visual c++ - inconsistent dll linkage & definition of dllimport static data member not allowed

Question

Assuming I have these two files:

Header.h

class DLL ExportClass{
public:
  ExportClass();
  static int test;
};

Source.cpp

#ifdef EXPORT
    #define DLL __declspec(dllexport)
#else
    #define DLL __declspec(dllimport)
#endif

#include "Header.h"

int ExportClass::test = 0;
ExportClass::ExportClass(){
}

And I won't define EXPORT (to import a already exported class with a static member), why do I get these warnings:

1>source.cpp(11): warning C4273: 'test' : inconsistent dll linkage
1>          header.h(4) : see previous definition of 'public: static int ExportClass::test'
1>source.cpp(13): warning C4273: 'ExportClass::ExportClass' : inconsistent dll linkage
1>          header.h(3) : see previous definition of '{ctor}'

And this error:

1>source.cpp(11): error C2491: 'ExportClass::test' : definition of dllimport static data member not allowed

If I define EXPORT it works. I kind of understand the warnings, but I thought, that the static variable and the ctor could be ignored by the compiler, because the whole class is declared as __declspec(dllimport) anyway. I want to use the same codebase for the __declspec(dllexport) and __declspec(dllimport) - but it seems the compiler stll tries to define these symbols that are marked as __declspec(dllexport) in their declaration. What is the common practice to solve this problem?

Answer 1

You are expecting the compiler to ignore a very serious mishap. It encountered the __declspec(dllimport) attribute on the class declaration, that quite unequivocally states that the class implementation is present in different module that's going to bound at runtime. But then it encountered the definition as well, completely unexpected since the attribute contract says that it is compiled in an entirely different project.

The C4273 warning is generated to remind you that it is very unclear what function is actually going to execute at runtime. There are two, one that is busy compiling, another in the DLL. Which one will actually execute is a wild guess. C4273 is a level 1 warning, the kind that fit the "this is almost surely wrong" category. It is not entirely impossible to work okay since there's some expectation that the functions have at least the same code. The odds that will not cause trouble are however not great, it could only work if the function doesn't have any side effects that change the internal DLL state. Very hard to diagnose bug when it does btw.

Then it encountered the exported variable. Same case, there are two of them. This is where the compiler programmer put his foot down, having code randomly use one or the other is no longer something that can be ignored. That just cannot ever work, the variables cannot have the same value. So C2491 is a hard error.

No idea how you got in this pickle, clearly the road you're trying to travel will make you fall off a steep cliff.