Question

I have two files called file_utils.h and file_utils.cpp which contain some methods and variables which are used by different classes. Here’s an example of how it looks:

file_utils.h:

namespace my_namespace
{
extern Foo* foo;
extern Bar* bar;

void my_function(Blah* blah);
}

file_utils.cpp

#include "file_utils.h"

void my_namespace::my_function(Blah* blah)
{
    foo = 0;    // undefined reference to my_namespace::foo
    bar = 0;    // undefined reference to my_namespace::bar
    //...
}

some_class.cpp

#include "file_utils.h"

some_function()
{
    my_namespace::my_function(blah);
    this->foo = *my_namespace::foo; // will that work ok?
}

So the errors are in the comments. If I remove the extern keyword I get multiple definition of my_namespace::foo error. What is the problem? Is that even a good idea from design standpoint or should I try to use a class with static members and methods instead?

Answer 1

The problem is that you only declared but not defined the variables.

You need to provide a definition in a single implementation file:

file_utils.cpp

#include "file_utils.h"

//definition:
namespace my_namespace
{
   Foo* foo;
   Bar* bar;
}

//alternatively, to keep the same formatting you have
//Foo* my_namespace::foo;
//Bar* my_namespace::bar;

void my_namespace::my_function(Blah* blah)
{
    foo = 0;
    bar = 0;
    //...
}