Namespaces with external linkage

Question

The problem I have is basically the same as \'greentype\' mentions at http://www.cplusplus.com/forum/beginner/12458/

I\'m sharing variables through namespaces and a

Answer 1

Any global object, like i, must have exactly one definition somewhere in the program, but it can be declared multiple times.

Using extern without an initializer makes a declaration just a declaration. This is appropriate for your header file, but you must still define i somewhere. As well as making the header declaration extern you also need to add a definition (i.e. a copy of the declaration without extern) to one and only one of your source files.

Edit: Reading your question, you say that you want to pass a variable to a function. From a style and code structure point of view, this isn't usually a good reason for using a shared (global) variable. In the absence of any overriding reasons you should normally define a function which takes a parameter and pass a value (possibly from a local variable) from the calling site to that function via its parameter.