Support for Object-Oriented Programming (OOP) in C++

Support for OOP in C++
▸ General Characteristics:
1- Evolved from SIMULA 67.
• Simula is a name for two simulation languages,
Simula I and Simula 67, Developed in the 1960s, by
Ole-Johan Dahl and Kristen Nygaard.
• Simula is considered the first Object-Oriented
Programming language.
• Simula provided the framework for many of the
OOP languages today.
2- Most widely used OOP language.
2

3
3- Mixed typing system (Retains C types, adds classes).
• Static typing: A variable has a single type associated with it throughout it is life at run time (Types of all
variables/expressions are determined at compile time.
• Dynamic typing: Allow the type of a variable. As well as it is value, to change as the program runs.
4- Constructors and Destructors (Implicitly called when objects are created/cease to
exist).
• Constructor is a special member function of a class that is executed whenever we create new objects of
that class.
A constructor will have exact same name as the class and it does not have any return type at all, not even
void. Constructors can be very useful for setting initial values for certain member variables.
• Destructor is a special member function that is called when the lifetime of an object ends.
The purpose of the destructor is to free the resources that the object may have acquired during its lifetime.

4
5- Elaborate access controls to class entities.
Private Protected Public
Same Class YES YES YES
Derived Class NO YES YES
Friend Class YES YES YES
Other class NO NO YES
Access
ModifiersAccess
Locations
“Access controls”

5
 Inheritance
• A class need not be the subclass of any class
• Access controls for members are
– Private (visible only in the class and friends) (disallows subclasses from being subtypes)
– Public (visible in subclasses and clients)
– Protected (visible in the class and in subclasses, but not clients)

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Public base classes in C++ has the class declaration:
class < derived > : public < base >
{
< member-declarations >
};
Private base classes in C++ has the class declaration:
class < derived > : private < base >
{
< member-declaration >
};

7
Inheritance example in C++ :
class base_class
{
private:
int a;
float x;
protected:
int b;
float y;
public:
int c;
float z;
};
class subclass_1 : public base_class { … };
//b and y are protected and c and z are public
class subclass_2 : private base_class { … };
//b, y, c, and z are private, and no derived class of
//subclass_2 has access to any member of base_class
//Note that a and x are not accessible in either
//subclass_1 or subclass_2
class subclass_3 : private base_class {
base_class :: c; }
//Instances of subclass_3 can access c.
An object is
an instance of a class,
and may be called a
class instance or
class object.

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In addition, the subclassing process can be declared with access controls (private or public), which define
potential changes in access by subclasses.
> Private derivation: inherited public and protected members are private in the subclasses (Does
not represent an is-a relationship (Inheritance).
> Public derivation: public and protected members are also public and protected in subclasses.
 In Private derivation:
- By default, all members inherited from < base > become private members of <derived > .
• Privacy principle:
- The private members of a class are accessible only to member functions of the class.
- Functions in a derived class cannot access the private members of it’s base class.

9
Multiple inheritance is supported
> If there are two inherited members with the same name, they can both be referenced using
the scope resolution operator.
> Multiple inheritance allows a new class to inherit from two or more classes.
class A { … };
class B : public A { … };
class C : public A { … };
class D : public B, public C { … };

10
Common problem with multiple Inheritance
> Diamond Problem
Is an ambiguity that arises when two classes B and C inherited from class A, and class D inherited
from both class B and class C.
If a method in D calls a method defined in A , and B and C overridden that method differently,
then from which class does it inherit: B or C?
D
B C
A

11
 Dynamic Binding
> Method can be defined to be virtual, which means that they can be called
through polymorphic variables and dynamically bound to messages.
> A pure virtual function has no definition at all.
“It cannot be called, unless it is redefined in the derived class.”
> A class that has at least one pure virtual function is an abstract class.
“An abstract class cannot be instantiated.”

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class shape
{
public:
virtual void draw()=0;
};
Dynamic Binding Example
class rectangle : public shape
{
public:
void draw()
{
cout<<"rect n";
}
};
class square : public rectangle
{
public:
void draw()
{
cout<<"square n";
}
};
virtual void draw()=0;
“=0” in function definition
indicates a pure virtual
function.

13
Dynamic Binding Example
int main()
{
square *sq = new square;
rectangle *rect = new rectangle;
shape *ptr_shape = sq;
ptr_shape -> draw(); //Square
rect ->draw(); //Rect
rect = sq;
rect ->draw(); //Square
square sq2;
rectangle r2 = sq2;
r2.draw(); //Rect
}
Even thought it
contains a square

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 Evaluation
> C++ provides extensive access controls (unlike other OO language such
as Smalltalk).
> C++ provides multiple inheritance.
> In C++, the programmer must decide at design time which methods
will be statically bound and which must be dynamically bound.
- Static binding is faster!
- Design Decision may be wrong, requiring change later.
- Dynamic binding in C++ is faster than Smalltalk.

14
Static binding
The choice of
which function to
call in a class that
employs
inheritance is made
at compile time.
Dynamic binding
The choice is
made at run
time.
“If you use dynamic binding, The program will look up which function to use in a virtual function table,
which takes a little time, making dynamic binding slower.”

THANKS!
Any questions?
Prepared by: Ameen Shaarawi
ameencom4u@gmail.com

Support for Object-Oriented Programming (OOP) in C++

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Support for Object-Oriented Programming (OOP) in C++