圆形包含C中的头文件

Question

is it possible to make something like round include in C? example: in ial.h - #include "adt.h" and in adt.h - #include "ial.h"

Answer 1

This is called a circular dependency , and while it's possible, you should not . Avoid designing around the concept at all costs.

What you should do is extract the common ground from both headers and create a third one, that both include .

   bad         good

a <---> b     a     b
              |     |
              -> c <-

If you find you can't do this, then most likely a and b are the same semantic unit, and belong in the same header.

Answer 2

Obviously, if you did that, and didn't do anything else, you'd have an infinitely long file as the circular dependency kept including its components.

But lots of people include lots of files in C, so this sort of thing is bound to happen.

Enter include guards . In each (well-designed) header file, there must be a way to make sure that you break the circular dependency. The two most popular ways use different precompiler macros:

#pragma once for compilers that accept this (which is now most of them), it makes sure that the header is only included once per compilation. Put it at the top of each header, and you're ok.

ifdefs - something like this

#if !defined(__MY_HEADER_H__)
#define __MY_HEADER_H__
/* all the code */
#endif

You'll see include guards on any standard header.

I do like uʍop ǝpısdn's answer, though - if it's your code, do everything you can to avoid creating this kind of dependency.

Answer 3

I guess you should be able to do it by using #ifndef :

In ial.h :

#ifndef IAL_H
#define IAL_H

...

#include "adt.h"

...

#endif

In adt.h :

#ifndef ADT_H
#define ADT_H

...

#include "ial.h"

...

#endif

Whether it is a good idea or not, I would say don't do it.