Re: derived class access to virtual base class variables?

Why do I get an error when trying to access x from Inner1 and Inner2?

#include <cstdio>

template<class T> class Data {
public:
    int x;
};

template<class U> class Inner1: virtual public Data<U> {
public:
    void operator () () {
        printf("nespecializat1\n");
        x = 3;
    }
};

template<class U> class Inner2: virtual public Data<U> {
public:
    void operator () () {
        printf("nespecializat2\n");
        x = 5;
    }
};

template<class T> class Base: virtual public Data<T>, public
Inner1<T>, public Inner2<T> {
public:
    Inner1<T> a;
    Inner2<T> b;
};

int main() {
    Base<char> x;
    x.a();
    printf("%d\n", x.x);
    x.b();
    printf("%d\n", x.x);

    return 0;
}