Why Does My C Code Produce a 'Multiple Definition of `wat'' Error, and How Can I Fix It?

Multiple Definitions of 'wat' in a C Project

The given code consists of four files: FileA.cpp, FileA.h, FileB.cpp, and FileB.h. When compiled, it encounters an error stating "multiple definition of `wat'".

FileB.h defines an integer variable named 'wat' as follows:

#ifndef FILEB_H_
#define FILEB_H_

int wat;
void world();


#endif /* FILEB_H_ */

Since this header file is included in both FileA.cpp and FileB.cpp, the compiler detects that 'wat' is being defined multiple times. This occurs because the macro FILEB_H_ is not properly defined in all of the header files.

To resolve this issue, the header files should be modified as follows:

FileA.h:

#ifndef FILEA_H_
#define FILEA_H_

#include "FileB.h"
void hello();

#endif /* FILEA_H_ */

FileB.h:

#ifndef FILEB_H_
#define FILEB_H_

extern int wat;  // Declare 'wat' as external
void world();


#endif /* FILEB_H_ */

FileB.cpp:

#include "FileB.h"

int wat = 0;  // Define 'wat' with its initial value
void world()
{
    // More code
}

By using 'extern' in FileB.h, we inform the compiler that 'wat' is defined elsewhere and that it should not search for its definition within the current file. The actual definition of 'wat' is then provided in FileB.cpp, avoiding the multiple definition error.

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