Undefined reference to static class member

Question

Can anyone explain why following code won't compile? At least on g++ 4.2.4.

And more interesting, why it will compile when I cast MEMBER to int?

#include <vector>

class Foo {  
public:  
    static const int MEMBER = 1;  
};

int main(){  
    vector<int> v;  
    v.push_back( Foo::MEMBER );       // undefined reference to `Foo::MEMBER'
    v.push_back( (int) Foo::MEMBER ); // OK  
    return 0;
}

Answer 1

Regarding the second question: push_ref takes reference as a parameter, and you cannot have a reference to static const memeber of a class/struct. Once you call static_cast, a temporary variable is created. And a reference to this object can be passed, everything works just fine.

Or at least my colleague who resolved this said so.