Re: Allow DLLs to access a Singleton

From:
"Ben Pope" <benpope81_REMOVE_@gmail.com>
Newsgroups:
microsoft.public.vc.language
Date:
Mon, 10 Jul 2006 23:54:29 +0100
Message-ID:
<44b2da2b$0$16763$88260bb3@news-taz.teranews.com>
<fernando.a.gomez.f@gmail.com> wrote in message
news:1152571362.177971.51220@35g2000cwc.googlegroups.com...

Ben Pope wrote:

Hi,

Currently I have a main application that loads DLLs. The main
application
has a factory, that is a singleton. How do I allow the DLLs to access
that
singleton?

I have made the singleton pretty basic for brevity, and MyClass is not
much
of a factory, but it illustrates the problem.

Currently I have something like:

Test.hpp:
----
#include <iostream>

class MyClass {
   int val_;
public:
   MyClass() : val_(0) {
      std::cout << "MyClass()" << std::endl;
   }
   void value(int val) { val_ = val; }
   int value() { return val_; }
};

template<class T>
class Singleton {
private:
   static T* t_;
   Singleton();
public:
   static T& Instance() {
      if (!t_) {
         t_ = new T;
      }
      return *t_;
   }
};

typedef Singleton<MyClass> MySingleton;

Test.cpp
----
#include "test.hpp"

template<class T>
T* Singleton<T>::t_ = 0;

TestDll.cpp
----
#include "..\test\test.hpp"
//#include <windows.h>
_declspec(dllexport) void Register(MySingleton& mine) {
mine.Instance().value(9);
}

Main.cpp
----
#include "test.cpp"
#include <iostream>
#include <windows.h>

int main() {
   MyClass& mine = MySingleton::Instance();
   std::cout << mine.value();
   mine.value(5);
   std::cout << mine.value();
   typedef void(*FunctionRegister)(MySingleton&);

   HMODULE mod = LoadLibrary("TestDll.dll");
   if (!mod) {
      std::cout << "Libray Failed to Load" << std::endl;
      return 1;
   }
   FunctionRegister reg = (FunctionRegister)GetProcAddress(mod,
"Register");
   if (!reg) {
      std::cout << "Failed to Find Function \"Register\"" << std::endl;
      return 2;
   }
   reg(/* what goes here? */);
}

Since MyClass is never instantiated, I can't really pass them around. I
need for the DLL to know about the Singleton in the main application
without
creating it's own instance, completely defeating the point. At the
moment,
the static MyClass* is an unresolved external for the DLL.

Thanks for any pointers (pun not intended)!

Ben Pope
--
I'm not just a number. To many, I'm known as a string...


Hi Ben,

since MySingleton::Instance() returns the same always, why do you need
to pass a MySingleton instance as a parameter? I mean, I think that is
against the singleton pattern. I'd just do something like:

_declspec(dllexport) void Register()
{
//mine.Instance().value(9);
MySingleton::Instance().value(9);
}

Then in your main function I'd put something like:

// ...
FunctionRegister reg = (FunctionRegister)GetProcAddress(mod,
"Register");
if (!reg) {
   std::cout << "Failed to Find Function \"Register\"" << std::endl;
   return 2;
}
reg();
// ...


OK, I understand what you're saying, it was actually my first attempt. The
problem is how do I get the static MyClass* to not be an unresolved
external?

If I put the test.cpp bit into the header, then my main application and the
DLL will each have their own singleton... I need some way of dealing with
that, and I'm not sure what that is.

I'm a bit of a DLL newbie, although I'm quite comfortable with C++.

Cheers,

Ben Pope
--
I'm not just a number. To many, I'm known as a string...

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