Chapter 15

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Chapter 15

Polymorphism and Virtual Functions

This activity contains 10 questions.

Which of the following best describes what a virtual function is?

A virtual function is one that really does not exist; it is a placeholder for another function that will implement the virtual function.A virtual function refers to a function that we inherit automatically from a base class.A virtual function uses memory stored on disk and treats it as physical RAM.A virtual function is used when the same function name is used in different levels of a class hierarchy. The computer will invoke the function associated with an object at runtime instead of at compile time.

Which of the following is not another term that is used for the practice of using virtual functions?

overridingdynamic or late bindingpolymorphismredefining

What will the following code output when run?


// Animal class
class Animal
{
private:
	string type;
	int age;
	string intToString(int i);
public:
	Animal();
	Animal(string newType, int newAge);
	string getDescription();
	virtual string getNoise();
};

Animal::Animal()
{
	age=0; type="Unknown";
}

Animal::Animal(string newType, int newAge)
{
	age= newAge; type = newType;
}

// Converts an integer to a string
string Animal::intToString(int i)
{
	stringstream ss;
	ss << i;
	return (ss.str());
}

string Animal::getDescription()
{
	string sAge = intToString(age);
	return string("Animal type: ") + type +
		string(". Age: ") + sAge +
		string(". Noise made: ") + getNoise();
}

string Animal::getNoise()
{
	return(string("Unknown"));
}

// Cow class
class Cow : public Animal
{
private:
	string breed;
public:
	Cow();
	Cow(int newAge, string newBreed);
	string getBreed();
	string getDescription();
	string getNoise();
};

Cow::Cow() : Animal(), breed("")
{
}

Cow::Cow(int newAge, string newBreed)
  :Animal("cow", newAge), breed(newBreed) 
{
}

string Cow::getBreed()
{
	return breed;
}

string Cow::getDescription()
{
	return Animal::getDescription() + 
		". Breed: " + breed;
}

string Cow::getNoise()
{
	return (string("Moo"));
}

// Main
int main()
{
	Cow bessie(4, "holstein");	
	cout << bessie.getDescription() << endl;
	return 0;
}

This code will give a compiler error message because getNoise is defined but not implemented in the Animal class.. Breed: HolsteinAnimal type: cow. Age: 4. Noise made: Unknown. Breed: holsteinAnimal type: cow. Age: 4. Noise: Moo: Unknown. Breed: Holstein

In the following class definitions what will the following code output when run?


// Animal class
class Animal
{
private:
	string type;
	int age;
	string intToString(int i);
public:
	Animal();
	Animal(string newType, int newAge);
	virtual string getDescription();
	virtual string getNoise();
};

// Cow class
class Cow : public Animal
{
private:
	string breed;
public:
	Cow();
	Cow(int newAge, string newBreed);
	string getBreed();
	virtual string getDescription();
	virtual string getNoise();
};

Why is "virtual" used in the Cow class for getDescription() and getNoise()?

If virtual was left off in the derived class then these methods would redefine the methods in the base class instead of override the methods in the base class.Virtual is required in the derived class since these methods are virtual in the base class.Virtual is optional because the virtual property is inherited. However, including the keyword makes it clear that these are virtual functions.The methods should be declared virtual in the derived class, but this modifier could be omitted in the base class.

What will the following code output when run?


// Animal class
class Animal
{
private:
	string type;
	int age;
	string intToString(int i);
public:
	Animal();
	Animal(string newType, int newAge);
	string getDescription();
	virtual string getNoise();
};

Animal::Animal()
{
	age=0; type="Unknown";
}

Animal::Animal(string newType, int newAge)
{
	age= newAge; type = newType;
}

// Converts an integer to a string
string Animal::intToString(int i)
{
	stringstream ss;
	ss << i;
	return (ss.str());
}

string Animal::getDescription()
{
	string sAge = intToString(age);
	return string("Animal type: ") + type +
		string(". Age: ") + sAge +
		string(". Noise made: ") + getNoise();
}

string Animal::getNoise()
{
	return(string("Unknown"));
}

// Cow class
class Cow : public Animal
{
private:
	string breed;
public:
	Cow();
	Cow(int newAge, string newBreed);
	string getBreed();
	string getDescription();
	string getNoise();
};

Cow::Cow() : Animal(), breed("")
{
}

Cow::Cow(int newAge, string newBreed)
  :Animal("cow", newAge), breed(newBreed) 
{
}

string Cow::getBreed()
{
	return breed;
}

string Cow::getDescription()
{
	return Animal::getDescription() + 
		". Breed: " + breed;
}

string Cow::getNoise()
{
	return (string("Moo"));
}

// Main
int main()
{
	Animal pet;
	Cow bessie(4, "holstein");	
	pet = bessie;
	cout << pet.getDescription() << endl;
	return 0;
}

Animal type: cow. Age: 4. Noise made: MooAnimal type: cow. Age: 4. Noise made: UnknownThis program will not compile because the types are incompatible. The line pet = bessie will fail because the types do not match.Animal type: cow. Age: 4. Noise made: Moo. Breed: holstein.

