C++ syntax question, use of the class keyword

Can any one explain "class X x;" what actually it me开发者_开发技巧an ...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.

This is called an elborated type specifier and is useful in cases such as below:

$9.1/3 - "An elaborated-type-specifier (7.1.5.3) can also be used as a type-specifier as part of a declaration. It differs from a class declaration in that if a class of the elaborated name is in scope the elaborated name will refer to it."

int bad;

class bad{};

int main(){
   //bad b;
   class bad b;    // Elaborated type specifier
}

You should now be able to undersand the difference between class Z z and Z z;

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".

C++ started out as an extension of C.

In C class names are not full-fledged typenames. So in C, if you write

struct Blah
{
    int x;
};

then to declare a variable of this class, in C you have to write

struct Blah myBlahObject;

In C++ the class name is a full-fledge typename, so you can just write

Blah myBlahObject;

But mainly for compatibility with C, C++ retains the C-style declaration syntax, and generalizes it to also work with the C++ class keyword.

And that means that in C++ you can write e.g.

class NeverSeenBefore* p;

to declare a pointer variable p of type NeverSeenBefore*. Where NeverSeenBefore has not been declared anywhere yet. It's just an incomplete type, and use of the class (or struct) keyword informs the compiler, as in C, that there is such a type, even though it's not yet been declared.

C++ has a built in incomplete type, called void -- and you get the same kind of error message (from a good compiler) if you try to declare a void variable.

The main difference between void and other incomplete types is that void can not be completed.

By the way, the C technique for obtaining a full fledged typename is to use typedef. Since class names are full fledged typenames in C++, such typedef is not necessary in C++. So, when you see C++ code where a typedef is provided for a class instead of just naming the class, then you know that a C programmer has been there – or a person taught by a C programmer (or recursively, taught by a person taught by a C programmer, so on).

Cheers & hth.,

class X x; instantiates an object of type class X on the stack that is named x. Looking at your definition of X there are no methods or fields, so you really can't do much with it.

Do you know what classes are in c++?

When declaring an instance of a class, the class keyword is optional.

As per your new edit,

class Z z; when the compiler encounter this, it identifies that Z is a class since you specified it. But it cannot find the declaration of the class Z. Hence it is showing error: incomplete type is not allowed.

Now, when the compiler encounters Z z; it will not have any idea about what Z is since you didn't specify it anywhere. So it reports error: identifier "Z" is undefined.