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C++ Interview Questions

1. How C++ differs from C?
2. Difference between Overloading and Overriding
3. What are inline functions and how they differ from macros?
4. Problems of Multiple Inheritance
5. Write a C++ program to swap two numbers without using third variable.
6. Write C++ program to compute Fibonacci series using iteration and using recursion.
7. What is :: operator?
8. What is pass by reference and pass by value?
9. How do you name a class destructor?
10. Write a linked list program.
11. What is STL?

C++ Vs Java

Compared to C++, Java is programmer efficient but not as much time and memory efficient as C++. Java is slow as bytecode is executed in JVM which is a system software. Java is not memory efficient because memory reclaim is done when decided by Garbage Collector rather than when it should have been actually done. Moreover, JVM and Garbage Collector take memory themselves.
Java is programmer efficient as he need not fear C++ pointers and can code more easily through exhaustive classes available.

A table of comparison of C++ with Java will be made available soon

Standard Template Library (STL)

  • Going by the name, Standard means part of standard C++, Template means the classes and functions contained in the STL are generic and in C++ terms they are template classes or template functions, and Library means collection of these classes and functions.
  • Originally, STL was conceived by HP and now supported by almost all C++ compilers that support ISO/ANSI C++.
  • STL contains 3 things:
- Container: May be sequential like list and vector. Or, associative like map and set
- Iterator: For accessing elements of a container. May be forward only, bi-directional, random access, output iterators and input iterators.
- Algorithms: To perform functions on elements contained in a container. Examples are: find, merge, sort etc.

Points to Ponder
  • STL Algorithms are generic and not member function or friend function of any class. So these algorithms can be used with non STL containers also like you can use sort() for C++ arrays.
  • Although, STL is quite useful, efficient, and robust but your compiler may report some warnings etc. which can be handled easily like turning off warnings, not using pre-compiled headers etc.
Interested readers can either request me a complete tutorial on STL by emailing me at:
Or, wait for detailed post later.

Operator Overloading

  • Operator overloading feature makes C++ an extensible language
  • You can overload the built-in operators of C++ for user-defined data-types or combination of user-defined and built-in types. For e.g. using strcat(str1,str2) is not that intuitive as res = str1+str2 i.e. overloaded '+' for string concatenation.
  • However, you can overload most of the operators but not all.
  • By overloading an operator, you cannot change its semantics, associativity, and precedence. For e.g., you cannot overload a binary operator to become unary.
  • Syntax for operator overloading
Keyword 'operator' is used with following syntax(es):

//Binary Operator Overloading as member function
returnDataType className::operator oprSymbol(class a);

//Unary Operator Overloading as member function
returnDataType className::operator oprSymbol();

//Binary Operator Overloading as friend function
friend returnDataType oprSymbol(class a, class b);

//Unary Operator Overloading as friend function
friend returnDataType oprSymbol(class a);

Points to Ponder

  • If a binary operator is overloaded as a member function then function takes 1 parameter and if as a friend function then 2 parameters
  • If a unary operator is overloaded as a member function then function takes no parameter and if as a friend function then 1 parameter
  • In operator overloading as a member function, one parameter is provided by the invoking class.

Which operators cannot be overloaded? Why the first parameter should be an object and not a built-in type? How to overloaded postfix and prefix versions of increment and decrement operators? To know answers to these and many other such issues in operator overloading, you may request a presentation file (ppt) on operator overloading by e-mailing me at: zainvi[dot]sf[at]gmail[dot]com

More On Classes

  • OOP encapsulates data(attributes) and functions(behaviors) into packages called classes.
  • A class is a user-defined data type that acts as a blueprint for instantiating any number of objects of that class.
  • Classes have property of information hiding i.e. a class know how to communicate with another class across well-defined interface, however, they are not allowed to know how they are implemented i.e. implementation is hidden.
  • C & other procedural languages are action-oriented. In C, unit of programming is function. Verbs in system specifications help in determining required set of functions.
  • C++ is object-oriented language. Unit of programming is classes from which objects are eventually instantiated. Nouns in system specifications help in identifying required classes.
  • C++ classes are natural evolution of C struct which is a collection of related variables whereas a class contains both variables (data members) and functions (member functions) that manipulate those data.
  • Member functions are also called operations or methods. Methods are invoked in response to messages sent to an object. A message corresponds to a member function call sent from one object to another or sent from a function to an object.
  • General syntax of a class is

Class className
access-specifier 1: ---------------- can be public, private or protected
className( ); //constructor
member function prototypes;
access-specifier 2:
data member declarations;

};--------------Forgetting this is a syntax error

  • Member Access Specifiers
  • Public Accessible where the program has access to an object of that class
  • Private Accessible only to member functions and to friends of that class
A member access specifier can be used anywhere and any number of times but it is better to keep all declarations at the top and use a member access specifier only once
Data members are usually private and member functions are public. However, this is not a hard and fast rule.
Public member functions are also called interface of the class.

