Compatible Return Types for Overridden Methods in Java

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Compatible Return Types for Overridden Methods
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Return types present an important difficulty Consider the following operator, defined in a base class:
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virtual const Base
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operator++(
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The derived class inherits it as
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const Base
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operator++(
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but that is not really what we want If we have a return type in the derived class, it ought to be a constant reference to the derived type not to the base type Thus the operator++ inherited is not the one we want Instead, we would like to override operator++ with
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Recall that overriding a function means writing a new function with the same signature Under original C++ rules, the return type of the new and overridden function had to match exactly Under the new rules, the return type may be relaxed This means if the original return type is a pointer (or reference) to B, the new return type may be a pointer (or reference) to D, provided D is a publicly derived class of B This condition corresponds to our normal expectation of IS-A relationships
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If the original return 'ype is a pointer (Or reference) to 6, the new return type may be a pointer (or reference) to D9 provided D is a publicly derived of 6
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Private inheritance means that even public members of the base class are hidden
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Private lnheritance
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Composition is preferred to private inheritance In composition, class B is said to be composed of class A (and other objects)
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Although the default is private inheritance, it should be avoided
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Private inheritance means that even public members of the base class are hidden That seems like a silly idea, doesn't it In fact it is, if we are talking about implementing an IS-A relationship Private inheritance is thus generally used to implement a HAS-A relationship (that is, a derived class D has or uses a base class B) In many cases we can get by without using inheritance Instead, we can use composition, making an object of class B a member of class D and, if necessary, making D a friend of B Composition is preferable to private inheritance, but occasionally the latter is more expedient or slightly faster (because it avoids a layer of function calls) For the most part, you should avoid private inheritance unless it greatly simplifies some coding logic or can be justified on performance grounds However, in Section 533, we demonstrate an appropriate and typical use of private inheritance Recall that by default, private inheritance is used If we omitted the keyword public on line 3 of Figure 46, we would have private inheritance In that case the public member functions of exception would still be inherited, but they would be private members of BadIndex and could not be called by users of BadIndexThus the what method would not be visible The type compatibility of base class and derived class pointers and references described earlier does not apply for nonpublic inheritance Thus, in the following code, a BadIndex exception would not be caught:
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c a t c h ( const exception
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cout << e w h a t ( ) << e n d l ;
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Friendship is not inherited
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Friends
Are friends of a class still friends in a derived class The answer is no For example, suppose that F is a friend of class B, and D is derived from B Suppose also that D has nonpublic member M Then, in class D, F does not have access to M However, the inherited portion of B is accessible to F in class D Figure 421 summarizes the results Note that D can declare that F is also a friend, in which case all of D's members would be visible
447 Call by Value and Polymorphism Do Not Mix
Consider the following statement Assume that BadIndex is publicly inherited from exception and suppose that it has overridden the what method:
c a t c h ( exception e
c o u t << w h a t ( ) << e n d l ; 1