Sealing a Virtual Function

The latest version of this topic can be found at Sealing a Virtual Function.

The syntax for sealing a virtual function has changed from Managed Extensions for C++ to Visual C++.

The __sealed keyword is used in Managed Extensions to modify either a reference type, disallowing subsequent derivation from it (see Declaration of a Managed Class Type), or to modify a virtual function, disallowing subsequent overriding of the method in a derived class. For example:

__gc class base { public: virtual void f(); };  
__gc class derived : public base {  
public:  
   __sealed void f();  
};  

In this example, derived::f() overrides the base::f() instance based on the exact match of the function prototype. The __sealed keyword indicates that a subsequent class inherited from the derived class cannot provide an override of derived::f().

In the new syntax, sealed is placed after the signature rather than being allowed to appear anywhere before the actual function prototype, as was previously allowed. In addition, the use of sealed requires an explicit use of the virtual keyword as well. That is, the correct translation of derived, above, is as follows:

ref class derived: public base {  
public:  
   virtual void f() override sealed;  
};  

The absence of the virtual keyword in this instance results in an error. In the new syntax, the contextual keyword abstract can be used in place of the =0 to indicate a pure virtual function. This was not supported within Managed Extensions. For example:

__gc class base { public: virtual void f()=0; };  

can be rewritten as

ref class base { public: virtual void f() abstract; };  

See Also

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