Multiple copies of static members when mixing shared/static libs

Question

Situation:

I have:

class Platform { public: Platform() { count++; cout << getCount();} static int getCount() { return count; } private: static int count; }

which is created as static library.

Consider making a dynamic library extension

class __declspec(dllimport/dllexport) DerivedPlatform : public Platform { }

And yes I am aware that I'm deriving from a non-dll interface class.

Per: Are static fields inherited?, there should only be a single instance of count ever.

Here's the tricky part, I actually end up with two different copies of count (even though count is declared static). Ie, upon loading in the dll and calling registerPlatforms(), it increments a DIFFERENT count object:

int main() { Platform::count = 0; Platform A; // increases count by 1, cout shows 1 loadPlugin(); // loads the shared library DerivedPlatform DerivedPlatform D; // increases count by 1 again, cout shows 2 cout << Platform::getCount(); // shows 1 !!!!!! }

I have no idea how to resolve this, ie. how to ensure that only one static variable persists. Apparently DLLs have their own heap for static variables - so it sort of makes sense why this would happen.

Mark Ransom · Accepted Answer · 2012-12-13 23:32:13Z

Yes, that's what happens when you link a static library into both an executable and a DLL. Neither has any knowledge of the other at link time so they both get a copy. For the code itself that usually doesn't hurt anything, but for static variables it can be a real hassle.

You need to rearchitect your solution so that the static library is in a DLL instead, either the existing one or a brand new third one. Or eliminate all static variables.

So how come if I change the static library into a .dll, it then works? Do ALL .dlls share the same heap?
@proteneer, it has nothing to do with the heap - static variables are stored in a separate location near the code. Putting it into a DLL just ensures that only one copy exists.

Collectives™ on Stack Overflow

Multiple copies of static members when mixing shared/static libs

1 Answer 1

2 Comments

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Related