Undefined reference to static class member

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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;
}

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