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Virtual functions

Posted on 1998-07-22
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Last Modified: 2013-11-18
#include <iostream.h>

class Animal
{
      public:
      virtual char* speak(int) {return "Silence";}
};
class Cat:public Animal
{
      public:
      virtual char* speak(int) {return "Meow";}
};
class Dog:public Animal
{
      public:
      virtual char* speak(short) {return "Bow-wow";}
};

int main()
{
      Animal *p = new Dog;
      cout << p->speak( 0 ) << endl;
      return 0;
}

The speech of dogs is supposed to be 'Bow-wow' but instead the program prints 'silence'.Why did the dog not bark?
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Comment
Question by:sreejesh
6 Comments
 
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Accepted Solution

by:
alexo earned 20 total points
ID: 1168262
Because of different signatures.
Explanation comming...
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LVL 11

Expert Comment

by:alexo
ID: 1168263
In C, a function is known by its name.  In C++, a function is known by its "signature", which is the name and the types of the arguments.  Therefore f(int) and f(short) are two different functions.

Now, for your example:

        Animal *p = new Dog;

Although p points to Dog, its static type is Animal*.

        /* ... */    p->speak( 0 )    /* ... */

The compiler does not know what actual object p will point to at run-time.  So, it generates code to call the virtual function whose signature is known at compile-time, based on the information it knows about p.

        class Animal
        {
        public:
            virtual char* speak(int) {return "Silence";}
        };

Since p is a pointer to Animal, the compiler will (virtually) call the speak(int) function.

        class Dog:public Animal
        {
        public:
            virtual char* speak(short) {return "Bow-wow";}
        };

p points to a Dog object.  However, the Dog object does not define a speak(int) finction (the speak(short) function has a different signature and thus considered a different function).  Therefore, the inherited speak(int) function of the superclass (Animal) is called.

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LVL 22

Expert Comment

by:nietod
ID: 1168264
By the way, A nice thing about the Borland C++ compiler  (there aren't many) is that it warns you when you do this.  Most compilers don't warn you since this is legal C++.  However, almost no one every really wants to do this on purpose, so Borland has enough sense to warn you.
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LVL 3

Expert Comment

by:danny_pav
ID: 1168265
this looks like one of those PC-lint bug of the month ads.
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Author Comment

by:sreejesh
ID: 1168266
Alexo, You pointed out the correct answer. Congrats ! and Thank You. Same to nietod and danny_pav also.
0
 
LVL 11

Expert Comment

by:alexo
ID: 1168267
>> this looks like one of those PC-lint bug of the month ads.
Bingo!  No wonder the "Bow-wow" looked familiar...
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