C ++语法问题，使用class关键字

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

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 ;

Answer 2

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".

Answer 3

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 eg

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.,

Answer 4

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++?

Answer 5

声明类的实例时， class关键字是可选的。

Answer 6

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 .