What will the following program output when run?


// Animal class
class Animal
{
private:
	string type;
	int age;
	string intToString(int i);
public:
	Animal();
	Animal(string newType, int newAge);
	string getDescription();
	virtual string getNoise();
};

Animal::Animal()
{
	age=0; type="Unknown";
}

Animal::Animal(string newType, int newAge)
{
	age= newAge; type = newType;
}

// Converts an integer to a string
string Animal::intToString(int i)
{
	stringstream ss;
	ss << i;
	return (ss.str());
}

string Animal::getDescription()
{
	string sAge = intToString(age);
	return string("Animal type: ") + type +
		string(". Age: ") + sAge +
		string(". Noise made: ") + getNoise();
}

string Animal::getNoise()
{
	return(string("Unknown"));
}

// Cow class
class Cow : public Animal
{
private:
	string breed;
public:
	Cow();
	Cow(int newAge, string newBreed);
	string getBreed();
	string getDescription();
	string getNoise();
};

Cow::Cow() : Animal(), breed("")
{
}

Cow::Cow(int newAge, string newBreed)
  :Animal("cow", newAge), breed(newBreed) 
{
}

string Cow::getBreed()
{
	return breed;
}

string Cow::getDescription()
{
	return Animal::getDescription() + 
		". Breed: " + breed;
}

string Cow::getNoise()
{
	return (string("Moo"));
}

// Main
int main()
{
	Animal pet("cow",4);
	Cow bessie;	
	bessie = pet;
	cout << bessie.getDescription() << endl;
	return 0;
}

Animal type: cow. Age: 4. Noise made: MooThis program will not compile because the types are incompatible. The line bessie = pet will fail because the types do not match.Animal type: cow. Age: 4. Noise made: Moo. Breed:.Animal type: cow. Age: 4. Noise made: Unknown

What is the output of the following program?


// Animal class
class Animal
{
private:
	string type;
	int age;
	string intToString(int i);
public:
	Animal();
	Animal(string newType, int newAge);
	virtual string getDescription();
	virtual string getNoise();
};

Animal::Animal()
{
	age=0; type="Unknown";
}

Animal::Animal(string newType, int newAge)
{
	age= newAge; type = newType;
}

// Converts an integer to a string
string Animal::intToString(int i)
{
	stringstream ss;
	ss << i;
	return (ss.str());
}

string Animal::getDescription()
{
	string sAge = intToString(age);
	return string("Animal type: ") + type +
		string(". Age: ") + sAge +
		string(". Noise made: ") + getNoise();
}

string Animal::getNoise()
{
	return(string("Unknown"));
}

// Cow class
class Cow : public Animal
{
private:
	string breed;
public:
	Cow();
	Cow(int newAge, string newBreed);
	string getBreed();
	virtual string getDescription();
	virtual string getNoise();
};

Cow::Cow() : Animal(), breed("")
{
}

Cow::Cow(int newAge, string newBreed)
  :Animal("cow", newAge), breed(newBreed) 
{
}

string Cow::getBreed()
{
	return breed;
}

string Cow::getDescription()
{
	return Animal::getDescription() + 
		". Breed: " + breed;
}

string Cow::getNoise()
{
	return (string("Moo"));
}

// Main
int main()
{	
	Cow *pBessie = new Cow(4,"jersey");
	Animal *pAnimal = pBessie;
	cout << pAnimal->getDescription() << endl;
	delete pAnimal;	
	return 0;
}

Animal type: cow. Age: 4. Noise made: MooAnimal type: cow. Age: 4. Noise made: Moo. Breed: jerseyAnimal type: cow. Age: 4. Noise made: UnknownThis program will not compile because the types are incompatible. The line Animal *pAnimal = pBessie will fail because the types do not match.

What will the following program output? Note the cout statements in the destructors of both classes.


// Animal class
class Animal
{
private:
	string type;
	int age;
	string intToString(int i);
public:
	Animal();
	Animal(string newType, int newAge);
	~Animal();
	virtual string getDescription();
	virtual string getNoise();
};

Animal::Animal()
{
	age=0; type="Unknown";
}

Animal::Animal(string newType, int newAge)
{
	age= newAge; type = newType;
}

Animal::~Animal()
{
	cout << "Animal destructor" << endl;
}

// Converts an integer to a string
string Animal::intToString(int i)
{
	stringstream ss;
	ss << i;
	return (ss.str());
}

string Animal::getDescription()
{
	string sAge = intToString(age);
	return string("Animal type: ") + type +
		string(". Age: ") + sAge +
		string(". Noise made: ") + getNoise();
}

string Animal::getNoise()
{
	return(string("Unknown"));
}

// Cow class
class Cow : public Animal
{
private:
	string breed;
public:
	Cow();
	Cow(int newAge, string newBreed);
	~Cow();
	string getBreed();
	virtual string getDescription();
	virtual string getNoise();
};

