Virtual base class data members

Question

Why it is recommended not to have data members in virtual base class?

What about function members? If I have a task common to all derived classes is it OK for virtua

Answer 1

The core advice is to have a default-constructor in the virtual base. If you don't, then every most-derived class (ie. any subclass) must call the virtual base ctor explicitly, and that leads to angry colleagues knocking on your office door...

class VirtualBase {
public:
    explicit VirtualBase( int i ) : m_i( i ) {}
    virtual ~VirtualBase() {}

private:
    int m_i;
};

class Derived : public virtual VirtualBase {
public:
    Derived() : VirtualBase( 0 ) {} // ok, this is to be expected
};

class DerivedDerived : public Derived { // no VirtualBase visible
public:
    DerivedDerived() : Derived() {} // ok? no: error: need to explicitly
                                    // call VirtualBase::VirtualBase!!
    DerivedDerived() : VirtualBase( 0 ), Derived() {} // ok
};