ssrs ean 13 tout << "X0. "; > tout << "-X().\n"; > in Software

Make Denso QR Bar Code in Software tout << "X0. "; > tout << "-X().\n"; >

CHAP. 1 l]

class Y : public X { public: tout << "Y(): Y::q = H cc q cc ". "; } yo -t q = new int[1023]; -Y() { delete [] q; tout << 'I-Y ( > . " ; } private: int* q; ); main0 -I for (int i = 0; i c 8; i++){ X* r = new Y; delete r;

., . . ..y:( 1 .f y3..: q =.$. fJ3i$59..2.~.~.; . .. .+q( 3. * x0. x ( ) :. V(): n:cJ = ox5a29c; -x-(.) i wx ( ) (I. y ( ) t y : = q = fjx$bi,J& W -y ( ) + WI .: w 1 .: Y::q = OX5C22G. . y ( .), ; 31; , q z .0$5d2~c I -X ( .j , + x0 _ y i.).: yr z.q ; qy&&~c,, wx (j ~ : , : : _ XI) x ( .) -: Y() : . Y: :q = &5ggJ7,; . . . :. . Each iteration of the for loop creates a new dynamk object. As in Example 11.6, the cons&c&&&e

: : .* 5 -x( 1) l ( ,. ,. . : . ,, ,: : :. .,,;,,. ,

x() *

-X()

.::; ; . ._ :. ,.. , ;,.. , , 1.; ,.,. ;.: , I. ,:. ;; ,. , +.., I . . . , ...,., . .\ .. : ,. .I L , - .:, . ,.: , _. .: . . . , . ,+ .;/, : < : , , , . ; I : ,,; :. ,.~:-,~.:-,:;.~,~,~~:~~. .. ; :,;I., . : .; .., , ;: :I , ,.,;:::y, ;:. -: :, : r

_:. . . /:iji.,::.

invoked in top-down sequence: first x ( ) and then Y ( > , allocating 4100 bytes of storage (using 4 bytes for each int). But since r is declared to be a pointer to x objects, only the x destructor is invoked, deallocating only 8 bytes. SO on each iteration, 3992 bytes are lost! This loss is indicated by the actual yalues of the pointer Y : : q. To plug this leak, change the destructor -X ( ) into a virtual function: class X { public: tout cc "X0. 'I; } x0 1 p = new-int[2]; v i r t u a l -X() { delete [] p; tout cc "-X().\n"; private: int* p;

With the base class destructor declared virtual, each iteration of the for loop calls both destructors, thereby restoring all memory that was allocated by the new operator. This allows the same memory to be reused for the pointer r.

This example illustrates what is known as a memory leak. In a large-scale software system, this could lead to a catastrophe. Moreover, it is a bug that is not easily located. The moral is: declare the base class destructor virtual whenever your class hierarchy uses dynamic binding. As noted earlier, these examples are contrived to illustrate specific features of C++ and are not meant to exemplify typical programming practice. 11.9 ABSTRACT BASE CLASSES A well-designed object-oriented program will include a hierarchy of classes whose interrelationships can be described by a tree diagram like the one below. The classes at the leaves of this

tree (e.g., Owl, Fish, Dog) would include specific functions that implement the behavior of their respective classes (e.g., Fish. swim ( ) , Owl. fly ( ) , Dog. dig ( ) ). However, some of these functions may be common to all the subclasses of a class (e.g., Vertebrate.eat(), Mammal .suckle(), Primate.peel()). Such functions are likely to be declared virtual in these base classes, and then overridden in their subclasses for specific implementations. If a virtual function is certain to be overridden in all of its subclasses, then there is no need to implement it at all in its base class. This is done by making the virtual function pure. A pure virtual member function is a virtual function that has no implementation in its class. The syntax for specifying a pure virtual member function is to insert the initializer = o ; in place of the functions body, like this:

virtual int f() =O;

For example, in the Vertebrate class above, we might decide that the eat ( > function would be overridden in every one of its subclasses, and thus declare it as a pure virtual member function within its Vertebrate base class:

class Vertebrate { public: virtual void eat0 =O; 1;