Question

Is it possible to create one function which is only available if the class is instantiated with a specific type? (Without rewriting the whole class for that type?)

Answer 1

If I understand what you’re after, one possibility would be to use a base class that contains the other functions, then have a template and a specialization that both derive from that base class, with the specialization adding the extra function you want:

struct X {
    int x() { return 1; }
};

template<class T>
struct Y : public X {
};

template<>
struct Y<int> : public X {
    int y() { return 2; }
};

int main() {
    Y<long> y;
    y.x();
    Y<int> z;
    z.y();
    return 0;
}