Delegate in Standard C++

Environment: ANSI/ISO C++

Microsoft introduced a new feature called “delegates” in the .NET framework. It is actually a class that holds a list of function pointers. As long as they own the same function signature, the delegate object can hold static, global, or member function pointers. Now I’m going to do the same in a “unmanaged C++” by way of using the “external polymorphism” pattern.

1. Construct the abstract delegate base class

class Delegate {
public:
      virtual void Invoke()=0;
protected:
      Delegate(){}
      virtual ~Delegate(){}
}; 

2. Construct a derive class which accepts a static/global function pointer

//NonTypeDelegate.h
#include "Delegate.h"

class NonTypeDelegate : public Delegate
{
public:
   void Invoke();
   NonTypeDelegate(void (*pfn)(int),int iParam);
   virtual ~NonTypeDelegate(){}
private:
   void (*m_pfn)(int);
   int m_iParam;
};

//NonTypeDelegate.cpp
#include "NonTypeDelegate.h"
#include <iostream>

using namespace std;

NonTypeDelegate::NonTypeDelegate(void (*pfn)(int),
                                 int iParam):m_pfn(pfn),
                                 m_iParam(iParam)
{
}

void NonTypeDelegate::Invoke()
{
   cout << "NonTypeDelegate Invokern";
   m_pfn(m_iParam);
}

3. Construct another derive class which accepts a member function pointer

//TypeDelegate.hpp
#include "Delegate.h"
#include <iostream>

using namespace std;

template <typename T>

class TypeDelegate : public Delegate
{
public:
   void Invoke();
   TypeDelegate(T &t, void (T::*pfn)(int), int iParam);
   ~TypeDelegate(){}

private:
   T m_t;
   void (T::*m_pfn)(int);
   int m_iParam;
};

template<typename T>
TypeDelegate<T>::TypeDelegate(T &t,
                              void (T::*pfn)(int),
                              int iParam):m_t(t),
                              m_pfn(pfn),
                              m_iParam(iParam)
{
}

template<typename T>

void TypeDelegate<T7gt;::Invoke()
{
   cout << "TypeDelegate Invokern";
   (m_t.*m_pfn)(m_iParam);
}

4. Now glue up all the stuffs

#include <iostream>
#include "NonTypeDelegate.h"
#include "TypeDelegate.hpp"
#include <vector>

using namespace std;

void Test(int iParam)
{
   cout << "Test Invokedrn";
}

class A
{
 public:
    void Test(int iParam)
    {
       cout << "A::Test Invokedrn";
    }
};

int main(int argc, char* argv[])
{
   NonTypeDelegate nTDelegate(Test,1);

   A a;
   TypeDelegate<A> tDelegate(a,A::Test,2);

   vector<Delegate*> vecpDelegate;
   vecpDelegate.push_back(&nTDelegate);
   vecpDelegate.push_back(&tDelegate);

   for (vector<Delegate*>::const_iterator kItr=vecpDelegate.begin();
       kItr!=vecpDelegate.end();
       ++kItr)
   {
       (*kItr)->Invoke();
   }

   return 0;
}

5. And the output is

NonTypeDelegate Invoke
Test Invoked
TypeDelegate Invoke
A::Test Invoked

Conclusion

Actually, you can derive a class which can accept different signature of functions pointer. Thanks to the powerful “external polymorphism” pattern.

References

Chris Cleeland, Douglas C.Schmidt and Timothy H.Harrison External Polymorphism : An Object Structural Pattern for Transparently Extending C++ Concrete Data Types

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