2

In C++, I have some header files such as: Base.h, and some classes using Base.h:

//OtherBase1.h
#include "Base.h"
class OtherBase1{
    // something goes here
};

//OtherBase2.h
#include "Base.h"
class OtherBase2{
    // something goes here
};

And in main.cpp, I can only use one of those two OtherBase classes because of duplicate header. If I want to use both classes, in OtherBase2.h I have to #include "OtherBase1.h" instead of #include "Base.h". Sometimes, I just want to use OtherBase2.h and not OtherBase1.h, so I think it's really weird to include OtherBase1.h in OtherBase2.h. What do I do to avoid this situation and what's the best practice for including header file?

pb2q
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Kingfisher Phuoc
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3 Answers3

8

You should be using include guards in Base.h.

An example:

// Base.h
#ifndef BASE_H
#define BASE_H

// Base.h contents...

#endif // BASE_H

This will prevent multiple-inclusion of Base.h, and you can use both OtherBase headers. The OtherBase headers could also use include guards.

The constants themselves can also be useful for the conditional compilation of code based on the availability of the API defined in a certain header.

Alternative: #pragma once

Note that #pragma once can be used to accomplish the same thing, without some of the problems associated with user-created #define constants, e.g. name-collisions, and the minor annoyances of occasionally typing #ifdef instead of #ifndef, or neglecting to close the condition.

#pragma once is usually available but include guards are always available. In fact you'll often see code of the form:

// Base.h
#pragma once

#ifndef BASE_H
#define BASE_H

// Base.h contents...

#endif // BASE_H
pb2q
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    +1. Note that on most compilers you can also just use `#pragma once` at the top of the header file for the same effect. – Cameron Jun 12 '12 at 04:29
4

You have to use header guards to avoid duplication.

http://en.wikipedia.org/wiki/Include_guard

For example in your Base.h add following:

#ifndef BASE_H_
#define BASE_H_

// stuff in Base.h

#endif

See this SO question for heard guard formats

#include header guard format?

Community
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Sanish
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    All symbols starting with double underscores, and those starting with an underscore followed by an uppercase character are reserved by the implementation. You should use `#ifndef BASE_H__` instead. – Praetorian Jun 12 '12 at 04:36
  • @Prætorian, +1, thanks for reminding that, I have edited my answer accordingly. – Sanish Jun 12 '12 at 04:39
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    @Prætorian: Names *containing* a double underscore are reserved. `BASE_H` or `BASE_H_` are fine, but not `BASE_H__`. – Mike Seymour Jun 12 '12 at 06:31
0

To avoid having problems related to importing the same header file more than once, you can use the preprocessor to guard against that. The usual way of doing this is by adding these bits to Base.h:

#ifndef BASE_H_CONSTANT
#define BASE_H_CONSTANT

// Stick the actual contents of Base.h here

#endif
Smashery
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