwise we'd get an error telling us that (in main) that N is an undefined symbol. And, X is a double, because it uses 8 bytes of storage whereas float is a 4-byte type.

Also, you can see exactly what your preprocessor does by taking a look at your tool chain reference to see how to run just the preprocessor and then stop and look at its output. If you want to test macros for proper/expected expansion, you can comment-out any header files that you include, run the preprocessor and look at its output.

What book are you using and what tool chain?

:davis:

Red83 · #3 21-Feb-2006, 07:38

My book is "C by dissection" by Al Kelley and Ira Pohl. I'm sorry, but I'm not sure what the tool chain is.

#4 21-Feb-2006, 07:52

Quote:

Originally Posted by Red83

My book is "C by dissection" by Al Kelley and Ira Pohl. I'm sorry, but I'm not sure what the tool chain is.

What compiler/software are you using?

...also, when mentioning books, you may want to include the copyright date and/or any relevant revision/edition information. If you're using the 4th edition, the copyright date is 2000. The reason for this is so that we know whether or not you're using an outdated (pre-standards) book. If you were, we'd want to point you toward something more recent.

:davis:

Red83 · #5 21-Feb-2006, 07:59

The book is a 4th edition, and my software is VisualStudio.NET.
I just wasn't familiar with the tool chain terminology.

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