C

Today we shall discuss about a simple C program.

I would like to explain the difference between array and &array address.

So, let's start by taking a live question. 

Q. Find the o/p:

main()

{

    char *ptr="hello";

    char ary[]="world";

printf("Ans:\n");

printf("PTR: &ptr=%x, ptr=%x, *ptr=%x, ptr[0]=%x\n", &ptr,ptr, *ptr, ptr[0]);

printf("ARY: &ary=%x,ary=%x, *ary=%x, ary[0]=%x\n", &ary, ary, *ary, ary[0]);

}

Ans:

PTR: &ptr=bfb86990, ptr=8048524, *ptr=68, ptr[0]=68

ARY: &ary=bfb8698a,ary=bfb8698a, *ary=77, ary[0]=77

When you look at the output of array and &array you will find that they both are same. Why? But, whereas in the case of a character pointer (not a character array), they seems to differ. Why again?

We all know that an array is like a constant pointer, and the address of the arrary is also indicated by the name of the array itself. Hence, when we mention arrary and when we mention &array they both mean the same and point to the same location. Memory allocation also happens in the same area as the starting address of the array.

But, but in case of a character pointer, prt and &ptr points to two distinct and different locations. &ptr is the place which is used by the compiler itself to store the actual data string that is pointed by ptr. &ptr points to a .rodata area inside the data segment, accessible and allocated by the compiler itself.

That is the only reason, why you get a segmentation fault while trying to modify the data pointed by the ptr character pointer.

C++ questions: Part 8

Q. Why the size of empty Class is one byte?
Ans:
Yes, the compiler will generate 1 byte of memory to mark the existence of the class.  This doesn't answer WHY though.  The reason is the language standard states that all classes must have a memory size of at least 1 byte so that the class doesn't occupy the same memory space with another class.  This is to prevent name mangling.  i.e., if I declare a class A {};, the compiler will still generate an entry in its table to something called "A".  If behind that I declare another class, say class B, if A takes 0 bytes of memory, and B's data gets written in the place where A was declared.  In this case, an instantiation of A would take on the properties of B. 

Q. What are the default methods of every object?
Ans:
http://qiang-ma.blogspot.com/2007/06/c-default-methods.html
 +---------------------------------------------------------+
|What Name |
+---------------------------------------------------------+
|Default Constructor A::A() |
|Copy Constructor A::A(const A &) |
|Assignment [const] A &A::operator=(const A &) |
|Destructor A::~A() |
+---------------------------------------------------------+


Q. Why a constructor can't be virtual?
Ans:

A constructor can not be virtual because at the time when the constructor is invoked the virtual table (vtable) would not be available in the memory. Virtual allows us to call a function knowing only the interfaces and not the exact type of the object. To create an object you need complete information. In particular, you need to know the what you want to create exactly. Hence call to a constructor can't be virtual.