accessing base class public member from derived class

Question

Is it possible to access base class public member from instance of derived class in some other locations in the program.

class base {
public:
    int x;

    base(int xx){
    x = xx;
    }
};

class derived : base {
public:
    derived(int xx) : base(xx){
    }
};

class main {
public:
    derived * myDerived;      

    void m1(){
        myDerived = new derived(5);
        m2(myDerived);  
    }

    void m2(derived * myDerived){
        printf("%i", myDerived->x);
    }    
};

After above code, I got following error.

`error: 'int base::x' is inaccessible`

score 8 · Answer 1 · answered Oct 31 '12 at 13:59

8

The problem is that you accidentally use private inheritance here

class derived : base {

This makes all base class members private in the derived class.

Change this to

class derived : public base {

and it will work as expected.

answered Oct 31 '12 at 13:59

Bo Persson

90,663
31
146
203

score 6 · Accepted Answer · answered Oct 31 '12 at 13:59

6

You inherit privately from the base class. What you typically need is public inheritance:

class derived : public base

Here is the FAQ on private inheritance.

answered Oct 31 '12 at 13:59

nogard

9,432
6
33
53

score 3 · Answer 3 · answered Oct 31 '12 at 14:00

You should inherit from base publicly, then.

class derived : public base {
public:
    derived(int xx) : base(xx){
    }
};

Private inheritance is used in very specific circumstances, such as when you have a has-a relationship between two classes, but you also need to override a member of the base class.

score 1 · Answer 4 · answered Oct 31 '12 at 14:00

From outside the class, you can only access public members of public base classes; and inheritance is private by default when you define a class using the class keyword.

To make it accessible, you need public inheritance:

class derived : public base
                ^^^^^^

score 1 · Answer 5 · answered Oct 31 '12 at 14:00

1

Try:

class derived : public base {
    ...
};

answered Oct 31 '12 at 14:00

cjamesm

109
7

score 0 · Answer 6 · edited Feb 05 '14 at 14:41

Use public inheritance:

class derived : public base {
  ...
};

or

Make x private instead of public and use following code:

class Base {
        int x;
        public:
            Base (int xx) {
                x = xx;
            }
            void display() {
                cout << "x = " << x << endl;
            }
    };

    class Derived : public Base {
        public:
            Derived (int xx) : Base (xx) {
            }
    };

    int main() {
        Derived d1(2);
        Derived *d = new Derived(10);
        d->display();
        d1.display();
        return 0;
    }

accessing base class public member from derived class

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