C Prepocessor hard to understand and very confused.

[Programminglinguist]

I'm having a REALLY hard time understanding preprocessors.

Now I know the situation where you want to "define" something using the #define preprocessor such as #define PI 3.14

that would change all the ("PI") words in your code into 3.14 right?


Well what about #define _MATH_H_


that I have no idea what is happening. In the math library it says

#ifndef _MATH_H_
#define _MATH_H_


code here


#endif


can you explain to me what is happening? Please in detail!

[Zhuge]

That just basically sets some preprocessor "variable" if you want to call it that, to exist, so if you later check #ifdef it will be true.

[Programminglinguist]

so why would i ever need to use that in a program? What will happen if i don't.


I'm having some problems creating a RPG game because of this stuff. Can you explain in detail what's happening? Or possibly give me an easy to read tutorial on it?

[arathalion]

#ifdef and #ifndef check to see if the symbol has been defined. #ifdef then includes everything up to an #else or an #endif if the symbol has been defined, while #ifndef includes everything if the symbol has not been defined.

So, in your example, the first time you include math.h, assuming that is the name of that file, the pre-processor will find the #ifndef _MATH_H_ line, find that it has not been defined and continue including code.

The next line then defines _MATH_H_ as the default value, which should be 1.

If you then try to include math.h again, the pre-processor will get to the #ifndef line, discover that _MATH_H_ has been defined and skip to the #endif.

so why would i ever need to use that in a program? What will happen if i don't.

Well, People do this to stop a header file from being included multiple times.

example.h

#ifndef _EXAMPLE_H
#define _ EXAMPLE_H

int exampi = 0;

#endif 

example.c

#include "example.h"
#include "example.h"

...

This example then, tries to include example.h twice, however, the second time it tries, it discovers that _EXAMPLE_H has been defined, and skips to the #endif, thus stopping the file from being included twice. If you didn't have this code the compiler would spit errors about defining the same variable twice.

[Programminglinguist]

oh thank you arathalion that was very helpful. However one more question that has been bugging me in the back of my mind for a very long time. What does it mean to "define" something?

As you said right above your 2 examples and the quote "If you then try to include math.h again, the pre-processor will get to the #ifndef line, discover that _MATH_H_ has been defined and skip to the #endif."

what do you mean by "has been defined"

does that mean that the compiler has already compiled that? ... or.. what. i'm confused

[Disch]

Before I get started -- please note that macros are evil and you should generally avoid #define where possible. It's fine to use for include guards (ie: the whole _MATH_H_ thing), but apart from that it's kind of sketchy. So don't make a habit of putting #define everywhere.


"Define" doesn't really mean anything special, it just means that the preprocessor recognizes the word as a symbol. Here's an example:

#include <iostream>
using namespace std;

int main()
{
#ifndef TEST
  cout << "test 1\n";
#endif

#define TEST

#ifndef TEST
  cout << "test 2\n";
#endif
}

If you run this, you'll fine that "test 1" will print, but not "test 2". This is because TEST was not defined until the #define line.

[NGen]

Why, exactly, are macros evil? I use them all the time and rarely have any problems with them.

[Bazzy]

http://www.parashift.com/c++-faq-lite/newbie.html#faq-29.8

[chrisname]

I think they're just bad practice... Partly, I guess, because they're unnecessary. You'd be better off using inline functions and constants in most cases.

I use them, but not "all the time." I use them for things like header guards (obviously), and also for debugging. They're useful for trying out two pieces of code separately, like this:

#if __DEBUG == 1 || __DEBUG == 0
foo();
#endif

/* More code */

#if __DEBUG == 2 || __DEBUG == 0
bar();
#endif 

In that case you could compile foo(), bar() or foo and bar() by defining __DEBUG to 1, 2 or 0 respectively (I'd use 3, but making 0 the option to compile all code allows you to add as many of those #if-endif blocks as you want without changing those that are already there).

@Bazzy,
That guy says pointers are evil. I most heartily disagree.

[Bazzy]

http://www.parashift.com/c++-faq-lite/big-picture.html#faq-6.15 :

C programmers who are new to C++ often use pointers, arrays and/or #define more than they should.
...
The goal of farcical things like "pointers are evil" is to convince new C++ programmers that C++ really isn't "just like C except for those silly // comments."

Everything is evil if used improperly or when you have a better alternative

[chrisname]

I see...

[magnificence7]

Hey thanks , I didn't know that. :P
Know I can code normal again! I always had to strugle to find what made the header file be included multibple times! :P