Cow::Cow() : Animal(), breed("")
{
}

Cow::Cow(int newAge, string newBreed)
  :Animal("cow", newAge), breed(newBreed) 
{
}

Cow::~Cow()
{
	cout << "Cow destructor" << endl;
}

string Cow::getBreed()
{
	return breed;
}

string Cow::getDescription()
{
	return Animal::getDescription() + 
		". Breed: " + breed;
}

string Cow::getNoise()
{
	return (string("Moo"));
}

// Main
int main()
{	
	Cow *pBessie = new Cow(4,"jersey");	
	Animal *pAnimal = pBessie;
	delete pAnimal;	
	return 0;
}

Cow destructorCow destructor
Animal destructorAnimal destructor
Cow destructorAnimal destructor

What will the following program output? Note the cout statements in the destructors of both classes.


// Animal class
class Animal
{
private:
	string type;
	int age;
	string intToString(int i);
public:
	Animal();
	Animal(string newType, int newAge);
	virtual ~Animal();
	virtual string getDescription();
	virtual string getNoise();
};

Animal::Animal()
{
	age=0; type="Unknown";
}

Animal::Animal(string newType, int newAge)
{
	age= newAge; type = newType;
}

Animal::~Animal()
{
	cout << "Animal destructor" << endl;
}

// Converts an integer to a string
string Animal::intToString(int i)
{
	stringstream ss;
	ss << i;
	return (ss.str());
}

string Animal::getDescription()
{
	string sAge = intToString(age);
	return string("Animal type: ") + type +
		string(". Age: ") + sAge +
		string(". Noise made: ") + getNoise();
}

string Animal::getNoise()
{
	return(string("Unknown"));
}

// Cow class
class Cow : public Animal
{
private:
	string breed;
public:
	Cow();
	Cow(int newAge, string newBreed);
	virtual ~Cow();
	string getBreed();
	virtual string getDescription();
	virtual string getNoise();
};

Cow::Cow() : Animal(), breed("")
{
}

Cow::Cow(int newAge, string newBreed)
  :Animal("cow", newAge), breed(newBreed) 
{
}

Cow::~Cow()
{
	cout << "Cow destructor" << endl;
}

string Cow::getBreed()
{
	return breed;
}

string Cow::getDescription()
{
	return Animal::getDescription() + 
		". Breed: " + breed;
}

string Cow::getNoise()
{
	return (string("Moo"));
}

// Main
int main()
{	
	Cow *pBessie = new Cow(4,"jersey");	
	Animal *pAnimal = pBessie;
	delete pAnimal;	
	return 0;
}

Cow destructor
Animal destructorAnimal destructorAnimal destructor
Cow destructorCow destructor

What will the following program output?


// Animal class
class Animal
{
private:
	string type;
	int age;
	string intToString(int i);
public:
	Animal();
	Animal(string newType, int newAge);
	virtual string getDescription();
	virtual string getNoise()=0;
};

Animal::Animal()
{
	age=0; type="Unknown";
}

Animal::Animal(string newType, int newAge)
{
	age= newAge; type = newType;
}

// Converts an integer to a string
string Animal::intToString(int i)
{
	stringstream ss;
	ss << i;
	return (ss.str());
}

string Animal::getDescription()
{
	string sAge = intToString(age);
	return string("Animal type: ") + type +
		string(". Age: ") + sAge +
		string(". Noise made: ") + getNoise();
}


// Cow class
class Cow : public Animal
{
private:
	string breed;
public:
	Cow();
	Cow(int newAge, string newBreed);
	string getBreed();
	virtual string getDescription();
	virtual string getNoise();
};

Cow::Cow() : Animal(), breed("")
{
}

Cow::Cow(int newAge, string newBreed)
  :Animal("cow", newAge), breed(newBreed) 
{
}

string Cow::getBreed()
{
	return breed;
}

string Cow::getDescription()
{
	return Animal::getDescription() + 
		". Breed: " + breed;
}

string Cow::getNoise()
{
	return (string("Moo"));
}

// Main
int main()
{	
	Cow bessie(4,"jersey");	
	Animal dog("dalmatian", 5);
	
	cout << bessie.getDescription() << endl;
	cout << dog.getDescription() << endl;
	return 0;
}

It will not compile because getNoise() is defined as a pure virtual function in class Animal. This makes the Animal class abstract, preventing the instantiation of a variable of type Animal.Animal type: cow. Age: 4. Noise made: moo. Breed:jersey
Animal type: dog. Age: 5. Noise made: moo.Animal type: cow. Age: 4. Noise made: moo. Breed:jersey
Animal type: dog. Age: 5. Noise made:It will not compile because there is a getNoise() method defined for class Animal, but no implementation of a getNoise() function for class Animal.

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