Using protected members

Question

I'm having trouble understanding how to have a child class use a protected member from a parent class.

Suppose that in the parent class, I have a function, in public, that will set data_parent, a protected member, equal to some value.

Now, in the child class, I have a protected member called data_child and have a function, in public, that will return data_parent + data_child.

But, when I try to compile I receive the error " use of undeclared identifier 'data_parent' ". What am I doing wrong?

Please post an [MCVE](http://www.stackoverflow.com/help/mcve). The error suggests there is no such variable `data_parent`... — Barry, Apr 04 '15 at 23:29
Please provide the code you tried to run. We can't understand what you meant exactly without code. — CodeMonkey, Apr 04 '15 at 23:55
You should really provide a source as your description of problem seems t be incorrect. — Predrag Manojlovic, Apr 05 '15 at 00:18
Duplicate of http://stackoverflow.com/q/4643074/560648, as it turns out. — Lightness Races in Orbit, Apr 05 '15 at 00:53

score 3 · Answer 1 · answered Apr 04 '15 at 23:51

I can't explain your error message, except that your code differs somehow from your description.

The following is essentially what you have described except that I added constructors (which are optional), picked names for the member functions, and added a driver program to test. It works without error.

#include <iostream>

class Parent
{
    public:
       Parent() : data_parent(0) {};
       void SetDataParent(int x) {data_parent = x;};
    protected:
        int data_parent;
};

class Child: public Parent
{
    public:
       Child() : Parent(), data_child(0) {};
       void SetDataChild(int x) {data_child = x;};
       int GetSum() const {return data_child + data_parent;};
    protected:
        int data_child;
};

int main()
{
    Child child;
    child.SetDataParent(42);
    child.SetDataChild(100);
    std::cout << child.GetSum() << '\n';
}

My guess is that you forgot to have Child inherit from Parent, making for a fairly tense family relationship.

This answer is deeply offensive to every adopted child on the planet. — Lightness Races in Orbit, Apr 05 '15 at 00:50

score 0 · Answer 2 · answered Apr 05 '15 at 00:31

0

Solved the problem. I did not realize that there was a difference between inheriting from a normal class and a template class. I needed to include

using Parent<type>::data_parent;

answered Apr 05 '15 at 00:31

scrubhelpplz

1
1

Or write `this->data_parent`. – Lightness Races in Orbit Apr 05 '15 at 00:51
That's a key bit of information you left out .... that the base class is a template specialisation. – Peter Apr 05 '15 at 07:12

Using protected members

2 Answers2