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ExtTime(); // Default constructor,
// setting time to
private: // 0;0;0 EST
ZoneType zone;
}; |
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class ExtTime : public Time |
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states that ExtTime is derived from Time. The reserved word public declares Time to be a public base class of ExtTime. This means that all public members of Time (except constructors) are also public members of ExtTime. In other words, Time's member functions Set, Increment, and Write can also be invoked for ExtTime objects.* However, the public part of ExtTime specializes the base class by reimplementing (overriding) the inherited functions Set and Write and by providing its own constructors. |
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Overriding Reimplementing a member function inherited from a parent class. |
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The private part of ExtTime declares that a new private member is added: zone. The private members of ExtTime are therefore hrs, mins, secs (all inherited from Time), and zone. Figure 16-5 pictures the relationship between the ExtTime and Time classes. |
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This diagram shows that each ExtTime object has a Time object as a subobject. Every ExtTime is a Time, and more. C++ uses the terms base class and derived class instead of superclass and subclass. The terms superclass and |
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*If a class declaration omits the word public and begins as class DerivedClass : BaseClass or if it explicitly uses the word private, class DerivedClass : private BaseClass then BaseClass is called a private base class of DerivedClass. Public members of BaseClass are not public members of DerivedClass. That is, clients of DerivedClass cannot invoke BaseClass operations on DerivedClass objects. We do not work with private base classes in this book. |
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