Explicit Override of an Interface Member

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The latest version of this topic can be found at Explicit Override of an Interface Member.

The syntax for declaring an explicit override of an interface member within a class has changed from Managed Extensions for C++ to Visual C++.

You often want to provide two instances of an interface member within a class that implements the interface – one that is used when class objects are manipulated through an interface handle, and one that is used when class objects are used through the class interface. For example:

public __gc class R : public ICloneable {  
   // to be used through ICloneable  
   Object* ICloneable::Clone();  
  
   // to be used through an R  
   R* Clone();  
};  

In Managed Extensions we do this by providing an explicit declaration of the interface method with the method's name qualified with the name of the interface. The class-specific instance is unqualified. This eliminates the need to downcast the return value of Clone, in this example, when explicit called through an instance of R.

In the new syntax, a general overriding mechanism has been introduced that replaces the Managed Extensions syntax. Our example would be rewritten as follows:

public ref class R : public ICloneable {  
public:  
   // to be used through ICloneable  
   virtual Object^ InterfaceClone() = ICloneable::Clone;  
  
   // to be used through an R  
   virtual R^ Clone();  
};  

This revision requires that the interface member being explicitly overridden be given a unique name within the class. Here, I've provided the awkward name of InterfaceClone. The behavior is still the same – an invocation through the ICloneable interface invokes the renamed InterfaceClone, while a call through an object of type R invokes the second Clone instance.

See Also

Member Declarations within a Class or Interface (C++/CLI)
Explicit Overrides