The base class tt(Vehicle) is provided with its own concrete implementations
of its virtual members (tt(mass) and tt(setMass)).  However, virtual member
functions do not necessarily em(have) to be implemented in base classes.

When the implementations of virtual members are omitted from base classes the
class imposes requirements upon derived classes. The derived classes are
required to provide the `missing implementations'.

This approach, in some languages (like bf(C#, Delphi)
 hi(Java interface) and bf(Java)) known as an emi(interface), defines a
 emi(protocol). Derived classes em(must) obey the protocol by implementing the
as yet not implemented members. If a class contains at least one member whose
implementation is missing no objects of that class can be defined.

Such incompletely defined classes are always base classes. They enforce a
protocol by merely declaring names, return values and arguments of some of
their members. These classes are call hi(class: abstract)
 emi(abstract classes) or em(abstract base classes).  Derived classes become
non-abstract classes by implementing the as yet not implemented members.

Abstract base classes are the foundation of many em(design patterns)
 hi(design pattern) (cf. em(Gamma et al.) (1995))
 hi(Gamma, E.), allowing the programmer to create highly
 emi(reusable software). Some of these design patterns are covered by the
annotations() (e.g, the emi(Template Method) in section ref(FORK)), but for a
thorough discussion of design patterns the reader is referred to Gamma em(et
al.)'s book.

Members that are merely declared in base classes are called
 emi(pure virtual functions). A virtual member becomes a pure virtual member
by postfixing ti(= 0) to its declaration (i.e., by replacing the semicolon
ending its declaration by `tt(= 0;)'). Example:
        verb(    #include <iosfwd>
    class Base
    {
        public:
            virtual ~Base();
            virtual std::ostream &insertInto(std::ostream &out) const = 0;
    };
    inline std::ostream &operator<<(std::ostream &out, Base const &base)
    {
        return base.insertInto(out);
    })

All classes derived from tt(Base) em(must) implement the tt(insertInto)
member function, or their objects cannot be constructed. This is neat: all
objects of class types derived from tt(Base) can now always be inserted into
ti(ostream) objects.

    Could the i(virtual destructor) of a base class ever be a pure virtual
function? The answer to this question is no. First of all, there is no need to
enforce the availability of destructors in derived classes as destructors are
provided by default (unless a destructor is declared with the tt(= delete)
attribute). Second, if it is a pure virtual member its implementation does not
exist. However, derived class destructors eventually call their base class
destructors. How could they call base class destructors if their
implementations are lacking? More about this in the next section.

    Often, but not necessarily, pure virtual member functions are
tt(const) hi(const member functions) member functions. This allows the
construction of constant derived class objects. In other situations this might
not be necessary (or realistic), and
 i(non-constant member functions) might be required. The general rule for
tt(const) member functions also applies to pure virtual functions: if the
member function alters the object's data members, it cannot be a tt(const)
member function.

Abstract base classes frequently don't have
 hi(class: without data members) data members.  However, once a base class
declares a pure virtual member it em(must) be declared identically in derived
classes. If the implementation of a pure virtual function in a derived class
alters the derived class object's data, then em(that) function cannot be
declared as a tt(const) member. Therefore, the author of an abstract base
class should carefully consider whether a pure virtual member function should
be a tt(const) member function or not.
