Static object in struct

Question

I have class A with method display(). I create a struct B with a static variable of type object A:

class A{
public :
    void display()
    {
        cout << "in A " << endl;
    }
};
typedef struct B{
    static A a;
} bb;

//B::a.display();
int main() {
    bb b;
    bb::a.display();
    return 0;
}

Now I get an error when try to access a.

How can I define a static object in this case?

What is the error message, did you use a `using`-directive or `using`-declarations to make `std`'s namespace symbols available ? — Rerito, Nov 13 '14 at 10:03
I think it just declare object A. B::a.display(); not work because undefined reference to `B::a'. — Memory Leak, Nov 13 '14 at 10:04
@Rerito could you show more specific about using-directive or using-declarations — Memory Leak, Nov 13 '14 at 10:07
@MemoryLeak, here is an article that should enlighten you about namespaces in C++ : http://winterdom.com/dev/cpp/nspaces — Rerito, Nov 13 '14 at 10:10

score 4 · Accepted Answer · answered Nov 13 '14 at 10:03

4

You declared static A a; but did not define it. Add the following line before int main() and it'll link successfully:

A B::a;

answered Nov 13 '14 at 10:03

Paul

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Since no constructor for `A` is defined, won't the compiler automatically call a default constructor to initialize a ? – Rerito Nov 13 '14 at 10:05
1

@Rerito: Yes. Doesn't mean you don't have to define it. – Lightness Races in Orbit Nov 13 '14 at 10:06
@LightnessRacesinOrbit : Tested on Ideone without adding anything except a `using`-directive : http://ideone.com/0LkbNE. Why did it work ? – Rerito Nov 13 '14 at 10:07
@Rerito: No access to data members. Therefore no _odr-use_. [Here's the real testcase](http://ideone.com/VwP6Pc). – Lightness Races in Orbit Nov 13 '14 at 10:28

Static object in struct

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