#include <vector>       

class c_A {
    public:
        std::vector<int> v;
        c_A one_element(void) {
            c_A res; 
            res.v = v;
            res.v.resize(1);
            return res;
        };
};

class c_Aext : public c_A { 
public:
    c_Aext() : c_A() { } // just call parent's default
    c_Aext(const c_A c) : c_A(c) { } // call parent's copy

    c_Aext one_element() {
        return c_Aext(c_A::one_element());
    }
};

int main () {
 
   c_Aext Aext;
   Aext.v = {0, 1, 2};
   c_Aext B = Aext.one_element(); // The problem
   return 0;
};