  • A member function with the same name as the class.
  • It initializes the data members of a class object.
  • Called automatically when an object of that class is created.
  • There can be many constructors accomplished through function overloading.
  • Specifying a return type or a return value for a constructor is a syntax error.
  • Default Constructor is a constructor with no parameters
  • The default constructor takes no parameters, whether you declare it or get it free from the compiler
  • If you declare any constructor, the compiler will not provide a default constructor

  • Once the class has been defined, it can be used as a type in object, array, pointer or a reference.
Time sunset, arrayoftimes[5], *ptrToTime,&dTime=sunset <------- C++ is extensible
  • Data members may be private as they may be of no concern to class' clients. Thus, implementation is hidden and this information hiding promotes program modifiability and simplifies the clients' perception of a class.
  • Member functions are usually shorter than non-object oriented programs because the data stored in data members have ideally been validated by a constructor and/or by member functions that store new data. Because the data are already in the object, the member function calls often have no arguments or atleast few arguments than typical function calls in non object oriented languages. Thus, the calls are shorter, function definitions are shorter and function prototypes are shorter.
  • Attempting to initialize a data member of a class explicitly in a class definition is a syntax error.

  • A member function the same name as the class and with prefix ~ i.e. ~className( );
  • If no destructor is specified then the system 'plugs-in' one destructor.
  • The destructor does 'termination housekeeping' on each class object before the memory for the object is reclaimed by the system.
  • Destructors cannot take arguments and hence, cannot be overloaded.
  • Destructors have no return value.
  • Declaring member functions inside a class definition (via their function prototypes) & defining those member functions outside the class definition separates the interface from of a class from its implementation. This promotes good Software Engineering as it hides implementation from class' clients and also class' need not recompile if implementation changes; provided interface remains same. Member functions declared outside in a class definition can be defined outside class definition using binary scope resolution operator ::. If member function is defined outside then use of class name and :: is mandatory as it ties function to a class and two classes may have functions with same name. However, scope of such functions is that of class only.
  • Member functions defined in class definition are automatically inlined whereas those defined outside can be inlined using inline keyword. However, compiler reserves the right whether to inline a function or not.
  • From performance point of view, smaller functions can be defined in class definition for making inline but it is not good from Software Engineering point of view because it exposes code to class' clients and also clients need to recompile if inline function changes.
  • Only simplest and most stable member functions should be defined in class header. Member function calls generally take either no arguments or substantially fewer arguments than conventional function calls. This reduces likelihood of passing wrong arguments. Number of arguments are reduced because of encapsulation of data members and member functions within an object gives the member functions within an object gives member functions the right to access the data members.
  • Member functions are usually designed to be client-oriented rather than implementation oriented. OOPs imply re-use i.e. crafting valuable classes & creating valuable "software assests". Often classes need not be created from scratch. They may be derived from other classes that provide attributes and behaviors the new classes can use (inheritance) OR classes can include objects of other classes as members (aggregation or composition). Such software re-use can greatly enhance programmer's productivity.

Class Scope and Accessing Class Members
  • Logically, it may seem that objects may be quite larger because they contain data and functions.
  • Physically, this is not true. Applying sizeof to a class name or to an object of that class will report size of class' data only. This is because the compiler creates different copies of class' data for each object because data can vary among objects. However, the compiler creates only one copy of member functions to be shared by all objects of that class. It can be shared because function code is non-modifiable (also called re-entrant code or pure procedure). Data is placed in memory when objects are defined.

  • A class' data members and member functions belong to that class' scope
  • Non member functions have file scope
  • Within a class scope, class members are immediately accessible by all of that class' member functions and can be referenced by name. Outside class scope, class members are referenced through one of the handles on an object - an object name, a reference or a pointer to an object.
  • An implicit handle is inserted by t he compiler on every reference to a data member or member function in an object.
  • Member functions of a class can ONLY be overloaded by other member functions of the class. Include prototype for each overloaded function in class definition and provide a separate function definition for each version.
  • Variables defined in member functions have function scope.
  • If class data also contains variable with same name then this local variable will hide class scope variable. To access class scope variable, use :: operator. Hidden global variables can be accessed using unary scope resolution operator.
  • To access members of a class, use dot operator with object name or reference and arrow operator with pointer to object.

