C/C++ Interview Question&Answer Part 2

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.
- 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.
- 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.

Suppose that data is an array of 1000 integers. Write a single function call that will sort the 100 elements data [222] through data [321].

quicksort ((data + 222), 100);

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.

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.

Which recursive sorting technique always makes recursive calls to sort subarrays that are about half size of the original array?

Mergesort always makes recursive calls to sort subarrays that are about half size of the original array, resulting in O(n log n) time.

What is abstraction?

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

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.

What is the difference between an external iterator and an internal iterator? Describe an advantage of an external iterator.

An internal iterator is implemented with member functions of the class that has items to step through. .An external iterator is implemented as a separate class that can be "attach" to the object that has items to step through. .An external iterator has the advantage that many difference iterators can be active simultaneously on the same object.

What is a scope resolution operator?

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

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; };

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.

How can you quickly find the number of elements stored in a a) static array b) dynamic array ?
Why is it difficult to store linked list in an array?
How can you find the nodes with repetetive data in a linked list?

Write a prog to accept a given string in any order and flash error if any of the character is different. For example : If abc is the input then abc, bca, cba, cab bac are acceptable but aac or bcd are unacceptable.
Write out a function that prints out all the permutations of a string. For example, abc would give you abc, acb, bac, bca, cab, cba. You can assume that all the characters will be unique.

What’s the output of the following program? Why?

#include 
main()
{
typedef union
{
int a;
char b[10];
float c;
}
Union;
Union x,y = {100};
x.a = 50;
strcpy(x.b,\"hello\");
x.c = 21.50;
printf(\"Union x : %d %s %f \n\",x.a,x.b,x.c );
printf(\"Union y :%d %s%f \n\",y.a,y.b,y.c);
}
Given inputs X, Y, Z and operations | and & (meaning bitwise OR and AND, respectively)
What is output equal to in

output = (X & Y) | (X & Z) | (Y & Z)

Why are arrays usually processed with for loop?

The real power of arrays comes from their facility of using an index variable to traverse the array, accessing each element with the same expression a[i]. All the is needed to make this work is a iterated statement in which the variable i serves as a counter, incrementing from 0 to a.length -1. That is exactly what a loop does.

What is an HTML tag?

An HTML tag is a syntactical construct in the HTML language that abbreviates specific instructions to be executed when the HTML script is loaded into a Web browser. It is like a method in Java, a function in C++, a procedure in Pascal, or a subroutine in FORTRAN.

What problems might the following macro bring to the application?

#define sq(x) x*x

Anything wrong with this code?
T *p = new T[10];
delete p;

Everything is correct, Only the first element of the array will be deleted”, The entire array will be deleted, but only the first element destructor will be called.

Anything wrong with this code?
T *p = 0;
delete p;

Yes, the program will crash in an attempt to delete a null pointer.

How do you decide which integer type to use?

It depends on our requirement. When we are required an integer to be stored in 1 byte (means less than or equal to 255) we use short int, for 2 bytes we use int, for 8 bytes we use long int.
A char is for 1-byte integers, a short is for 2-byte integers, an int is generally a 2-byte or 4-byte integer (though not necessarily), a long is a 4-byte integer, and a long long is a 8-byte integer.

What’s the best way to declare and define global variables?

The best way to declare global variables is to declare them after including all the files so that it can be used in all the functions.

What does extern mean in a function declaration?

Using extern in a function declaration we can make a function such that it can used outside the file in which it is defined.
An extern variable, function definition, or declaration also makes the described variable or function usable by the succeeding part of the current source file. This declaration does not replace the definition. The declaration is used to describe the variable that is externally defined.
If a declaration for an identifier already exists at file scope, any extern declaration of the same identifier found within a block refers to that same object. If no other declaration for the identifier exists at file scope, the identifier has external linkage.

What can I safely assume about the initial values of variables which are not explicitly initialized?

It depends on complier which may assign any garbage value to a variable if it is not initialized.

What is the difference between char a[] = “string”; and char *p = “string”;?

In the first case 6 bytes are allocated to the variable a which is fixed, where as in the second case if *p is assigned to some other value the allocate memory can change.

What’s the auto keyword good for?

Answer1
Not much. It declares an object with automatic storage duration. Which means the object will be destroyed at the end of the objects scope. All variables in functions that are not declared as static and not dynamically allocated have automatic storage duration by default.
For example
int main()
{
int a; //this is the same as writing “auto int a;”
}

Answer2
Local variables occur within a scope; they are “local” to a function. They are often called automatic variables because they automatically come into being when the scope is entered and automatically go away when the scope closes. The keyword auto makes this explicit, but local variables default to auto auto auto auto so it is never necessary to declare something as an auto auto auto auto.

What is the difference between char a[] = “string”; and char *p = “string”; ?

Answer1
a[] = “string”;
char *p = “string”;

The difference is this:
p is pointing to a constant string, you can never safely say
p[3]=’x';
however you can always say a[3]=’x';
char a[]=”string”; - character array initialization.
char *p=”string” ; - non-const pointer to a const-string.( this is permitted only in the case of char pointer in C++ to preserve backward compatibility with C.)

Answer2
a[] = “string”;
char *p = “string”;
a[] will have 7 bytes. However, p is only 4 bytes. P is pointing to an adress is either BSS or the data section (depending on which compiler — GNU for the former and CC for the latter).

Answer3
char a[] = “string”;
char *p = “string”;

for char a[]…….using the array notation 7 bytes of storage in the static memory block are taken up, one for each character and one for the terminating nul character.
But, in the pointer notation char *p………….the same 7 bytes required, plus N bytes to store the pointer variable “p” (where N depends on the system but is usually a minimum of 2 bytes and can be 4 or more)……

How do I declare an array of N pointers to functions returning pointers to functions returning pointers to characters? 
Answer1
If you want the code to be even slightly readable, you will use typedefs.
typedef char* (*functiontype_one)(void);
typedef functiontype_one (*functiontype_two)(void);
functiontype_two myarray[N]; //assuming N is a const integral

Answer2
char* (* (*a[N])())()
Here a is that array. And according to question no function will not take any parameter value.

What does extern mean in a function declaration?

It tells the compiler that a variable or a function exists, even if the compiler hasn’t yet seen it in the file currently being compiled. This variable or function may be defined in another file or further down in the current file.

How do I initialize a pointer to a function?
This is the way to initialize a pointer to a function
void fun(int a)
{
}
void main()
{
void (*fp)(int);
fp=fun;
fp(1);
}

How do you link a C++ program to C functions?

By using the extern "C" linkage specification around the C function declarations.

Explain the scope resolution operator.

It permits a program to reference an identifier in the global scope that has been hidden by another identifier with the same name in the local scope.

What are the differences between a C++ struct and C++ class?

The default member and base-class access specifiers are different.

How many ways are there to initialize an int with a constant?

Two.
There are two formats for initializers in C++ as shown in the example that follows. The first format uses the traditional C notation. The second format uses constructor notation.
int foo = 123;
int bar (123);