C++ a singleton class with dll

Question

I created a static library with class:

class CLog
{
   private:
   CLog();
   ...
   ...
   public:
   static CLog& GetInstance()                                 


        

Answer 1

The problem is that every project that links the static library, be it main program or DLL, will get a separate copy of the static variable. This breaks the typical method of creating a singleton.

The simplest way around this is to create another DLL which holds the singleton, rather than a static library. Since only one linker output will contain the static variable, the problem is solved.

In my own case I created a singleton manager that identified each singleton by a unique GUID and ensured that only one copy existed application wide. The singleton manager existed as its own DLL.

Answer 2

The method I used was to export a function called GetLogger from the EXE which provides a pointer to the singleton. GetInstance() implementation is conditional on the _USRDLL preprocessor define. When _USRDLL is set (for the DLL compilation) GetInstance() calls GetModuleHandle() to get a handle to the EXE and loads the function called GetLogger. Here's the code based on your example:

Static lib has Log.h:

class Log
{
  private:
  Log();

  public:
  ~Log();

  static Log& GetInstance()                                
  {                 
  #ifdef _USRDLL
    typedef Log* (*GetLoggerFn)();
    HMODULE mod = GetModuleHandle( NULL );
    GetLoggerFn getLogger = (GetLoggerFn)::GetProcAddress( mod, "GetLogger" );
    Log* Instance = getLogger();
    return *Instance;
  #else
    static Log Instance;
    return Instance;
  #endif
  }
  void Write(const std::string& str );
};
#define LOG Log::GetInstance()

Static lib has Log.cpp:

#include "Log.h"

void Log::Write(const std::string& str )
{
    std::cout << this << "  " << str << std::endl;
}

Log::Log()
{
}

Log::~Log()
{
    std::cout << "Log destroyed" << std::endl;
}

DLL just has a log statement in DllMain:

#include "../static/Log.h"
BOOL APIENTRY DllMain( HMODULE hModule,
                       DWORD  ul_reason_for_call,
                       LPVOID lpReserved
                     )
{
    switch (ul_reason_for_call)
    {
    case DLL_PROCESS_ATTACH:
    case DLL_THREAD_ATTACH:
    case DLL_THREAD_DETACH:
    case DLL_PROCESS_DETACH:
        break;
    }
    LOG.Write("Hello from dll");
    return TRUE;
}

And EXE looks like this:

#include "stdafx.h"
#include "../static/Log.h"
#include <Windows.h>
extern "C"
{
    __declspec( dllexport ) Log* GetLogger()
    {
        return &LOG;
    }
}


int _tmain(int argc, _TCHAR* argv[])
{
    LOG.Write("Hello from exe");
    HMODULE mod = ::LoadLibraryA( "../Debug/tdll.dll");
    ::FreeLibrary( mod );
    LOG.Write("unloaded library");
    return 0;
}