Strange multiple definitions error with headers - c++

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?

Answer 1

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.