So,if I define an enum like:

class x{
public:
enum{
hey=0
};

}

Is it true that it doesn't occupy any space? And if yes,why?

The class doesn't store anything; look at the enum as a type and not a value.

You are just telling the compiler you are going to use the symbol `x::hey' as meaning 0. That may or may not "occupy any space", depending on what exactly you mean by that. (The size of an object of type x will not be larger because of the enum, the executable will not be larger (except maybe for debugging info), but the source code will use more space, and the compiler will use more memory.)

Thanks,got it.I was refering to using that code I showed you instead of a const inside a class.

So,Inside the class I could write int es[hey];

So should I use enums like that in a class instead of a static const if I need to write code like the array above? I understand that the performance I get by doing this is...very small,but just as a concept,is it better?

Ofcourse,I could just use int es[10],but when I need the same constant value in many places in a class,defining an enum like the one above,or a static const,might help understand the code faster.

Variants of a compile-time constant has no effect on the run-time, because it is compile-time only. So not only is it very small, but exactly zero run-time performance difference. Design-wise... well, that's a different question and depends on what the constant is used for and what it represents.