c++ - Override member field in derived classes

Question

I have a code snippet below:

#include <iostream>

using namespace std;

class Base {
public:
    Base() : b(0) {}
    int get();
    virtual void sayhello() { cout << "Hello from Base with b: " << b << endl; }
private:
    int b;
};

int Base::get() {sayhello(); return b;} 

class Derived : public Base {
public:
    Derived(double b_):b(b_){}
    void sayhello() { cout << "Hello from Derived with b: " << b << endl; }
private:
    double b;
};

int main() {
    Derived d(10.0);
    Base b = d;

    cout << "Derived b: " << d.get() << endl;
    cout << "Base b: " << b.get() << endl;
}

Run the compiled executable and I find the result is out of my expectation on my llvm-g++ 4.2 machine. The output on my box is as

Hello from Derived with b: 10
Derived b: 0
Hello from Base with b: 0
Base b: 0

What I want to do in the code is to override a member field (b) in Derived class. Since I think both Base and Derived need to access this field, I define a get member function in Base, thus Derived can inherit it. Then I try to get the member field from different objects.

The result shows that I still get original b in Base by d.get() instead of that in Derived, which is what I expected the code to do. Anything wrong with the code (or my understanding)? Is this behavior specified in the specification? What is the right way to override a member field and properly define its getter and setter?

Answer 1

The new b added in the derived class doesn't override base's b. It just hides it.

So, in the derived class you have two b and the virtual method prints corresponding b.