Question

how can I do that? I thought of an method in the complex class which copies every variable of the basic object to the complex object, but that seems a little bit to inconvenient.

class Basic
{
  //basic stuff
}

class Complex : public Basic
{
  //more stuff
}

Basic * basicObject = new Basic();
//now "extending" basicObject and "cast" it to Complex type
//which means copy everything in basicObject to an complexObject

or something like:

Complex * complexObject = new Complex();
complexObject.getEverythingFrom(basicObject);

seems to be too inconvenient, because everytime I change the Basic class, I have to change this “copy” method too.

Answer 1

Define the values you want to share between the classes in protected section like so:

class Base
{
public:
int myPublicShared1;
int myPublicShared2;

Base& operator = (Base& other)
{
    // Copy contents in base across
    return *this;
}
protected:
int myShared1;
int myShared2;

private:
int notShared1;
int notShared2;
};

class Derived : public Base
{
public:
Derived& operator = (Derived& other)
{
    Base::operator = (other);
    // copy the rest of variables specific to Derived class.
}
Derived& operator = (Base& other)
{
    Base::operator = (other);
}
// Derived now has all variables declared in Base's public and protected section
};