What does the compiler do with multiple #include of same header

[gnobber]

What does the compiler do? I mean if you include iostream it includes other #include. What if you included a header file already included in one of you earlier #include? The compiler replaces them right, but does the compiler detect and if so, avoid replacing repeating #includes?

[Bazzy]

The preprocessor doesn't care if you already included a header and it just replaces #include as usual.
When writing a header is good practice putting header guards:

//MyHeader.h
#ifndef MY_HEADER
#define MY_HEADER
    // Code here
#endif 

This prevents the code to be replaced multiple times

http://www.cplusplus.com/forum/articles/10627/

[gnobber]

Hi Bazzy,

Thanks, so that's what #ifndef is for... thanks again!

[Return 0]

Really? Visual Studio throws an error in this situation:

//headerfile1.h
   #include "headerfile2.h"

//headerfile2.h
   #include "headerfile1.h"

//main.cpp
   #include "headerfile1.h"
   #include "headerfile2.h" 

fatal error C1014: too many include files : depth = 1024

[helios]

Those headers aren't guarded and there's an infinitely recursive inclusion error. The preprocessor is not smart enough to realize when a header is being included more than once in the same recursion branch.

[gnobber]

Um sorry 'bout this but i've tried it and it appears im not doing it right? How would I do this:

 
// Wrong
#ifndef <climits>
#include <climits>
#endif 

Any help?

[helios]

In Bazzy's example, MY_HEADER is not a placeholder for an actual header. It's a macro the preprocessor defines and it's meant to be used in your headers. The language headers are already guarded, so including them twice does nothing.

For example,
header.h:

#ifndef __HEADER_H__
#define __HEADER_H__
//...
#endif 

[z05DSL3A]

climits, will be guarded, so just include it. It is the headers that you write that should be guarded by you.

#ifndef _HEADERFILE1_
#define _HEADERFILE1_

#include"headerfile2.h"

#endif 

^{headerfile1.h}

#ifndef _HEADERFILE2_
#define _HEADERFILE2_

#include"headerfile1.h"

#endif 

^{headerfile2.h}

#include <iostream>
#include <iostream> // is guarded,will have no ill effect

#include "headerfile1.h"
#include "headerfile2.h"

int main()
{

return 0;
}

[gnobber]

Ok, it's still not too clear to me but im just beginning and I was just forward-skimming the book im reading. I'll probably understand more after I read about it. thanks Bazzy and helios! And Grey Wolf!