Question

Can any one explain “class X x;” what actually it mean …in the program…Please give me some example also?
Ex:

#include <iostream>
using namespace std;

class X {
    public:
        X() {}
};

class Y {
    public:
        Y() {}
};

int main()
 {
    class X x; //what is this? explain it
    class Y y; //what is this? explain it
    class Z z; //what is this? explain it    //error
    return 0;
  }

Edited: May i know the exact difference between “Class Z z” and “Z z”.Because While
compiling this program in 2 ways

  class Z Z;   //here iam getting as Error: error: incomplete type is not allowed
  Z z; //error: identifier "Z" is undefined

Since iam asking the exact difference.

Answer 1

AFAIK

class X x;

is equivalent to

X x;

(EDIT: for those nitpickers: at least, in your case, when X is a previously defined class and there is no other variable named X in the same scope).

In plain-old C, whenever you define a struct without a typedef, you have explicitly use the struct keyword when creating variables. In C++, one can omit the struct keyword, but you also can write struct X x; if you prefer that. And since the only difference between class and struct in C++ is the default visibility, one can conclude that it is also legal to write class X x;

Answer to your edit:

 class Z;

is a forward declaration (often used in header files where you don’t want to #include "Z.hpp"). This is also called an incomplete type declaration. It allows pointers to Z to be declared, for example

 class Z *z;

is legal code, even when the compiler has not seen any class body declaration of Z. What is not allowed is to create an instance of z like class Z z; as long as Z is incomplete from the compiler’s view.

The code Z z;, however cannot be interpreted as a forward declaration by the compiler (not even as a disallowed forward declaration). It just shows up as “Z is undefined”.