DELEGATES and C++

Environment: [eg VC6 SP4, W98 W2K NT …]

Presentation:


This article shows an implementation of delegates. What is a delegate? An object
that acts like a multi-function pointer with a subscription system. It really
simplifies the use of static or ‘object’ member function pointers for callback
notifications and event handling. While implementation is a bit complicated, the
use is very intuitive:

Quick Sample:

class CFoo {
public:
   // declaration of member callback
   void Foo( int );
};
// declaration of static callback
void Foo( int i );
void main( ) {
   CFoo obj;
   …
   // Declare a delegate for functions
   // with one argument of type int
   CDelegate1< int > delegate;
   // Add Foo static function
   delegate.Add( Foo );
   // Add Foo member function based on foo object
   delegate.Add( obj, CFoo::Foo );
   …
   // Execute the callbacks
   delegate( 0 );
   …
   // Remove Foo static function
   delegate.Remove( Foo );
   // Remove Foo member function
   delegate.Remove( obj, CFoo::Foo );
}

How to?


You just have to add the headers to your project and include the “Delegates.h” header
in your C++ code (in stdafx.h for example) to gain access to these classes:

Notes:


a) STL is used for multi-platform implementation.

b) C++ does not authorize variable length template argument lists and that’s
why I made different implementations for delegates with zero, one or two arguments.
If someone knows how to do in another way, just let me know.

c) C++ only defines scope resolution operator for base classes. We have to
specify the conversion in this particular case:

class CFoo
{
   …
   void Foo( int ) { }
   …
   void Init( CDelegate1& delegate )
   {
      // ERROR: CFoo:: operator does not exist here
      delegate.Add( *this, CFoo::Foo );
      // CORRECT: type cast is available
      delegate.Add( *this, (void (CFoo::* )( int )) Foo );
   }
   …
};

To solve this little “read/write complication” we can define a typedef in our class declaration:

class CFoo
{
   …
   typedef MEMBER_FUNCTION_CALL_TYPEDEF1( CFoo, int ) OwnHandler;
   …
};

So we could have:

delegate.Add( *this, (OwnHandler) Foo );

Actually the

‘

base

’

macros are:

MEMBER_FUNCTION_CALL_TYPEDEF0 for 0 arg member function
MEMBER_FUNCTION_CALL_TYPEDEF1 for 1 arg member function
MEMBER_FUNCTION_CALL_TYPEDEF2 for 2 args member function

You could also define a macro with MEMBER_FUNCTION_CALLx in your cpp file…



Conclusion:


I hope this code will help you in your programming. I’m working at EOLE SA and this method
helped me in my work because it’s lighter and less restrictive than interface definition/derivation
style of notifications (connection points).

Thanks for your attention. Please report any bugs!

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