s DOWNCASTING AND UPCASTING in Software

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You might think some sort of type casting would allow you to easily get around the slicing problem However, things are not that simple The following is illegal:
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Pet vpet; Dog vdog; //Dog is a derived class with base class Pet vdog = static_cast<Dog>(vpet); //ILLEGAL!
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However, casting in the other direction is perfectly legal and does not even need a casting operator:
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vpet = vdog; //Legal (but does produce the slicing problem)
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Casting from a descendant type to an ancestor type is known as upcasting, since you are moving up the class hierarchy Upcasting is safe because you are simply disregarding some information (disregarding member variables and functions) So, the following is perfectly safe:
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vpet = vdog;
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upcasting
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Polymorphism and Virtual Functions
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Display 158 Review of the Classes PFArrayD and PFArrayDBak
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class PFArrayD { public: PFArrayD( ); ~PFArrayD( ); protected: double *a; //for an array of doubles int capacity; //for the size of the array int used; //for the number of array positions currently in use }; PFArrayD::PFArrayD( ) : capacity(50), used(0) { Some details about the base class PFArrayD a = new double[capacity]; A more complete definition of PFArrayD is } PFArrayD::~PFArrayD( ) { delete [] a; }
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given in Displays 148 and 149, but this display has all the details you need for this chapter
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class PFArrayDBak : public PFArrayD The destructors should be virtual, but { we had not yet covered virtual public: functions when we wrote these classes PFArrayDBak( ); ~PFArrayDBak( ); private: double *b; //for a backup of main array int usedB; //backup for inherited member variable used }; PFArrayDBak::PFArrayDBak( ) : PFArrayD( ), usedB(0) { b = new double[capacity]; Some details about the derived class PFArrayDBak } A complete definition of PFArrayDBak is given in PFArrayDBak::~PFArrayDBak( ) { delete [] b; }
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Displays 1410 and 1411, but this display has all the details you need for this chapter
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Casting from an ancestor type to a descended type is called downcasting and is very downcasting dangerous, since you are assuming that information is being added (added member variables and functions) The dynamic_cast that we discussed brie y in 1 is used for downcasting It is of some possible use in defeating the slicing problem but is dangerously unreliable and fraught with pitfalls A dynamic_cast may allow you to downcast, but it only works for pointer types, as in the following:
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Pet *ppet; ppet = new Dog; Dog *pdog = dynamic_cast<Dog*>(ppet); //Dangerous!
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We have had downcasting fail even in situations as simple as this, and so we do not recommend it The dynamic_cast is supposed to inform you if it fails If the cast fails, the dynamic_cast should return NULL (which is really the integer 0)2 If you want to try downcasting keep the following points in mind: 1 You need to keep track of things so that you know the information to be added is indeed present 2 Your member functions must be virtual, since dynamic_cast uses the virtual function information to perform the cast
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You need not know how a compiler works in order to use it That is the principle of information hiding, which is basic to all good program design philosophies In particular, you need not know how virtual functions are implemented in order to use virtual functions However, many people nd that a concrete model of the implementation helps their understanding, and when reading about virtual functions in other books you are likely to encounter references to the implementation of virtual functions So, we will give a brief outline of how they are implemented All compilers for all languages (including C++) that have virtual functions typically implement them in basically the same way If a class has one or more member functions that are virtual, the compiler creates what is called a virtual function table for that class This table has a pointer (memory virtual address) for each virtual member function The pointer points to the location of the function correct code for that member function If one virtual function was inherited and not table changed, then its table entry points to the de nition for that function that was given in the parent class (or other ancestor class if need be) If another virtual function had a new de nition in the class, then the pointer in the table for that member function points to that de nition (Remember that the property of being a virtual function is
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The standard says The value of a failed cast to pointer type is the null pointer of the required result type A failed cast to a reference type throws a bad_cast