1. What is virtual constructors/destructors?

Virtual destructors: If an object (with a non-virtual destructor) is destroyed explicitly by applying

the delete operator to a base-class pointer to the object, the base-class destructor function

(matching the pointer type) is called on the object.

There is a simple solution to this problem – declare a virtual base-class destructor. This makes all

derived-class destructors virtual even though they don’t have the same name as the base-class

destructor. Now, if the object in the hierarchy is destroyed explicitly by applying the delete operator

to a base-class pointer to a derived-class object, the destructor for the appropriate class is called.

Virtual constructor: Constructors cannot be virtual. Declaring a constructor as a virtual function is

a syntax error. Does c++ support multilevel and multiple inheritance?

Yes.

What are the advantages of inheritance?

• It permits code reusability.

• Reusability saves time in program development.

• It encourages the reuse of proven and debugged high-quality software, thus reducing problem

after a system becomes functional.

What is the difference between declaration and definition?

The declaration tells the compiler that at some later point we plan to present the definition of this

declaration.

E.g.: void stars () //function declaration

The definition contains the actual implementation.

E.g.: void stars () // declarator

{

for(int j=10; j>=0; j--) //function body

cout<<”*”;

cout<<endl;

}

2. What do you mean by pure virtual functions?

A pure virtual member function is a member function that the base class forces derived classes to

provide. Normally these member functions have no implementation. Pure virtual functions are

equated to zero.

class Shape { public: virtual void draw() = 0; };

3. What is namespace?

Namespaces allow us to group a set of global classes, objects and/or functions under a name. To say

it somehow, they serve to split the global scope in sub-scopes known

as namespaces.

The form to use namespaces is:

namespace identifier { namespace-body }

Where identifier is any valid identifier and namespace-body is the set of classes, objects and

functions that are included within the namespace. For example:

namespace general { int a, b; } In this case, a and b are normal variables integrated within the

general namespace. In order to access to these variables from outside the namespace we have to

use the scope operator ::. For example, to access the previous variables we would have to put:

general::a general::b

The functionality of namespaces is specially useful in case that there is a possibility that a global

object or function can have the same name than another one, causing a

redefinition error.

4. What is RTTI?

Runtime type identification (RTTI) lets you find the dynamic type of an object when you have only a

pointer or a reference to the base type. RTTI is the official way in

standard C++ to discover the type of an object and to convert the type of a pointer or reference

(that is, dynamic typing). The need came from practical experience with

C++. RTTI replaces many homegrown versions with a solid, consistent approach.

5. What is a template?

Templates allow to create generic functions that admit any data type as parameters and return value

without having to overload the function with all the possible data types. Until certain point they fulfill

the functionality of a macro. Its prototype is any of the two following ones:

template <class indetifier> function_declaration; template <typename indetifier>

function_declaration;

The only difference between both prototypes is the use of keyword class or typename, its use is

indistinct since both expressions have exactly the same meaning and behave exactly the same way.

6. What do you mean by inline function?

The idea behind inline functions is to insert the code of a called function at the point where the

function is called. If done carefully, this can improve the application's

performance in exchange for increased compile time and possibly (but not always) an increase in the

size of the generated binary executables.

7. What is virtual class and friend class?

Friend classes are used when two or more classes are designed to work together and need access

to each other's implementation in ways that the rest of the world

shouldn't be allowed to have. In other words, they help keep private things private. For instance, it

may be desirable for class DatabaseCursor to have more privilege to the internals of class Database

than main() has.

8. What is function overloading and operator overloading?

Function overloading: C++ enables several functions of the same name to be defined, as long as

these functions have different sets of parameters (at least as far as their types are concerned). This

capability is called function overloading. When an overloaded function is called, the C++ compiler

selects the proper function by examining the number, types and order of the arguments in the call.

Function overloading is commonly used to create several functions of the same name that perform

similar tasks but on different data types.

Operator overloading allows existing C++ operators to be redefined so that they work on objects

of user-defined classes. Overloaded operators are syntactic sugar for

equivalent function calls. They form a pleasant facade that doesn't add anything fundamental to the

language (but they can improve understandability and reduce

maintenance costs).