Separating Interface from Implementation

  • Keep class declaration in a header file to be included by any client who want to use the class
  • Implementation is kept in a separate source file
  • Clients don't need source code; they need only object code of class implementation. This way no proprietary information is revealed by provided class libraries.
  • However
- Inline functions are revealed
- Private members are not accessible but visible, thereby provide hint about implementation
  • Information that is internally used by class and not required by class' clients should be kept in separate unpublished files. <---Principle of Least Privilege
  • Use #ifndef, #define and #endif preprocessor directives to prevent header files from being included more than once in a program like
#ifndef TIME1_H // header file being time1.h
#define TIME1_H

Controlling Access to Members

  • Use access specifiers public, private and protected to enforce information hiding & the principle of least privilege
  • Don't use an access specifier multiple times
  • Keep public members first which acts as public interface
  • Private members can only be accessed by member functions or friend classes. All other accesses results in syntax error.
  • Client of a class can be any member function of other class or a global function.
  • In struct, all members are public by default whereas in class, they are public. Using explicit access specifier is a good practice.
  • 'Access functions (set/get)' are used to read or modify the value of private data members.

Advanatges of set functions
  • Provide data validation capabilities such as range checking to ensure that the value is set properly.
  • Translate between the form of data used in the interface and form used in implementation.

Advantages of get functions (also called query functions)
  • Need not expose the data in 'raw' format
  • Can edit the data and limit the view of the data the client will see
  • Get/set functions are not required for every private member
  • Making data member private and providing public member access functions facilitates debugging because problems with data manipulation are localized to either the class' member functions or the friends of the class.
  • Set/Get make the data public. It is good from software engineering point of view as get and set put required constraints
  • Such functions should also provide feedback to client for any wrong setting etc. //elaborate

Access Functions and Utility Functions
  • Not all member functions need to be public to serve as part of a class interface. Some of these can be private to serve as utility functions to other functions of the class.
  • Predicate functions test the truth or falsity of conditions e.g. isEmpty() or isTrue() for container class - class capable of holding many objects such as linked list, stack or queue
  • Utility functions also called helper functions support the operation of the class' public member functions. They are not intended to be used by clients of a class.
  • Once a class is defined, creating and manipulating objects of that class usually involves issuing only a simple sequence of member functions calls-few, if any, control structures are needed. By contrast, it is common to have control structures in the implementation of a class' member functions.

Initializing class objects: Constructors
  • It is good to initialize all data members of a class before using member function.
  • Such initialization can be done in default constructor, overloaded constructors or through set functions after object is created. It cannot be done in class definition.

Using default arguments with constructors

  • A programmer-supplied constructor that default all its arguments (or explicitly requires no arguments) is also a default constructor i.e. a constructor that can be invoked with no arguments.
  • There can be only one default constructor per class
  • Syntax for default arguments in constructor can be:

  • If there is an access function doing same initialization as done by constructor, then it is better to call that access function from constructor passing arguments to it. Using data members before initializing them properly can cause logic errors. This way 'Avoid Repeating Code' is followed, maintenance becomes easy by reducing likelihood of programming error when altering the implementation.
  • Performance of a constructor is enhanced by explicitly declaring the constructor inline or by defining the constructor in class definition.
  • Declare default function arguments only in function prototype within the class definition in header file
  • Specifying default initializers for the same member function in both header file and member function definition is a syntax error.
  • Any change to default arguments of a method requires the client code to be re-compiled. It islikely that he default argument values will change, use overloaded functions instead. Thus, if the implementation of a member function changes, the client code need not be recompiled.
  • If no constructor is defined the compiler creates a default constructor; however it does not creates perform any initialization, so when the object is created, it is not guaranteed to be in a consistent state.
  • It is possible for a class not to have a default constructor if any constructors are defined
and none of them is explicitly a default constructor

Using Destructors
  • Declaration: ~className( ); <--- No arguments, return types and return values
  • A class may have only one destructor - destructor overloading is not allowed
~ is a bitwise compliment operator => Destructor is compliment of constructor

  • Destructor is called automatically when an object is destroyed. E.g. for automatic objects when program execution leaves the scope in which an object of that class was instantiated. The destructor does not actually destroy the object - it performs termination house-keeping before the system reclaims the memory so that memory may be re-used to hold new objects.
  • Destructors are appropriate for classes whose objects contain dynamically allocated memory (for arrays & strings for e.g.) for de-allocation of storage.

