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The sealed modifier may be applied to methods, classes, or structs. Methods marked sealed may not be overridden by derived classes. Classes and structs marked sealed cannot be inherited from. The syntax for a sealed class is as in Listing 8-9.
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CHAPTER 8 INHERITANCE
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Listing 8-9. Sealed Class Syntax // sealed.cpp ref class C sealed { }; ref class B : C // Error: cannot inherit from a sealed class. { }; Value classes are implicitly sealed; the use of sealed in the class declaration is not an error, although it is not necessary. Like abstract, the sealed modifier may also be used on an individual function. A sealed type indicates that all the functions in the class are sealed, but when used on an individual function, sealed indicates that only that function cannot be overridden. Derived classes can use new on a sealed base class function; however, they may not use override. If you use new on a sealed base class function, you are creating a totally unrelated method. The abstract or sealed modifiers apply to individual overloads of overloaded functions. Thus, one of a series of function overloads may be sealed, but this does not prevent the other overloads from being overridden in derived classes.
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Abstract and Sealed
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You can use both abstract and sealed. Applied to a type, this makes the type rather like a namespace or static class. You cannot create any instances of such a type, but you can define static fields and methods on the type, as in Listing 8-10. Listing 8-10. An Abstract Sealed Class // abstract_sealed.cpp using namespace System; ref class A abstract sealed { static int i = 1; static int j = 2; public: static A() { Console::WriteLine("A::A()"); } static void f() { Console::WriteLine("A::f " + i); } static void g() { Console::WriteLine("A::g " + j); } };
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CHAPTER 8 INHERITANCE
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int main() { A::f(); A::g(); } The output of Listing 8-10 is as follows: A::A() A::f 1 A::g 2 The modifiers abstract and sealed may be reversed in order.
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Properties participate in inheritance-like methods, but it s worth discussing some particulars. Property getter and setter methods may be virtual. The virtual keyword may be applied to the property, and thereby to both the get and the set methods, or to the individual get and set methods. A property that overrides a base class virtual property cannot be written out as a trivial property, because there would be no way to make it any different from the base class property. You can successfully override a base class trivial property with a fully specified property, as in Listing 8-11. The get and set methods must be fully specified, and the override modifier must be applied to them individually. Listing 8-11. Overriding a Trivial Property // virtual_properties.cpp using namespace System; ref class Base { public: virtual property int Prop; }; ref class Derived : Base { int prop; public:
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CHAPTER 8 INHERITANCE
virtual property int Prop { int get() override { return prop; } void set(int value) override { prop = value; } } }; The virtual keyword may also be applied to the individual get and set methods rather than to the property, as in Listing 8-12. This might be useful if you are only overriding one of the accessors, or if you need to make one nonvirtual and one virtual. Listing 8-12. Using Virtual Accessors // virtual_properties2.cpp using namespace System; using namespace System::Collections::Generic; value class Isotope { public: property unsigned int IsotopeNumber; property unsigned int AtomicNumber; property double Mass; }; ref class Element { double atomicWeight; public: property unsigned int AtomicNumber; property String^ Name; property String^ Symbol; property double AtomicWeight { virtual double get() { return atomicWeight; } void set(double a) { atomicWeight = a; } } Element(String^ name, String^ symbol, double a, double n)
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