Jason Baker Jason Baker - 1 month ago 6
C++ Question

Accessing protected members from subclasses: gcc vs msvc

In visual C++, I can do things like this:

template <class T>
class A{
protected:
T i;
};

template <class T>
class B : public A<T>{
T geti() {return i;}
};


If I try to compile this in g++, I get an error. I have to do this:

template <class T>
class B : public A<T>{
T geti() {return A<T>::i;}
};


Am I not supposed to do the former in standard C++? Or is something misconfigured with gcc that's giving me errors?

Answer

This used to be allowed, but changed in gcc 3.4.

In a template definition, unqualified names will no longer find members of a dependent base (as specified by [temp.dep]/3 in the C++ standard). For example,

    template <typename T> struct B {
      int m;
      int n;
      int f ();
      int g ();
    };
    int n;
    int g ();
    template <typename T> struct C : B<T> {
      void h ()
      {
        m = 0; // error
        f ();  // error
        n = 0; // ::n is modified
        g ();  // ::g is called
      }
    };

You must make the names dependent, e.g. by prefixing them with this->. Here is the corrected definition of C::h,

    template <typename T> void C<T>::h ()
    {
      this->m = 0;
      this->f ();
      this->n = 0
      this->g ();
    }