Accessing protected members in a derived class

Question

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 memb

Answer 1

protected members can be accessed:

• through this pointer
• or to the same type protected members even if declared in base
• or from friend classes, functions

To solve your case you can use one of last two options.

Accept Derived in Derived::DoSomething or declare Derived friend to Base:

class Derived;

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

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

You may also consider public getters in some cases.

Answer 2

You have access to the protected members of Derived, but not those of Base (even if the only reason it's a protected member of Derived is because it's inherited from Base)

Answer 3

class Derived : public Base
{
  protected:
    int d;
  public:
    void DoSomething()
    {
      b+=this->b;
      d=0;
    }
};

//this will work

Answer 4

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

Answer 5

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;
    }
};

Answer 6

Use this pointer to access protected members

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