#ifndef

**pktcperlc++java** · 03-26-2005

I was looking through system header files a while back, and I noticed that each one has some statement along the lines of:

Code:

#ifndef _CPP_

or something similar, I dont remember exactly what it looked like.

what do those statements mean, and how does one use them?

**Codeplug** · 03-26-2005

preprocessor directives

gg

**pktcperlc++java** · 03-26-2005

oh har har

I know what hte commands ARE, but I don't know the syntax for #ifndef, ie: the statements seem to be able to determine what language is being used, or what OS is being used. How does one do that kind of thing

**Dante Shamest** · 03-26-2005

but I don't know the syntax for #ifndef

The first link in the google page Codeplug posted has information on that.

the statements seem to be able to determine what language is being used, or what OS is being used. How does one do that kind of thing

By using it with common predefined macros.

**Micko** · 03-28-2005

consider this simple example:

Code:

#include <cstdlib>
#include <iostream>

using namespace std;

#ifdef __cplusplus
#define CPP 1
#else
#define CPP 0
#endif

int main()
{
   if (CPP)
   {
           cout<<"It's C++ compiler";
   }
   else
   {
    
       cout<<"It's C compiler";
   }
   system("pause");
}

**pktcperlc++java** · 03-28-2005

would this work?

Code:

#ifdef _cplusplus
#include <iostream>
void printHello(){

cout<<"hello\n";

}//end printHello
#elseif
#include <stdio.h>
void printHello(){

printf("Hello\n");

}//end printHello
#endif

**Micko** · 03-28-2005

Why not try it yourself and see?
But never mind, here analyze this:

Code:

#ifdef __cplusplus
#include <iostream>
#include <cstdlib>
void __foo()
{
     std::cout<<"CPPKivla";
}
#define foo __foo
#else
#include <stdio.h>
#include <stdlib.h>
void __foo()
{
     printf("CKivla");
}
#define foo __foo
#endif
int main()
{
    foo();
   system("pause");
}

**ubergeek** · 03-28-2005

almost. you have the concept right, but the preprocessor directives slightly wrong (or Dev-C++ doesn't support all of the preprocessor directives)

anyways, this works in Dev-C++ 4.9.9.2:

Code:

#if (defined __cplusplus)
#include <iostream>
using namespace std;
int main()
{
	cout << "c++" << endl;
	cin.get();
	return 0;
}
#else
#include <stdio.h>
int main(void)
{
	printf("c\n");
	getchar();
	return 0;
}
#endif

by "works," i mean that it compiles as both C and C++, and when the C program is run it outputs c and when the C++ program is run it outputs c++. not sure why i couldn't get #ifdef to work.

**pktcperlc++java** · 03-28-2005

what does foo stand for?

kind of random, but whenever I create a new carbon project, my about menu is always called about foo unitl I change it, and then Micko used it, what does it mean?

**ubergeek** · 03-28-2005

foo is just the generic example variable name etc. like if someone is explaining pointers sometimes they will make an example like:

Code:

int foo = 5; //this is an int variable with the value 5
int * pfoo = &foo; //pointer to foo
*pfoo = 6; //now foo has the value of 6

i don't know what it actually stands for, though.

actually, google is amazing. explore this page and the links from it.
http://www.dictionary.net/foo
apparently it originated from the slang abbreviation FUBAR, the meaning of which you can find out for yourself (not appropriate for the forums)

now i know. and so do you.

**MadCow257** · 03-28-2005

As a note, the FAQ, like google, is also amazing.
http://faq.cprogramming.com/cgi-bin/...&id=1043284351

**samGwilliam** · 03-28-2005

A useful technique is to stop the same code being compiled twice:

Code:

#ifndef MYFILE_H
#define MYFILE_H

int myFunc (int arg1);
int hello (void);

#endif

If MYFILE_H has not been defined, the code in your header will be compiled and its definition is flagged. So if for some reason, the file is included more than once, all subsequent inclusions will note that MYFILE_H has already been defined and skip over the code.

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