9. Difference between realloc() and free()?

The free subroutine frees a block of memory previously allocated by the malloc subroutine.

Undefined results occur if the Pointer parameter is not a valid pointer. If the Pointer parameter is a

null value, no action will occur. The realloc subroutine changes the size of the block of memory

pointed to by the Pointer parameter to the number of bytes specified by the Size parameter and

returns a new pointer to the block. The pointer specified by the Pointer parameter must have been

created with the malloc, calloc, or realloc subroutines and not been deallocated with the free or

realloc subroutines. Undefined results occur if the Pointer parameter is not a valid pointer

10. What do you mean by binding of data and functions?

Encapsulation.

11. What is abstraction?

Abstraction is of the process of hiding unwanted details from the user.

12. What is encapsulation?

Packaging an object’s variables within its methods is called encapsulation.

13. What is the difference between an object and a class?

Classes and objects are separate but related concepts. Every object belongs to a class and every

class contains one or more related objects.

Ø A Class is static. All of the attributes of a class are fixed before, during, and after the execution of

a program. The attributes of a class don't change.

Ø The class to which an object belongs is also (usually) static. If a particular object belongs to a

certain class at the time that it is created then it almost certainly will still belong to that class right

up until the time that it is destroyed.

An Object on the other hand has a limited lifespan. Objects are created and eventually destroyed.

Also during that lifetime, the attributes of the object may undergo significant change.

14. What is polymorphism? Explain with an example?

"Poly" means "many" and "morph" means "form". Polymorphism is the ability of an object (or

reference) to assume (be replaced by) or become many different forms of

object.

Example: function overloading, function overriding, virtual functions. Another example can be a plus

‘+’ sign, used for adding two integers or for using it to concatenate two strings.

15. What do you mean by inheritance?

Inheritance is the process of creating new classes, called derived classes, from existing classes or

base classes. The derived class inherits all the capabilities of the base class, but can add embellishments and refinements of its own.

16. What is a scope resolution operator?

A scope resolution operator (::), can be used to define the member functions of a class outside the

class.

17. What are virtual functions?

A virtual function allows derived classes to replace the implementation provided by the base class.

The compiler makes sure the replacement is always called whenever

the object in question is actually of the derived class, even if the object is accessed by a base pointer

rather than a derived pointer. This allows algorithms in the base class to be replaced in the derived

class, even if users don't know about the derived class.

18. What is friend function?

As the name suggests, the function acts as a friend to a class. As a friend of a class, it can access its

private and protected members. A friend function is not a member of

the class. But it must be listed in the class definition.

19. What is the difference between class and structure?

Structure: Initially (in C) a structure was used to bundle different type of data types together to

perform a particular functionality. But C++ extended the structure to contain functions also. The

major difference is that all declarations inside a structure are by default public.

Class: Class is a successor of Structure. By default all the members inside the class are private.

20. What is public, protected, private?

Ø Public, protected and private are three access specifiers in C++.

Ø Public data members and member functions are accessible outside the class.

Ø Protected data members and member functions are only available to derived classes.

Ø Private data members and member functions can’t be accessed outside the class.

However there is an exception can be using friend classes.

21. What is an object?

Object is a software bundle of variables and related methods. Objects have state and behavior.

22. What is a class?

Class is a user-defined data type in C++. It can be created to solve a particular kind of problem.

After creation the user need not know the specifics of the working of a class.

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Viva Questions:
What is the difference between an Interface and an Abstract class?

A: An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract. An interface has all public members and no implementation. An aQ: What is the purpose of garbage collection in Java, and when is it used?bstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods.
What is the purpose of garbage collection? 
A: The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used.

Q: Describe synchronization in respect to multithreading.

A: With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchonization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating same shared variable. This usually leads to significant errors.

What are pass by reference and passby value?

A: Pass By Reference means the passing the address itself rather than passing the value. Passby Value means passing a copy of the value to be passed.

What is the difference between a constructor and a method?

A: A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator.
A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator.

Q: State the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers.

A: public : Public class is visible in other packages, field is visible everywhere (class must be public too)
private : Private variables or methods may be used only by an instance of the same class that declares the variable or method, A private feature may only be accessed by the class that owns the feature.
protected : Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature.This access is provided even to subclasses that reside in a different package from the class that owns the protected feature.
default :What you get by default ie, without any access modifier (ie, public private or protected).It means that it is visible to all within a particular package.
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Q: What is an abstract class?

