Inheritance

“the mechanism by which one class acquires

the properties of another class”
 Arrange concepts into an
inheritance hierarchy
• Concepts at higher levels are more general
• Concepts at lower levels are more specific (inherit
properties of concepts at higher levels)

Vehicle

Wheeled vehicle Boat

Car Bicycle

2-door 4-door
 C++ and inheritance
• The language mechanism by which one class
acquires the properties (data and operations)
of another class
• Base Class (or superclass): the class being
inherited from
• Derived Class (or subclass): the class that
inherits
 Advantages of inheritance
• When a class inherits from another class,
there are three benefits:
• (1) You can reuse the methods and data of
the existing class
(2) You can extend the existing class by
adding new data and new methods
(3) You can modify the existing class by
overloading its methods with your own
implementations
Deriving One Class from Another
 Deriving One Class from Another (cont’d)

• Define a new class CountedQue from QueType

such that it has a new data member (length) that
records the number of items in the queue
template<class ItemType>
class CountedQue : public QueType<ItemType> {
public:
CountedQue();
void Enqueue (ItemType newItem);
void Dequeue (ItemType& item);
int LengthIs() const;
private:
int length;
};
 Inheritance and accessibility

• A class inherits the behavior of another

class and enhances it in some way
• Inheritance does not mean inheriting
access to another class’ private members
Rules for building a class hierarchy
• Derived classes are special cases of base classes
• A derived class can also serve as a base class for new
classes.
• There is no limit on the depth of inheritance allowed
in C++ (as far as it is within the limits of your
compiler)
• It is possible for a class to be a base class for more
than one derived class
 Modifying class behavior
template<class ItemType>
void CountedQue<ItemType>::Enqueue(ItemType newItem)
{
length++;
QueType<ItemType>::Enqueue(newItem);
}

template<class ItemType>
void CountedQue<ItemType>::Dequeue(ItemType& item)
{
length--;
QueType<ItemType>::Dequeue(item);
}
template<class ItemType>
int CountedQue<ItemType>::LengthIs() const
{
return length;
}
// class constructor
template<class ItemType>
CountedQue<ItemType>::CountedQue() : QueType<ItemType>()
{
length=0;
}
 Polymorphism
• Any code you write to manipulate a base class will
also work with any class derived from the base class.
• C++ general rule for passing objects to a function:
“the actual parameters and their corresponding
formal parameters must be of the same type”
• With inheritance, C++ relaxes this rule:
“the type of the actual parameter can be a class
derived from the class of the formal parameter”
 An example
template<class ItemType>
void Test(QueType& q, ItemType item)
{
q.Enqueue(item);
....
}
• Any object of a class derived from QueType can be
passed to the function !!
• Which Enqueue() function should be used? (the
compiler does not know that at compile time)
 Static vs. dynamic binding

• Static Binding: the determination of which

method to call at compile time
• Dynamic Binding: the determination of which
method to call at run time
 Virtual Functions
• C++ uses virtual functions to implement run-
time binding.

• To force the compiler to generate code that

guarantees dynamic binding, the word virtual
should appear before the function
declaration in the definition of the base class.
 Queue Implementation
template<class ItemType> private:
class QueueType { int front;
public:
QueueType(int);
int rear;
QueueType(); ItemType* items;
~QueueType(); int maxQue;
void MakeEmpty(); };
bool IsEmpty() const;
bool IsFull() const;
virtual void Enqueue(ItemType);
virtual void Dequeue(ItemType&);
 Virtual Functions (cont.)

• Rules for static/dynamic binding:

1) If the member function of the base class is not
a virtual function, the type of the formal
parameter determines which function to call.
2) If the member function of the base class is a
virtual function, the type of the actual
parameter determines which function to call.
 An example
class ItemType {
public:
...
virtual bool operator<(ItemType) const;
private: protected:
StrType lastName;
};

bool ItemType::operator<(ItemType item) const

{
int result;
result = strcmp(lastName, item.lastName);
if(result < 0)
return true;
else
return false;
}
Let's derive a new class from it:
class NewItemType : public ItemType {
public:
...
bool operator<(NewItemType) const;
private:
StrType firstName;
};
Let's derive a new class from it: (cont.)
bool NewItemType::operator<(NewItemType item) const
{
int result;
result = strcmp(lastName, item.lastName);
if(result < 0)
return true;
else if(result > 0)
return false;
else { // same last name
result = strcmp(firstName, item.firstName);
if(result < 0)
return true;
else
return false;
}
}
Let's assume that the client program
includes the following function:

void PrintResult(ItemType& first, ItemType& second)

{
if(first < second) // first.operator<(second)
cout << "First comes before second";
else
cout << "First does not come before second";
}
Let's assume that the client program
executes the following code:

ItemType item1, item2;

NewItemType item3, item4;

....

PrintResult(item1, item2);
PrintResult(item3, item4);
 Protected class members
• Derived classes cannot access the private data
of the base class
• Declaring methods and data of the base class
as protected (instead of private) allows
derived classes to access them
• Objects outside the class, however, cannot
access them (same as private)
 Warning: call by reference
vs. call by value
• If the object of the derived class is passed by
reference, everything works fine.

• If the object of the derived class is passed by

value, only the sub-object of the base class is
passed to the function (slicing problem)!!
 Protected and Private Inheritance
class X : protected Y { Y
...
}; X

• With protected inheritance, public and protected

members of Y become protected in X (i.e., classes
derived from X inherit the public members of Y as
protected)
• With private inheritance, public and protected
members of Y become private in X (i.e., classes
derived from X inherit the public members of Y as
private)
• Default inheritance: private
 Constructors and destructors
• You cannot override a base class constructor with a
derived class constructor (rather, the derived class
constructor calls the base class constructor first)
• All base class destructors should be declared virtual
• Virtual destructors are called in reverse order from
the constructors for derived class objects
 Multiple Inheritance
• Derived classes can inherit from more than
one base classes
X (base for Y)

Y (base for Z)

Z
 Example

Define a new class LookAheadStack that is

derived from class StackType.

(1) A look-ahead stack differs from the standard stack

only in the push operation.

(2) An item is added to the stack by the push method

only if its different from the top stack element.
template<class ItemType>
struct NodeType;

template<class ItemType>
class StackType {
public:
StackType();
~StackType();
void MakeEmpty();
bool IsEmpty() const;
bool IsFull() const;
void Push (ItemType);
void Pop(ItemType&);
private:
NodeType<ItemType>* topPtr;
};
template<class ItemType>
class LookAheadStack : public StackType<ItemType>
{
public:
void Push(ItemType);
LookAheadStack();
~LookAheadStack();
};
b) Implement the new push function and the
derived class’ constructor.
template<class ItemType>
void LookAheadStack <ItemType>::Push(ItemType newItem)
{
ItemType item;

if ( !StackType<ItemType>::IsEmpty() ) {
StackType<ItemType>::Pop(item);
StackType<ItemType>::Push(item);
if (item != newItem)
StackType<ItemType>::Push(newItem);
}
else
StackType<ItemType>::Push(newItem);
}

Constructor:
template<class ItemType>
LookAheadStack <ItemType>:: LookAheadStack():StackType()
{
}
c) Which functions and from which class
should be declared as virtual?
The functions that should be declared as
virtual are:

Push from base class (StackType)

Destructor from base class (StackType)