Base and derived classes C++

Question

A few days ago, I wanted to dive into the C++ world. I'm studying the base and derived class concepts. Could someone explain the nuance going on with the following two code snippets?

class A
{
    private:
    virtual int GetValue() { return 10; }

    public:
    int Calculate() { return GetValue()*1.5; }
};

class B: public A
{
    private:
    virtual int GetValue() { return 20; }
};

int main()
{
    B b;
    std::cout << b.Calculate() << std::endl;

    return 0;
}

The output is 30 but 15 was expected

class A
{
    private:
    int m_data;

    public:
    A(): m_data(GetValue()) {}
    int Calculate() { return m_data*1.5; }
    virtual int GetValue() { return 10; }
};

class B: public A
{
    public:
    virtual int GetValue() { return 20; }
};

int main()
{
    B b; A* ap;
    ap=&b; 
    std::cout << ap->Calculate() << std::endl;

    return 0;
}

The output is 15 but 30 was expected

Can someone explain and help me understand the reasoning? Something is wrong with my thinking on this concept, but I am unable to figure it out.

Explain why you thought your code would give those values. – Mar 20 '17 at 16:21 — , Mar 20 '17 at 16:21

score 5 · Accepted Answer · edited May 23 '17 at 12:00

First case:

This is trivial. You have an instantiated instance of B, and you compute return GetValue() * 1.5; which uses B::GetValue() as you've marked GetValue() to be virtual in the base class. Hence 20 * 1.5 is evaluated.

Second case:

Not so trivial. You are calling GetValue() in the base member initialiser to set a value for m_data. Standard C++ dictates that the base class GetValue() method will be called in that case. (Informally think of this as being due to class B not being constructed until class A is fully constructed). Hence 10 * 1.5 is evaluated. Interestingly, if GetValue() was pure virtual, then the behaviour of the program would have been undefined.

Reference: Why a virtual call to a pure virtual function from a constructor is UB and a call to a non-pure virtual function is allowed by the Standard?

Your reply is so helpful, clear, and complete. It was very important for me to understand this before going forward in the course. Thank you so much. I got it now. — QFi, Mar 20 '17 at 16:45

score 2 · Answer 2 · answered Mar 20 '17 at 16:31

Try the following code for the second example:

class A
{
private:
int m_data;

public:
A(): m_data(GetValue()) { std::cout << "Init m_data and A ";}
int Calculate() { return m_data*1.5; }
virtual int GetValue() { std::cout << "GetValue from A ";return 10; }
};

class B: public A
{
public:
B() { std::cout << "Init B "; }
virtual int GetValue() { std::cout << "GetValue from B"; return 20; }
};

int main()
{
B b; A* ap;
ap=&b; 
std::cout << ap->Calculate() << std::endl;

return 0;
}

It is the same as you already have but with outputs. You should get GetValue from A Init m_data and A Init B 15. I hope now you see why you have an output of 15. With the outputs you should be able to reconstruct the execution order.

Base and derived classes C++

2 Answers2