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In chapter ref(IOStreams) we encountered the class tt(fstream), one class
offering features of tt(ifstream) and tt(ofstream). In chapter
ref(INHERITANCE) we learned that a class may be derived from multiple base
classes. Such a derived class inherits the properties of all its base
classes. Polymorphism can also be used in combination with multiple
inheritance.
Consider what would happen if more than one `path' leads from the derived
class up to its (base) classes. This is illustrated in the next (fictitious)
example where a class tt(Derived) is doubly derived from tt(Base):
verb( class Base
{
int d_field;
public:
void setfield(int val);
int field() const;
};
inline void Base::setfield(int val)
{
d_field = val;
}
inline int Base::field() const
{
return d_field;
}
class Derived: public Base, public Base
{};)
Due to the double derivation, tt(Base)'s functionality now occurs twice in
tt(Derived). This results in i(ambiguity): when the function tt(setfield()) is
called for a tt(Derived) class object, which function will that be as there
are two of them? The scope resolution operator won't come to the rescue and so
the bf(C++) compiler cannot compile the above example and (correctly)
identifies an error.
The above code clearly duplicates its base class in the derivation, which can
of course easily be avoided by not doubly deriving from tt(Base) (or by using
composition (!)). But duplication of a base class can also occur through
i(nested inheritance), where an object is derived from, e.g., a tt(Car) and
from an tt(Air) (cf. section ref(VehicleSystem)). Such a class would be needed
to represent, e.g., a flying car+footnote(such as the one in James Bond
vs. the Man with the Golden Gun...). An tt(AirCar) would ultimately contain
two tt(Vehicles), and hence two tt(mass) fields, two tt(setMass())
functions and two tt(mass()) functions. Is this what we want?
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