Figure 183 Multiple Inheritance Panda Hierarchy in Java

Encoding QR-Code in Java Figure 183 Multiple Inheritance Panda Hierarchy
Figure 183 Multiple Inheritance Panda Hierarchy
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The base class constructors are invoked in the declaration order within the class derivation list For ying_yang, for example, the order of constructor invocation is the following: the Bear constructor (because Bear is derived from ZooAnimal; however, prior to the execution of the Bear constructor, the ZooAnimal constructor is invoked), the Endangered constructor, then the Panda constructor As we discussed in Section 174, the order of constructor invocation is not affected by either the presence of the base class within the member initialization list or the order in which they are listed That is, were Bear's default constructor to be invoked implicitly and therefore not listed within the member initialization list, such as the following,
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// Bear's default constructor is invoked prior to // Endangered's two-argument constructor Panda::Panda() : Endangered( Endangered::environment, Endangered::critical ) { }
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Bear's default constructor is still invoked prior to the explicitly listed two-argument constructor of Endangered Similarly, the order of destructor invocation is always the reverse of the constructor order In our example, the order of destructor invocation is the following: ~Panda(), ~Endangered(), ~Bear(), ~ZooAnimal() Under single inheritance, as we saw in Section 173, the public and protected members of the base class can be accessed directly as if they were members of the derived class The same holds true with multiple inheritance Under multiple inheritance, however, there is the possibility of inheriting a member with the same name from two or more base classes In this case, direct access is ambiguous and results in a compile-time error The compile-time error, however, is not triggered by the potential ambiguity of an unqualified access of the two members, but only by an actual attempt to access them (see the discussion in Section 1744) For example, if both Bear and Endangered define a print() member function, then a statement such as the following,
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ying_yangprint( cout );
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results in a compile-time error, even if the two inherited member functions define different parameter types:
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Error:
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ying_yangprint( cout ) -- ambiguous, one of Bear::print( ostream& ) Endangered::print( ostream&, int )
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The reason for this is that the inherited member functions do not form a set of overloaded functions within the derived class (see Section 173 for a discussion) print(), therefore, is resolved only using name resolution on the name print rather than using overload resolution based on the actual argument types passed to print() (We'll look at how this might be resolved in Section 184) Under single inheritance, a pointer, a reference, or an object of a derived class, if necessary, is converted automatically to a pointer, a reference, or an object of a publically derived base class Again, the same holds true with multiple inheritance A Panda pointer, reference, or object, for example, can be converted to a pointer, a reference, or an object of a ZooAnimal, Bear, or Endangered class For example:
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extern void display( const Bear& ); extern void highlight( const Endangered& ); Panda ying_yang; display( ying_yang ); // ok highlight( ying_yang ); // ok extern ostream& operator ( ostream&, const ZooAnimal& ); cout ying_yang endl; // ok
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Once again, however, under multiple inheritance, the possibility of an ambiguous conversion is much greater For example, consider the following two functions:
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extern void display( const Bear& ); extern void display( const Endangered&);
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An unqualified invocation of display() with a Panda object, such as
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Panda ying_yang; display( ying_yang ); // error: ambiguous
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results in a compile-time error of the following general form:
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display( ying_yang ) -- ambiguous, one of
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extern void display( const Bear&); extern void display( const Endangered&);
There is no way for the compiler to distinguish between the immediate base classes in terms of a derived class conversion Each conversion is equally applicable (We'll look at how this might be resolved in Section 1841) To see how the virtual function mechanism is affected by multiple inheritance, let's define a set of virtual functions for each of Panda's immediate base classes (Virtual functions were introduced in Section 172, and were discussed in detail in Section 175)
class Bear : public ZooAnimal { public: virtual ~Bear(); virtual ostream&print( ostream&) const; virtual string isA() const; // }; class Endangered { public: virtual ~Endangered(); virtual ostream&print( ostream&) const; virtual void highlight() const; // };
Let's now define Panda to provide its own instance of print(), its own destructor, and to introduce a new virtual function, cuddle(), as follows:
class Panda : public Bear, public Endangered { public: virtual ~Panda(); virtual ostream&print( ostream&) const; virtual void cuddle(); // };
The set of virtual functions that can be invoked directly from a Panda object are the following:
Table Active Panda Virtual Functions Name of Virtual Function destructor print(ostream&) const isA() const highlight() const cuddle() Active Instance Panda::~Panda() Panda::print(ostream&) Bear::isA() Endangered::highlight() Panda::cuddle()
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