THE VIRTUAL PROPERTY IS INHERITED in Software

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The property of being a virtual function is inherited For example, since bill was declared to be virtual in the base class Sale (Display 151), the function bill is automatically virtual in the derived class DiscountSale (Display 153) So, the following two declarations of the member function bill would be equivalent in the definition of the derived class DiscountSale:
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double bill( ) const; virtual double bill( ) const;
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Thus, if SuperDiscountSale is a derived class of the class DiscountSale that inherits the function savings, and if the function bill is given a new definition for the class SuperDiscountSale, then all objects of the class SuperDiscountSale will use the definition of the function bill given in the definition of the class SuperDiscountSale Even the inherited function savings (which includes a call to the function bill) will use the definition of bill given in SuperDiscountSale whenever the calling object is in the class SuperDiscountSale
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WHEN
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VIRTUAL FUNCTION
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There are clear advantages to using virtual functions and no clear disadvantages that we have seen so far So, why not make all member functions virtual In fact, why not define the C++ compiler so that (like some other languages, such as Java) all member functions are automatically virtual The answer is that there is a large overhead to making a function virtual Doing so uses more storage and makes your program run slower than if the function were not virtual That is why the designers of C++ gave the programmer control over which member functions are virtual and which are not If you expect to need the advantages of a virtual member function, then make that member function virtual If you do not expect to need the advantages of a virtual function, then your program will run more efficiently if you do not make the member function virtual
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Virtual Function Basics
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1 Explain the difference among the terms virtual function, late binding, and polymorphism 2 Suppose you modify the definitions of the class Sale (Display 151) by deleting the reserved word virtual How would that change the output of the program in Display 155
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It is wise to develop incrementally This means code a little, then test a little, then code a little more and test a little more, and so forth However, if you try to compile classes with virtual member functions but do not implement each member, you may run into some very-hard-tounderstand error messages, even if you do not call the undefined member functions! If any virtual member functions are not implemented before compiling, the compilation fails with error messages similar to this:
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Undefined reference to Class_Name virtual table
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Even if there is no derived class and there is only one virtual member function, but that function does not have a definition, this kind of message still occurs What makes the error messages very hard to decipher is that without definitions for the functions declared virtual, there will be further error messages complaining about an undefined reference to default constructors, even if these constructors really are already defined Of course, you may use some trivial definition for a virtual function until you are ready to define the real version of the function This caution does not apply to pure virtual functions, which we discuss in the next section As you will see, pure virtual functions are not supposed to have a definition
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s ABSTRACT CLASSES AND PURE VIRTUAL FUNCTIONS
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You can encounter situations in which you want to have a class to use as a base class for a number of other classes, but you do not have any meaningful de nition to give to one or more of its member functions When we introduced virtual functions we discussed one such scenario Let s review it now Suppose you are designing software for a graphics package that has classes for several kinds of gures, such as rectangles, circles, ovals, and so forth Each gure might be an object of a different class, such as the Rectangle class or the Circle class In a welldesigned programming project, all of these classes would probably be descendants of a single parent class called, for example, Figure Now, suppose you want a function to draw a gure on the screen To draw a circle, you need different instructions from those you need to draw a rectangle So, each class needs to have a different function to draw
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