Accessing protected members in a derived class

user_33152 0
Viewed 104121

I ran into an error yesterday and, while it's easy to get around, I wanted to make sure that I'm understanding C++ right.

I have a base class with a protected member:

class Base
{
  protected:
    int b;
  public:
    void DoSomething(const Base& that)
    {
      b+=that.b;
    }
};

This compiles and works just fine. Now I extend Base but still want to use b:

class Derived : public Base
{
  protected:
    int d;
  public:
    void DoSomething(const Base& that)
    {
      b+=that.b;
      d=0;
    }
};

Note that in this case DoSomething is still taking a reference to a Base, not Derived. I would expect that I can still have access to that.b inside of Derived, but I get a cannot access protected member error (MSVC 8.0 - haven't tried gcc yet).

Obviously, adding a public getter on b solved the problem, but I was wondering why I couldn't have access directly to b. I though that when you use public inheritance the protected variables are still visible to the derived class.

c++
8 Answers

You can try with static_cast< const Derived*>(pBase)->Base::protected_member ...

class Base
{
  protected:
    int b;

  public:
    ...
};

class Derived : public Base
{
  protected:
    int d;

  public:
    void DoSomething(const Base& that)
    {
      b += static_cast<const Derived*>(&that)->Base::b;
      d=0;
    }
    void DoSomething(const Base* that)
    {
      b += static_cast<const Derived*>(that)->Base::b;
      d=0;
    }
};
user_101192570 
class Derived : public Base
{
  protected:
    int d;
  public:
    void DoSomething()
    {
      b+=this->b;
      d=0;
    }
};

//this will work
user_81604070

Following the hack for stl I wrote a small code which seems to solve the problem of accessing the protected members in derived class

#include <iostream>

class B
{
protected:
    int a;
public:
    void dosmth()
    {
        a = 4;
    }

    void print() {std::cout<<"a="<<a<<std::endl;}
};

class D: private B
{
public:
    void dosmth(B &b)
    {
        b.*&D::a = 5;
    }
};

int main(int argc, const char * argv[]) {

    B b;
    D d;
    b.dosmth();
    b.print();
    d.dosmth(b);
    b.print();

    return 0;
}

Prints

a=4
a=5
user_126909320
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