Multiple inclusions of header files

Question

I have A.cpp and B.cpp which both include the header file header.h.

Later both A.cpp and B.cpp are included in a main.cpp.

This causes error when main.cpp is compiled saying that the header file header.h has been included twice.

How to solve this ?

@KillianDS I know. Later I do plan to move the source into the cpp files and include the class definitions in a separate header file. — Chani, Feb 23 '13 at 20:38

score 4 · Accepted Answer · answered Feb 23 '13 at 20:38

4

You should surround your header files in

#ifndef MYFILE_H
#define MYFILE_H

// Contents of your file

#endif

These are called include guards.

Second point: you shouldn't be including .cpp files, only .h files.

answered Feb 23 '13 at 20:38

alestanis

21,519
4
48
67

The header files are from a library. So I cannot add header guards there. What must i do now ? – Chani Feb 23 '13 at 20:41
Header files not mandatory have .h extentions, so please change your suggestion, it can confuse some novice programers. – Slava Feb 23 '13 at 20:42
1

@RitwikG That sounds like the "library" is thorougly broken. My advice: drop that piece of crap and use something that can actually be called a library. – R. Martinho Fernandes Feb 23 '13 at 20:44

juanchopanza · Answer 2 · 2013-02-23T20:48:17.137

2

Use include guards. in your headers, for example:

// Header.h
#ifndef HEADER_H_
#define HEADER_H_

// code from original header.h

#endif

And don't include .cpp files in other .cpp files. Include the necessary headers only.

Edit If the header files come from a 3rd party library, and do not have include guards, I would be very suspect of that library. I would drop it. However, you can make your own headers, including the library header in an include guard:

// FixedHeader.h
#ifndef HEADER_H_
#define HEADER_H_

#include "header.h"

#endif

Then #include "FixedHeader.h". But I would drop the library, seriously.

edited Feb 23 '13 at 20:48

answered Feb 23 '13 at 20:37

juanchopanza

223,364
34
402
480

The header files are from a library. So I cannot add header guards there. What must i do now ? – Chani Feb 23 '13 at 20:40
@RitwikG if they are from a library, they should already have include guards. If they don't, then drop the library, it is no good! Although note that, on Windows at least, include guards can be expressed with `#pragma once` or something like that. – juanchopanza Feb 23 '13 at 20:41

Slava · Answer 3 · 2013-02-23T21:05:03.127

If you cannot modify header file to include guards, there are 3 possible solutions (sorted from the best to worse):

1 Do not use that garbage.
2 Use wrapper my_header.h

#ifndef MY_HEADER_H
#define MY_HEADER_H
#include <header.h>
#endif // MY_HEADER_H

include my_header.h instead of header.h in your code

3 Use guards in .cpp file

 #ifndef HEADER_H  
 #define HEADER_H  
 #include <header.h>  
 #endif // HEADER_H

you have to be consistent though and use the same guards everywhere (that's why it is solution 3)

Multiple inclusions of header files

3 Answers3

Linked

Related