When Constructors and Destructors are called
  • Called automatically.
  • The order in which these function calls are made depends on the order in which execution enters & leaves the scope in which objects are instantiated.
  • Generally, destructors are called in reverse order of the constructor calls
  • However, the storage class of objects can alter the order in which the destructors are called.
  • Constructors are called for objects defined in global scope before any other function (including main) in that file begins execution (although the order of execution of global object constructors between files is not guaranteed.
  • Corresponding destructors are called when main terminates or the exit function is called.
  • Destructors are not called for global objects if the program is terminated with a call to function abort.
  • Constructors are called for automatic local objects when execution reaches the point where the objects are defined.
  • Corresponding destructors are called when the objects leave scope i.i. block in which they are defined exists.
  • Destructors are not called for automatic objects if the program is terminated with a call to functions exit or abort.
  • Constructors are called for static local objects only once when execution first reaches the point where the objects are defined.
  • Corresponding destructors are called when main terminates or the exit function is called.
  • Destructors are not called for static objects if the program is terminated with a call to function abort.

Using Data Members and Member Functions
  • Advantages of accessing private data through Get/Set
  • Protects the data members from receiving invalid values
  • Also insulates clients of the class from the representation of the data members. Thus, if the representation of the data changes for some reason (typically to reduce the amount of storage required or to improve performance), only the member functions need to change as long as the interface provided by the member functions remains the same. The clients may, however, need to be recompiled.

A subtle Trap: Returning a Reference to a Private Data Member
  • Never have a public member function return a non-const reference (or a pointer) to a private data member. Returning such a reference violates the encapsulation of the class. In fact, returning any reference or points to private data still makes the client code dependent on the representation of the class' data. So, returning pointers or references to private data should be avoided.

Assignment by Default Memberwise Copy
  • '=' can be used to assign an object to another object of the same type.
  • It is by default performed by memberwise copy-each member of one object is copied (assigned) individually to the same member in another object.
  • Memberwise copy can cause serious problems when used with a class whose data members contain dynamically allocated storage. However, operator overloading can be used in this case.
  • Objects, by default, are passed by call-by-value. (Compare with call-by-reference, security, performance, use of const)
  • Objects can be used function arguments

Software Re-usability
  • Software is increasingly being constructed from existing, well-defined, carefully tested, well-documented, portable, widely available components (class libraries). Software re-usability speeds the development of powerful, high quality software. RAD makes use of components.

C++ As Better C

Before moving on further in advanced C++, it is better to ensure us that C++ is nothing but an advanced form of C. Those of you who came from Pascal background may have not found learning C difficult as it was mere syntax change. But when you go to C++ from C, the thinking process needs to be changed. I have seen many people use just C++ environment and literally code in C only. I request you again to understand below features which were added to C++ as a solution to make code more efficient. The best way to learn a thing is to put it to practice as soon as you need it. So, here is your revision.

  • Programming Paradigms

* Procedural Programming Top-Down Approach
* Object-Based Programming - Classes, Objects, Encapsulation,Operator Overloading BUT NO INHERITANCE e.g. Visual Basic
* Object-Oriented Programming - Above + Inheritance, Polymorphism e.g. VC++
* Generic Programming - Function Templates, Class Templates
* Another Aspect - event-driven programming, graphics programming, GUI & multimedia programming

  • Introduction to C++

* Developed by Bjarne Stroustrup at Bell Labs
* C++ = C with Classes

  • Difference between C & C++

* C is procedural and does not support object-oriented features (PIE)
* C does not have call-by-reference
* C does not have single line comment
* C++ does not allow omission of function return-vale type
* C does not have new and delete
* C++ has better I/O.. cout, cin, stream insertion extraction operators can be

used for own data-types (operator overloading)

* In C++, variables can be declared anywhere before their use
* C++ is enhanced version of C. So, C++ compiler will compile C program but not vice-versa.

  • Advantages of OOPs

* Complexity Management

Through Abstraction, Encapsulation, and Information Hiding. No Global Variables.

* Software Reuse (of classes/functions)

Productivity Increases: Development time is reduced as no need to define own classes.

Quality Increases: Re-usable software are efficiently & well designed, developed,tested and maintained.

  • Library and Header Files

* Each library has a corresponding header file
* Header file contains function prototypes, data-types and constants

Old Style: New Style: "iostream"

* Inline Functions

Increased Program Size Function code is substituted wherever inline function is called

Reduced Execution Time No overhead of function call

Use only for small and frequently used functions

Change in inline function requires all clients to be re-compiled.

