Definition of the static data member

Question

I'm reading Scott Meyers' C++ and come across this example:

class GamePlayer{
private:
    static const int NumTurns = 5;
    int scores[NumTurns];
    // ...
};

What you see above is a declaration for NumTurns, not a definition.

Why not a definition? It looks like we initialize the static data member with 5.

I just don't understand what it means to declare but not define a variable with the value 5. We can take the address of the variable fine.

class A
{
public:
    void foo(){ const int * p = &a; }
private:
    static const int a = 1;
};

int main ()
{
    A a;
    a.foo();
}

DEMO

The body of `foo` is being optimized away. Try printing the address of `a` and you'll run into linker errors. http://coliru.stacked-crooked.com/a/eff67551fb50c778 Or compile your example with `-O0` instead of `-O2` — Praetorian, Jun 19 '15 at 05:12
http://stackoverflow.com/a/11301299/6610 says some very useful things about this: you have _declaration_, _definition_ and _initialization_. — xtofl, Jun 19 '15 at 05:21

user657267 · Accepted Answer · 2015-06-19T05:16:29.213

2

Because it isn't a definition. Static data members must be defined outside the class definition.

[class.static.data] / 2

The declaration of a static data member in its class definition is not a definition and may be of an incomplete type other than cv-qualified void. The definition for a static data member shall appear in a namespace scope enclosing the member’s class definition.

As for taking the address of your static member without actually defining it, it will compile, but it shouldn't link.

edited Jun 19 '15 at 05:16

answered Jun 19 '15 at 04:36

user657267

20,568
5
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So what did we do by writing `static const int NumTurns = 5`? Looks like we just declare a variable with and initialize it with the value 5. – St.Antario Jun 19 '15 at 04:39
@St.Antario Scott Meyers just told you, you declared the member inside the class definition, but that doesn't define it because it's `static`. – user657267 Jun 19 '15 at 04:40
I think OP's mixing _initialization_ with _definition_. Cfr. also http://stackoverflow.com/a/11301299/6610 – xtofl Jun 19 '15 at 05:20

score 1 · Answer 2 · answered Jun 19 '15 at 04:36

1

you need to put a definition of NumTurns in source file, like

const int GamePlayer::NumTurns;

answered Jun 19 '15 at 04:36

Xiaotian Pei

3,210
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Definition of the static data member

2 Answers2

Linked