C++ – cannot call base class protected functions

cinheritance

I cant call protected function in my base class. Why? It looks something like this:

class B : B2
{
public:
  virtual f1(B*)=0;
protected:
  virtual f2(B*) { codehere(); }
}
class D : public B
{
public:
  virtual f1(B*b) { return f2(b); }
protected:
  virtual f2(B*b) { return b->f2(this); }
}

In msvc I get the error error C2248: 'name::class::f2' : cannot access protected member declared in class 'name::class'

In gcc I get error: 'virtual int name::class::f2()' is protected.

Why is that? I thought the point of protected members is for derived classes to call.

Best Answer

Protected member functions can only be called inside the base class or in its derived class. You cannot call them outside your class. Outside calling means calling a member function of a class-typed variable.

virtual f1(B*b) { return f2(b); }

is ok, because f2 operates on the class itself. (called inside)

But

virtual f2(B*b) { return b->f2(this); }

won't compile, because f2 operates on b not the class D itself. (called outside) It's illegal.

To fix it B::f2 should be public.

Related Solutions

C# – Calling the base constructor in C#

Modify your constructor to the following so that it calls the base class constructor properly:

public class MyExceptionClass : Exception
{
    public MyExceptionClass(string message, string extrainfo) : base(message)
    {
        //other stuff here
    }
}

Note that a constructor is not something that you can call anytime within a method. That's the reason you're getting errors in your call in the constructor body.

C++ – How to call a parent class function from derived class function

I'll take the risk of stating the obvious: You call the function, if it's defined in the base class it's automatically available in the derived class (unless it's private).

If there is a function with the same signature in the derived class you can disambiguate it by adding the base class's name followed by two colons base_class::foo(...). You should note that unlike Java and C#, C++ does not have a keyword for "the base class" (super or base) since C++ supports multiple inheritance which may lead to ambiguity.

class left {
public:
    void foo();
};

class right {
public:
    void foo();
};

class bottom : public left, public right {
public:
    void foo()
    {
        //base::foo();// ambiguous
        left::foo();
        right::foo();

        // and when foo() is not called for 'this':
        bottom b;
        b.left::foo();  // calls b.foo() from 'left'
        b.right::foo();  // call b.foo() from 'right'
    }
};

Incidentally, you can't derive directly from the same class twice since there will be no way to refer to one of the base classes over the other.

class bottom : public left, public left { // Illegal
};

Best Answer

Related Solutions

C# – Calling the base constructor in C#

C++ – How to call a parent class function from derived class function

Related Topic