earl

OK I have to apologize in advance for this obscure question, but hopefully somebody can point me in the right direction...

Consider a class with a static member, initialized in that class' header file. i.e.,


Now assume this header is included in various places of an application, as well as runtime libraries loaded by that application. In terms of the entire process, will there exist one single Obj::num, or will each module have its own copy of Obj::num?

I've been doing related testing, and I'm confused because I've seen both behaviors, but I can't narrow down what causes each. I've been trying different ways of building shared libraries (i.e., using ld myself, using g++ -shared, etc.). Is there a C++ language mechanism I can use to explicitly force the "single copy for the entire process" scenario, despite how the application and its modules are compiled/linked? For instance, changing my header to:


I presume would only declare num in my main module (assuming I define MAIN_MODULE for that compilation), but I'm not familiar enough with what's happening behind the scenes to know if this is correct. I would like a way to code this so I don't have to worry about how it is compiled and linked on a bunch of different UNIX platforms. Any insight (or directions to any related linking tutorials) is appreciated...

Eggbert

Good question! The answer is that the library has no choice but to guarantee that the definition of a static member is performed only once. If a static member is defined multiple times, the behavior is undefined. As with anything falling under the One Definition Rule, the easiest fix is to provide a header with your declarations for inclusion in however many files you want:
And then a source file that is linked with the project once and provides definitions for declared names in the header file:
This is a safe and easy way to avoid multiple definitions, especially for situations where an error or warning is not required, such as this one.

earl

Thanks a lot eggbert.

That clears things up quite a bit. The whole multiple definition being undefined thing clears up the different behavior I've been seeing.

So I'm assuming using something like that compile flag would have the same effect as moving the definition to a source file that gets linked into the main module (and no other modules in the application), so long as it's only defined once.

Thanks again.

EDIT:

I forgot to ask if you or anybody happened to know where I could find this documented? Or anything related to the "one definition rule" in general. Thanks.