A: Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract may not be instantiated (ie, you may not call its constructor), abstract class may contain static data. Any class with an abstract method is automatically abstract itself, and must be declared as such.
A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.
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Q: What is static in java?

A: Static means one per class, not one for each object no matter how many instance of a class might exist. This means that you can use them without creating an instance of a class.Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are attached to a class, not an object. A static method in a superclass can be shadowed by another static method in a subclass, as long as the original method was not declared final. However, you can't override a static method with a nonstatic method. In other words, you can't change a static method into an instance method in a subclass.
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Q: What is final?

A: A final class can't be extended ie., final class may not be subclassed. A final method can't be overridden when its class is inherited. You can't change value of a final variable (is a constant).

What are Wrapper Classes ? 
A They are wrappers to primitive data types. They allow us to access primitives as objects.

Q Can we call finalize() method ? 
A Yes. Nobody will stop us to call any method , if it is accessible in our class. But a garbage collector cannot call an object's finalize method if that object is reachable.

What is the initial state of a thread when it is created and started?
A The thread is in ready state.

Can we declare an anonymous class as both extending a class and implementing an interface? 
A No. An anonymous class can extend a class or implement an interface, but it cannot be declared to do both

What are the differences between boolean & operator and & operator 
A When an expression containing the & operator is evaluated, both operands are evaluated. And the & operator is applied to the operand. When an expression containing && operator is evaluated, the first operand is evaluated. If the first operand returns a value of true then only the second operand is evaluated otherwise the second part will not get executed. && is also called short cut and.

Q What is the use of the finally block? 
A Finally is the block of code that executes always. The code in finally block will execute even if an exception is occurred. finally will not execute when the user calls System.exit().

How can you call a constructor from another constructor ? 
A By using this() reference.

How can you call the constructor of super class ? 
A By using super() syntax.

What's the difference between normal methods and constructors? 
A Constructors must have the same name of the class and can not have a return type. They are called only once, while regular methods can be called whenever required. We cannot explicitly call a constructor.

What is the use of packages in java ? 
A Packages are a way to organize files in java when a project consists of more than one module. It helps in resolving name conflicts when different modules have classes with the same names.

What if the main method is declared as private?

The program compiles properly but at runtime it will give “Main method not public.” message.
What is Byte Code?

Or

What gives java it’s “write once and run anywhere” nature?

All Java programs are compiled into class files that contain bytecodes. These byte codes can be run in any platform and hence java is said to be platform independent.

Explain the reason for each keyword of public static void main(String args[])?

public- main(..) is the first method called by java environment when a program is executed so it has to accessible from java environment. Hence the access specifier has to be public.

static: Java environment should be able to call this method without creating an instance of the class , so this method must be declared as static.

void: main does not return anything so the return type must be void

What are the differences between == and .equals() ?

Or

what is difference between == and equals

Or

Difference between == and equals method

Or

What would you use to compare two String variables - the operator == or the method equals()?

Or

How is it possible for two String objects with identical values not to be equal under the == operator?

The == operator compares two objects to determine if they are the same object in memory i.e. present in the same memory location. It is possible for two String objects to have the same value, but located in different areas of memory.

== compares references while .equals compares contents. The method public boolean equals(Object obj) is provided by the Object class and can be overridden. The default implementation returns true only if the object is compared with itself, which is equivalent to the equality operator == being used to compare aliases to the object. String, BitSet, Date, and File override the equals() method. For two String objects, value equality means that they contain the same character sequence. For the Wrapper classes, value equality means that the primitive values are equal.

What is implicit casting?

Implicit casting is the process of simply assigning one entity to another without any transformation guidance to the compiler. This type of casting is not permitted in all kinds of transformations and may not work for all scenarios.

Example

int i = 1000;

long j = i; //Implicit casting

Is sizeof a keyword in java?

The sizeof operator is not a keyword.
What is explicit casting?

Explicit casting in the process in which the complier are specifically informed to about transforming the object.

Example

long i = 700.20;