Eclipse/GCC: Undefined Reference to Extern Variable

Question

sorry if this is a repeated question, but I've been searching around for a couple of hours, and I'm getting conflicting answers... and what's worse, none of them are working.

It's a simple matter. I have many source files, and I have some common parameters that I want to be in a single file, say "Parameters.h". I want to set these parameters (once) at runtime, by passing them as arguments to the program.

PS: I know that a better way of doing it is to pass everything as arguments to functions, but it's a chunky piece of code and I need to get a result soon without making too many changes.

Here is a minimal working example:

Parameters.h

#ifndef PARAMETERS_H_
#define PARAMETERS_H_

extern int Alpha;

#endif

main.cpp

#include <iostream>
#include "Parameters.h"

int main(int argc, char * argv[])
{
    const int Alpha = 12.0;
}

Functions.cpp

#include "Parameters.h"

double Foo(const double& x)
{
    return Alpha*x;
}

When I compile with

gcc main.cpp Functions.cpp

I get the error "Functions.cpp:(.text+0xa): undefined reference to `Alpha'".

Answer 1

You have declared a global variable named Alpha, but you haven't defined it. In exactly one source file, write at file scope:

int Alpha;

or with an initializer:

int Alpha = 42;

Note that the local variable named Alpha you have defined within main is distinct from and completely unrelated to this global variable.