Delegates and Events

Article
12/18/2017

The latest version of this topic can be found at Delegates and Events.

The way to declare delegates and events has changed from Managed Extensions for C++ to Visual C++.

The double underscore is no longer needed, as shown in the following sample. Here a sample code in Managed Extensions:

__delegate void ClickEventHandler(int, double);  
__delegate void DblClickEventHandler(String*);  
  
__gc class EventSource {  
   __event ClickEventHandler* OnClick;    
   __event DblClickEventHandler* OnDblClick;    
};

The same code in the new syntax looks as follows:

delegate void ClickEventHandler( int, double );  
delegate void DblClickEventHandler( String^ );  
  
ref class EventSource {  
   event ClickEventHandler^ OnClick;   
   event DblClickEventHandler^ OnDblClick;   
};

Events (and delegates) are reference types, which is clear in the new syntax because of the use of the hat (^). Events support both an explicit declaration syntax and the trivial form shown in the preceding code. In the explicit form, the user specifies the add, raise, and remove methods associated with the event. (Only the add and remove methods are required; the raise method is optional.)

Under Managed Extensions, if you provide these methods, you do not also provide an explicit event declaration, but you must decide on a name for the event that is not present. Each method is specified in the form add_EventName, raise_EventName, and remove_EventName, as in the following example taken from the Managed Extensions specification:

// explicit implementations of add, remove, raise  
public __delegate void f(int);  
public __gc struct E {  
   f* _E;  
public:  
   E() { _E = 0; }  
  
   __event void add_E1(f* d) { _E += d; }  
  
   static void Go() {  
      E* pE = new E;  
      pE->E1 += new f(pE, &E::handler);  
      pE->E1(17);   
      pE->E1 -= new f(pE, &E::handler);  
      pE->E1(17);   
   }  
  
private:  
   __event void raise_E1(int i) {  
      if (_E)  
         _E(i);  
   }  
  
protected:  
   __event void remove_E1(f* d) {  
      _E -= d;  
   }  
};

The new syntax simplifies the declaration, as the following translation demonstrates. An event specifies the two or three methods enclosed in a pair of braces and placed immediately after the declaration of the event and its associated delegate type, as shown here:

public delegate void f( int );  
public ref struct E {  
private:  
   f^ _E; // delegates are also reference types  
  
public:  
   E() {  // note the replacement of 0 with nullptr!  
      _E = nullptr;   
   }  
  
   // the new aggregate syntax of an explicit event declaration  
   event f^ E1 {  
   public:  
      void add( f^ d ) {  
         _E += d;  
      }  
  
   protected:  
      void remove( f^ d ) {  
         _E -= d;  
      }  
  
   private:  
      void raise( int i ) {  
         if ( _E )  
            _E( i );  
      }  
   }  
  
   static void Go() {  
      E^ pE = gcnew E;  
      pE->E1 += gcnew f( pE, &E::handler );  
      pE->E1( 17 );   
      pE->E1 -= gcnew f( pE, &E::handler );  
      pE->E1( 17 );   
   }  
};

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