* Convention

– std::cout

– using std::cout

– using namespace std

* Keywords: All of C + others

* Passing Arguments to Functions

Call-By-Value: Use for small and non-modifiable arguments


– Increased Performance =¿ No overhead of coping large amount of data

– Weaker Security =¿ Caller’s data can be corrupted

(a) Using Pointers

(b) using Reference Parameters

– Alias (another name) for corresponding argument

– Pass large data by const reference =¿ Performance + Security

– Prototype: void SqByRef(int &) Call: SqByRef(z) Defn: void SqByRef(int &a)a*=a;

- Reference is obtained by pre-pending the variable name with &

_ By looking at calling, you cannot tell if variable is passed by value or by reference i.e. whether argument will be modified or not.

* More on Reference

– References can also be used as alias for other variables within a function

– References must be initialized in their declarations. Not doing so is a syntax error

int count = 1;

int &cref = count;

– Neither taking the address of a reference nor comparing references cause syntax

errors => Each operation actually occurs on the variable for which reference is an alias.

– A reference argument must be an lvalue and not a constant or expression that returns an rvalue.

int &x=a,y=b,z=c;

int &x,y,z; //wrong


– Returning a pointer or reference to an automatic variable in a called function is a logical error i.e.compiler may WARN. References to undefined variables are called Dangling References.

* Empty parameter list disables checking. C not equal to C++

* Default Arguments

– Simple but not clean approach

– If ommitted then auto-inserted by compiler & and passed in call.

– Must be rightmost (trailing) arguments in function parameter list

– If XXX then others should also be omitted

* Unary Scope Resolution Operator

Used for local and global variables with the same name

* Parameterized Stream Manipulator


setprecision(20)-- after decimal places


setiosflag (ios::fixed|ios::showpoint) - Inclusive OR

* Function Overloading:

Used to create several functions of the same name that perform similar tasks on different data types. Should differ in numbers, types and orderof arguments in call. Different only in return types is not function overloading - It issyntax error Operator Overloading/Function Templates = auto generating overloading functions that perform identical tasks on different data types signatures (function name+parameter type)-name mangling or name decoration to enable type-safe linkage- ensures that the proper overloaded function is called and the arguments conform toparameters. Linkage errors are detected and reported by the compiler. Overloadedfunctions with default parameters may cause ambiguity.

* Function Templates

Enable creation of functions that perform the same (identical)

operations on different types of data but the function template is defined only once.

C++ automatically generates separate template function to handle each type of call



T maximum(T i, T j)


int max;


return max;


Formal type parameters are built-in types or user-defined types used to specify the

types of the arguments to the function, to specify return type of function and todeclare variables within the body of the function definition.

* Class = Structure + Function

– Structures: Group data elements

– Functions: Organize program actions into named entities

– Classes: Group data elements + Organize program actions into named entities

* Role of Variable Type

– Size

– Information it can hold

– Supported operations

* Creating New Types

– C++ allows creating new types as powerful as built-in types

– Why new types: Closer rep resentation of real world makes writing programs easier e.g. heating systems have variables like rooms, heat sensors

In next post, we will see in detail classes with respect to data abstraction.

Source: C++ - How To Program by Deitel & Deitel

Inline Functions

  • Functions with small code can be declared as inline.
  • Wherever an inline function is called, actual function code will be replaced
  • Syntax: Put keyword inline before function definition and not in declaration. For e.g.

void swap(int& a,int& b); //declaration
inline void swap(int& a,int& b) //defintion
int temp;
temp = a;
a = b;
b= temp;

Inline Function Vs Macros
  • Unlike inline functions, macros do not have type-checking and also do not check if arguments are well-formed.
  • You cannot return a value as computation result in a macro.
  • Macro uses textual substitution, so there may be side-effects and inefficiencies due to re-evaluation of arguments and order of operators.
  • Macro expanded code is difficult to understand for compilation errors.
  • Debugging information for inline functions is more helpful
  • Sometimes, required construct cannot be written using macros or may be it will awkward to do so.

Points to Ponder
  • Member functions defined inside the class definition are inline by default.
  • Compiler decides which function to use as inline. So, even if you have declared a function inline, compiler can treat it as normal function. In this case, compiler will use calling instead of replacing the function code at the calling places.
  • Inline functions may increase the performance significantly.
  • May because inline functions increases code size to be compiled and therefore compilation time.
Bjarne Stroustrup suggests to avoid use of macros and instead use inline functions for frequently executed small functions.