Understanding the origin of a linker duplicate symbol error

Question

I have a c++ program that compiled previously, but after mucking with the Jamfiles, the program no longer compiled and ld emitted a duplicate symbol error

Answer 1

This is often the result of defining an object in a header file, rather than merely declaring it. Consider:

h.h :

#ifndef H_H_
#define H_H_
int i;
#endif

a.cpp :

#include "h.h"

b.cpp :

#include "h.h"
int main() {}

This will produce a duplicate symbol i. The solution is to declare the object in the header file: extern int i; and to define it in exactly one of the source-code files: int i;.