be aware of the static variable in inline functions

I've learned a lesson Friday when debugging a test case -- the static variable in inline function is not always static.

Here is the use case. I have a singleton impl in a dll like below. In the DLL, I created a singleton instance and in my main program I also referred my singleton instance using Singleton::Instance(). Well it turned out that the instance referred in main program is different from the one created in the DLL!

// some .h file
class Singleton 
{
      T& Instance()  
      {
           static T  instance;
           return instance;
      }
};

// some cpp file

Singleton m_t = Singleton::Instance();

Why would it happen and how to resolve it?

• If the singleton is in a static library, this is OK. 
• DLL is same as exe that they are a standalone linker unit, which means they are complete -- all symbols have to be resolved, while static library is not. So when static variable is in the inline function and the function is invoked in a linker unit, a storage is allocated by the linker for that static variable. In my case, I have two linker units, so I have two copies of that instance.
• Solution 1: do not inline
• Solution 2: export the static variable in DLL and import it in the main program. A common way in visual c++ is like below

// in dll

// stdafx.h
#define MYAPI __declspec(dllexport)

// header file singleton.h
class MYAPI Singleton (};

# in other linker units

// stdafx.h
#ifndef MYAPI
#define MYAPI __declspec(dllimport)
#endif

// cpp of header file

#include "stdafx.h"
#include "singleton.h"