static member variables initialization c++

Question

Why the compiler gives me an error when I don't initialize a value to a static member variable? Shouldn't it be initialized to 0?
Why I have to initialize member variable outside the class? (is this illegal because if you do so, and change the value of this static member variable inside the main function and you create an object of this clas, it will re-assign the static mamber variable to the old value) whereas const static member variable are legal to be initilized inside the class (and this is possible because you can't change the value of this static member variable anyway)?

Error: undefined reference to class_name::a

Note: that's the linker giving you the error, not the compiler. It's the same effect as declaring a (free) global variable as extern in a header but not actually defining it in a .cpp file. A global variable must have a consistent memory location known to all compilation units, otherwise they won't see each other's changes to its value. As such, it must have external linkage and be defined exactly once. It is not like a static (compilation unit local) variable. — pmdj
– pmdj, Commented Mar 16, 2012 at 21:33
@pmjordan : it's not the linker, the error is during the compiling time . — AlexDan
– AlexDan, Commented Mar 16, 2012 at 21:40
undefined reference is normally a linker error. If you're using an IDE, you might not realise that compiling has finished and it's progressed to linking. — pmdj
– pmdj, Commented Mar 16, 2012 at 22:12

hmjd · Accepted Answer · 2012-03-16 22:01:56Z

5

From the error posted, the linker is stating that the variable has not been defined, not that it has not been explicitly initialised:

class A
{
    // declaration.
    static int x;
};

// definition (in this case without explicit initialisation).
int A::x;

The linker should not emit an error and the compiler should not emit a warning, as long as no attempt is made to use the static variable before it has been assigned an initial value.

edited Mar 16, 2012 at 22:01

answered Mar 16, 2012 at 21:32

hmjd

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6 Comments

David Rodríguez - dribeas Over a year ago

... you were doing fine and then.... In the last code sample, there is no definition. There is a declaration and the initialization value, but no definition. This can be fine in many cases as static integral constants have special rules, but you will still need to define the constant if you (odr-)use it.

hmjd Over a year ago

@DavidRodríguez-dribeas, just deleted that for now. I thought at declaration if a variable was assigned an initial value , extern int x = 1; for example, that this made it a definition?

Johannes Schaub - litb Over a year ago

Huh. A::x by default has value 0, like any other class-static int variable.

Johannes Schaub - litb Over a year ago

@hmjd there is no such general rule like that. For extern int x if you directly initialize it, that makes it a definition. But for in-class initializations of static, it won't make it a definition.

AlexDan Over a year ago

@JohannesSchaub-litb I guess I'm misunderstood the difference between declare, define and initialize a variable. can you tell me the difference between them ?

|

Bo Persson · Accepted Answer · 2012-03-16 21:33:58Z

0

A static member is not really stored in any of the objects created, because it is shared between all objects of that class.

It should only be created once, even if you create many objects of that class. Or even if you create no objects of the class. Therefore you have to do it separately.

Compilers warn about all kinds of uninitalized variables, not only the static ones. Having a variable without a value is generally not very useful, so these warnings are good. Adding an = 0 is not too hard, is it?

answered Mar 16, 2012 at 21:33

Bo Persson

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