Re: Allow DLLs to access a Singleton

"Ben Pope" <>
Tue, 11 Jul 2006 00:32:55 +0100
<> wrote in message

Ben Pope wrote:

<> wrote in message

Ben Pope wrote:


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

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

Currently I have something like:

#include <iostream>

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

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

typedef Singleton<MyClass> MySingleton;

#include "test.hpp"

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

#include "..\test\test.hpp"
//#include <windows.h>
_declspec(dllexport) void Register(MySingleton& mine) {

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

int main() {
   MyClass& mine = MySingleton::Instance();
   std::cout << mine.value();
   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,
   if (!reg) {
      std::cout << "Failed to Find Function \"Register\"" <<
      return 2;
   reg(/* what goes here? */);

Since MyClass is never instantiated, I can't really pass them around.
need for the DLL to know about the Singleton in the main application
creating it's own instance, completely defeating the point. At the
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()

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

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

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

If I put the test.cpp bit into the header, then my main application and
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++.


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

Hi Ben,

you'll be needing neither a MyClass pointer nor a reference because
this is encapsulated by the Register function. However, if you're
planning to use MyClass elsewhere outside the DLL, you'll need to
export MyClass as well by adding the __declspec(dllexport) modificator
-just as you did with Register function. However, for loading it,
you'll need to declare the class again now with the
__declspec(dllimport) modifier. Then you'll have to reference the DLL
in the project settings (reference the .lib file that is generated in
the C++ options).

Finally, note that usually you'll do something like:

#define MYAPP __declspec(dllexport)
#define MYAPP __declspec(dllimport)
class MYAPP MyClass { ... };

so that you can use the same header file from both the DLL project and
the client project (The DLL project called MyApp usually #defines the

Thanks for that.

I've just figured out seperately that I have to export my Singleton and
MyClass, and then link the DLL against the lib file produced. I've also
seen that macro trick before, and sometimes do a similar thing with extern,
so I have something similar.


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

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