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Two function calls p- > f ( > are made. Both calls invoke the same version of f ( > that is defined in the base class x because p is declared to be a pointer to x objects. Having p point to y has no effect on the second call p- > f ( ) . Transform X: : f ( > into a virtualfinction by adding the keyword virtual" to its declaration:
class X { public:
virtual void
tout
"X::f()
executing\n";
With the rest of the code left unchanged, the output now becomes
Now the second call p- > f ( ) invokes
: :f() insteadof x: :f (>.
This example illustrates polymorphism: the same call p- >f ( > invokes different functions. The function is selected according to which class of object p points to. This is called dynamic binding because the associatioti (i.e., binding) of the call to the actual code to be executed is
CHAP. 111
COMPOSITION AND INHERITANCE
deferred until run time. The rule that the pointer s statically defined type determines which member function gets invoked is overruled by declaring the member function virtual. Here is a more realistic example: .
EXAMPLE 11.11 Polymorphism through virtual Functions
Here is a Person class with a Student class Person { public: Person(char*
subclass and a Professor subclass:
s) { name = new char[strlen(s+l)];
strcpy(name, s);
void print0 protected: char* name; >;
{ tout << "My name is ' CC name << ".\n"; }
class Student : public Person { public: Student(char* s, float g) : Person(s), gpa(g) { } void print0 { tout << "My name is ' << name << ' and my G.P.A. is ' << gpa CC ".\n"; } private: float gpa; >; class Professor : public Person { public: Professor(char* s, int n) : Person(s), pubis(n) { } void print0 { tout CC "My name is ' CC name << ' and I have ' << publs << ' publications.\n"; private: int publs; >; main0 -t Person* p; Person x("Bob") ; p = &Lx; p->print(); Student y("Tom", 3.47); P = &y; p->print(); Professor z("Ann", 7); p = &z; p->print();
COMPOSITION AND INHERITANCE
[CHAP. 11
always The print ( ) function defined in the base class is not virtual. So the call p->print ( ) :print ( ) because p has type Person*. The pointer invokes that same base class function Person : p is statically bound to that base class function at compile time. Now change the base class function Person : :print ( ) into a virtual function, and run the same program:
class Person { public: Person(char*
s) { name = new
char[strlen(s+l)];
strcpy(name, s);
virtual void print0 protected: char* name;
{ tout << 'My name is ' << name << '.\n"; }
Now the pointer p is dynamically bound to the print ( ) function of whatever object it points to. So invokes the base class function Person : : print ( ) , the second call the first call p->print() invokes the derived class function student : : print ( ) , and the third call invokes the derived class is polymorphic because its function Professor: :print (>. We say that the call p->print() meaning changes according to circumstance.
In general, a member function should be declared as virtual whenever it is anticipated that at least some of its subclasses will define their own local version of the function.
11.8 VIRTUAL DESTRUCTORS
Virtual functions are overridden by functions that have the same signature and are defined in subclasses. Since the names of constructors and destructors involve the names of their different classes, it would seem that constructors and destructors could not be declared virtual. That is indeed true for constructors. However, an exception is made for destructors. Every class has a unique destructor, either defined explicitly within the class definition or implicitly by the compiler. An explicit destructor may be defined to be virtual. The following example illustrates the value in defining a virtual destructor:
EXAMPLE 11.12 Memory Leak This program is similar to Example 11.6:
class X { public:
x0 1 p = new int[2];
-X() { delete [] p; private: int* p; >;
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