Re: Allow DLLs to access a Singleton

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10 Jul 2006 15:42:42 -0700
Ben Pope wrote:


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

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:

#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, "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()

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;
// ...

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