OOP

Interview

Basic concepts

What is Object Oriented Programming?

Object Oriented Programming (OOP) is a programming paradigm where the complete software operates as a bunch of objects talking to each other. An object is a collection of data and methods that operate on its data.

Why OOP?

The main advantage of OOP is better manageable code that covers following.
1. The overall understanding of the software is increased as the distance between the language spoken by developers and that spoken by users.
2. Object orientation eases maintenance by the use of encapsulation. One can easily change the underlying representation by keeping the methods same.

OOP paradigm is mainly useful for relatively big software. See this for a complete example that shows advantages of OOP over procedural programing.

What are main features of OOP?

Encapsulation
Polymorphism
Inheritance

• What is encapsulation?
  Encapsulation is referred to one of the following two notions.
  1) Data hiding: A language feature to restrict access to members of an object. For example, private and protected members in C++.
  2) Bundling of data and methods together: Data and methods that operate on that data are bundled together.

• What is Polymorphism? How is it supported by C++?
  Polymorphism means that some code or operations or objects behave differently in different contexts. In C++, following features support polymorphism.

  • Compile Time Polymorphism: Compile time polymorphism means compiler knows which function should be called when a polymorphic call is made. C++ supports compiler time polymorphism by supporting features like templates, function overloading and default arguments.

  • Run Time Polymorphism: Run time polymorphism is supported by virtual functions. The idea is, virtual functions are called according to the type of object pointed or referred, not according to the type of pointer or reference. In other words, virtual functions are resolved late, at runtime.

What is Inheritance? What is the purpose?

The idea of inheritance is simple, a class is based on another class and uses data and implementation of the other class.
The purpose of inheritance is Code Reuse.

What is Abstraction?

The first thing with which one is confronted when writing programs is the problem. Typically we are confronted with “real-life” problems and we want to make life easier by providing a program for the problem.

However, real-life problems are nebulous and the first thing we have to do is to try to understand the problem to separate necessary from unnecessary details: We try to obtain our own abstract view, or model, of the problem. This process of modeling is called abstraction.

C++ Concepts

What are the differences between C and C++?

1. C++ is a kind of superset of C.
2. C is a procedural programming language, but C++ supports both procedural and Object Oriented programming.
3. Since C++ supports object oriented programming, it supports features like function overloading, templates, inheritance, virtual functions, friend functions. These features are absent in C.
4. C++ supports exception handling at language level, in C exception handling is done in traditional if-else style.

5. C++ supports references, C doesn’t. (C里面 函数声明是参数是pointer， 调用函数时，参数取地址 addrss of operator)

   void fun(int *x, int *y){ /* ... */ }
   fun(&a, &b);
   

6. In C, scanf() and printf() are mainly used input/output. C++ mainly uses streams to perform input and output operations. cin is standard input stream and cout is standard output stream.

What are the differences between references and pointers?

Similarities:

Both references and pointers can be used to change local variables of one function inside another function. Both of them can also be used to save copying of big objects when passed as arguments to functions or returned from functions, to get efficiency gain.

Differences:

• References are less powerful than pointers
  
  1. Once a reference is created, it cannot be later made to reference another object; it cannot be reseated. This is often done with pointers.
  2. References cannot be NULL. Pointers are often made NULL to indicate that they are not pointing to any valid thing.
  3. A reference must be initialized when declared. There is no such restriction with pointers

Due to the above limitations, references in C++ cannot be used for implementing data structures like Linked List, Tree, etc. In Java, references don’t have above restrictions, and can be used to implement all data structures. References being more powerful in Java, is the main reason Java doesn’t need pointers.

• References are safer and easier to use:
  
  1. Safer: Since references must be initialized, wild references like wild pointers are unlikely to exist. It is still possible to have references that don’t refer to a valid location (See questions 5 and 6 in the below exercise )
  2. Easier to use: References don’t need de-referencing operator(*) to access the value. They can be used like normal variables. ‘&’ operator(address-of operator) is needed only at the time of declaration. Also, members of an object reference can be accessed with dot operator (‘.’), unlike pointers where arrow operator (->) is needed to access members.

What are virtual functions – Write an example?

Virtual functions are used with inheritance, they are called according to the type of object pointed or referred, not according to the type of pointer or reference. In other words, virtual functions are resolved late, at runtime. Virtual keyword is used to make a function virtual.
Following things are necessary to write a C++ program with runtime polymorphism (use of virtual functions)
1) A base class and a derived class.
2) A function with same name in base class and derived class.
3) A pointer or reference of base class type pointing or referring to an object of derived class.

#include<iostream>
using namespace std;
class Base {
public:
    virtual void show() { cout<<" In Base \n"; }
};
class Derived: public Base {
public:
    void show() { cout<<"In Derived \n"; } 
};
int main(void) {   
    Base *bp = new Derived;     
    bp->show();  // RUN-TIME POLYMORPHISM
    return 0;
}

Output:

In Derived

For example, in the following program bp is a pointer of type Base, but a call to bp->show() calls show() function of Derived class, because bp points to an object of Derived class.

What is this pointer?

The ‘this’ pointer is passed as a hidden argument to all nonstatic member function calls and is available as a local variable within the body of all nonstatic functions. ‘this’ pointer is a constant pointer that holds the memory address of the current object. ‘this’ pointer is not available in static member functions as static member functions can be called without any object (with class name).

Can we do “delete this”?

See http://www.geeksforgeeks.org/delete-this-in-c/

What are VTABLE and VPTR?

• vtable is a table of function pointers. It is - maintained per class.
• vptr is a pointer to vtable. It is maintained per object.

Compiler adds additional code at two places to maintain and use vtable and vptr.

1. Code in every constructor. This code sets vptr of the object being created. This code sets vptr to point to vtable of the class.
2. Code with polymorphic function call (e.g. bp->show() in above code). Wherever a polymorphic call is made, compiler inserts code to first look for vptr using base class pointer or reference (In the above example, since pointed or referred object is of derived type, vptr of derived class is accessed). Once vptr is fetched, vtable of derived class can be accessed. Using vtable, address of derived derived class function show() is accessed and called.