Strange multiple definitions error with headers

Question

I have a strange multiple definitions error in my project. I'm using the #ifndef preprocessor command to avoid including the same file multiple times. I cleared all other code. Here are my simplified files:

1 - main.cpp

#include "IP.hpp"

int main()
{
    return 0;
}

2 - IP.cpp

#include "IP.hpp"

//some codes!

3 - IP.hpp

#ifndef IP_HPP_INCLUDED
#define IP_HPP_INCLUDED

unsigned char LUTColor[2];

#endif // IP_HPP_INCLUDED

Using codeblocks & gnu gcc in win7, it says:

obj\Debug\main.o:C:\Users\aaa\Documents\prg\ct3\main.cpp|4|first defined here|

||=== Build finished: 1 errors, 0 warnings ===|

Before I deleted all of the other code, the error was:

||=== edgetest, Debug ===|

obj\Debug\IP.o||In function `Z9getHSVLUTPA256_A256_12colorSpace3b':|

c:\program files\codeblocks\mingw\bin..\lib\gcc\mingw32\4.4.1\include\c++\exception|62|multiple definition of `LUTColor'|

obj\Debug\main.o:C:\Users\aaa\Documents\prg\edgetest\main.cpp|31|first defined here|

||=== Build finished: 2 errors, 0 warnings ===|

And 'LUTColor' is in IP.hpp !

What's wrong?

possible duplicate of Repeated Multiple Definition Errors from including same header in multiple cpps — Michael Burr
– Michael Burr, Commented Dec 26, 2011 at 8:33

Community · Accepted Answer · 2017-05-23 10:34:26Z

3

The problem is in the header - you need:

#ifndef IP_HPP_INCLUDED
#define IP_HPP_INCLUDED

extern unsigned char LUTColor[2]; // Declare the variable

#endif // IP_HPP_INCLUDED

Do not define variables in headers!

You also need to nominate a source file to define LUTColor (IP.cpp is the obvious place).

See also: What are extern variables in C, most of which applies to C++ as well as C.

edited May 23, 2017 at 10:34

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answered Dec 26, 2011 at 8:33

Jonathan Leffler

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4 Comments

Vinayak Garg Over a year ago

Do not define variables in headers! unless they are const! +1

Jonathan Leffler Over a year ago

Are things that are const considered to be variable?

Vinayak Garg Over a year ago

const bool myBad_IsNotVariable = true;

Jonathan Leffler Over a year ago

But the point that constants are allowed in headers is worth making, and I didn't make any distinction between variables and constants, so your comment adds some value. Thanks.

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