What's the difference between a derived object and a base object in c++?

Question

What\'s the difference between a derived object and a base object in c++,

especially, when there is a virtual function in the class.

Does the derived object

Answer 1

What's the difference between a derived object and a base object in c++,

A derived object can be used in place of a base object; it has all the members of the base object, and maybe some more of its own. So, given a function taking a reference (or pointer) to the base class:

void Function(Base &);

You can pass a reference to an instance of the derived class:

class Derived : public Base {};
Derived derived;
Function(derived);

especially, when there is a virtual function in the class.

If the derived class overrides a virtual function, then the overridden function will always be called on objects of that class, even through a reference to the base class.

class Base
{
public:
    virtual void Virtual() {cout << "Base::Virtual" << endl;}
    void NonVirtual()      {cout << "Base::NonVirtual" << endl;}
};

class Derived : public Base
{
public:
    virtual void Virtual() {cout << "Derived::Virtual" << endl;}
    void NonVirtual()      {cout << "Derived::NonVirtual" << endl;}
};

Derived derived;
Base &base = derived;

base.Virtual();      // prints "Derived::Virtual"
base.NonVirtual();   // prints "Base::NonVirtual"

derived.Virtual();   // prints "Derived::Virtual"
derived.NonVirtual();// prints "Derived::NonVirtual"

Does the derived object maintain additional tables to hold the pointers to functions?

Yes - both classes will contain a pointer to a table of virtual functions (known as a "vtable"), so that the correct function can be found at runtime. You can't access this directly, but it does affect the size and layout of the data in memory.