The bf(C) programming language offers two methods for structuring data of
different types. The bf(C) tt(struct) holds data members of various types, and
the bf(C) tt(union) also defines data members of various types. However, a
union's data members all occupy the same location in memory and the programmer
may decide on which one to use.

In this chapter classes are introduced. A tt(class) is a kind of tt(struct),
but its content is by default inaccessible to the outside world, whereas the
content of a bf(C++) tt(struct) is by default accessible to the outside
world. In bf(C++) tt(struct)s find little use: they are mainly used to
aggregate data within the context of classes or to define elaborate return
values. Often a bf(C++) tt(struct) merely contains em(plain old data) (POD,
cf. section ref(POD)). In bf(C++) the tt(class) is the main data structuring
device, by default enforcing two core concepts of current-day software
engineering: em(data hiding) and em(encapsulation) (cf. sections ref(HIDING)
and ref(APPLICATION)).

The tt(union) is another data structuring device the language offers. The
traditional bf(C) union is still available, but bf(C++) also offers
em(unrestricted unions). Unrestricted unions are unions whose data fields may
be of class types. The annotations() covers these unrestricted unions in
section ref(UNIONS), after having introduced several other new concepts of
bf(C++),

bf(C++) extends the bf(C) tt(struct) and tt(union) concepts by allowing the
definition of em(member)
        hi(member function)
    functions (introduced in this chapter) within these data types.  Member
functions are functions that can only be used with objects of these data types
or within the scope of these data types. Some of these member functions are
special in that they are always, usually automatically, called when an object
starts its life (the so-called em(constructor)) or ends its life (the
so-called em(destructor)). These and other types of member functions, as well
as the design and construction of, and philosophy behind, classes are
introduced in this chapter.

We step-by-step construct a class tt(Person), which could be used in a
database application to store a person's name, address and phone number.

label(PERSON) Let's start by creating a tt(class Person) right away. From the
onset, it is important to make the distinction between the class
emi(interface) and its emi(implementation). A class may loosely be defined as
`a set of data and all the functions operating on those data'. This definition
is later refined but for now it is sufficient to get us started.

    A class interface is a definition, defining the organization of objects of
that class. Normally a definition results in memory reservation. E.g., when
defining tt(int variable) the compiler ensures that some memory is reserved in
the final program storing tt(variable)'s values.  Although it is a definition
no memory is set aside by the compiler once it has processed the class
definition. But a class definition follows the emi(one definition rule): in
bf(C++) entities may be defined only once. As a em(class definition) does not
imply that memory is being reserved the term em(class interface) is preferred
instead.

Class interfaces are normally contained in a class em(header file), e.g.,
tt(person.h). We'll start our tt(class Person) interface here (cf section
ref(ConstFunctions) for an explanation of the tt(const) keywords behind some
of the class's member functions):
        verbinclude(-a examples/person.h)
 The member functions that are em(declared) in the interface must still be
implemented. The implementation of these members is properly called their
definition.

In addition to member em(functions) classes also commonly define the em(data)
that are manipulated by those member functions. These data are called the
hi(data member)em(data members). In tt(Person) they are tt(d_name, d_address,
d_phone) and tt(d_mass). Data members should be given private 
    i(access rights). Since the class uses private access rights by default
they are usually simply listed at the top of the class interface.

All communication between the outer world and the class data is routed through
the class's member functions. Data members may receive new values (e.g., using
tt(setName)) or they may be retrieved for inspection (e.g., using
tt(name)). Functions merely returning values stored inside the object, not
allowing the caller to modify these internally stored values, are called
 hi(accessor)em(accessors).

Syntactically there is only a marginal difference between a class and a
struct. Classes by default define em(private) members, structs define
em(public) members. Conceptually, though, there are differences.  In bf(C++)
structs are used in the way they are used in bf(C): to aggregate data, which
are all freely accessible. Classes, on the other hand, hide their data from
access by the outside world (which is aptly called emi(data hiding))
and offer member functions to define the communication between the outer world
and the class's data members.

Following em(Lakos) (Lakos, J., 2001)
    hi(Lakos, J.) bf(Large-Scale C++ Software Design) (Addison-Wesley) I
suggest the following setup of class interfaces:
    itemization(
    it() All data members have em(private access rights), and are
placed at the top of the interface.
    it() All data members start with tt(d_), followed by a name suggesting
their meaning (in chapter ref(StaticDataFun) we'll also
encounter data members starting with tt(s_)).
    it() Non-private data members em(do) exist, but one should be hesitant to
define non-private access rights for data members (see also chapter
ref(INHERITANCE)).
    it() Two broad categories of member functions are
    hi(manipulator)em(manipulators) and em(accessors). Manipulators allow
the users of objects to modify the internal data of the objects. By
convention, manipulators start with tt(set). E.g., tt(setName).
    it() With em(accessors), a tt(get)-prefix is still frequently encountered,
e.g., tt(getName). However, following the conventions promoted by bi(Qt) (see
ti(https://doc.qt.io), e.g., its classes tt(QByteArray) and
tt(QString)), using the tt(get)-prefix is deprecated. So, rather than defining
a member tt(getAddress), it should simply be named tt(address).
    it() Normally (exceptions exist) the public member functions of a class
are listed first, immediately following the class's data members. They are the
important elements of the interface as they define the features the class is
offering to its users. It's a matter of convention to list them high up in the
interface.  The keyword tt(private) is needed beyond the public members to
switch back from public members to private access rights which
nicely separates the members that may be used `by the general public' from the
class's own support members.
    )
    Style conventions usually take a long time to develop. There is nothing
obligatory about them, though. I suggest that readers who have compelling
reasons em(not) to follow the above style conventions use their own. All
others are strongly advised to adopt the above style conventions.

Finally, referring back to section ref(INTRONAME) that
        verb(    using namespace std;)

must be used in most (if not all) examples of source code. As
explained in sections ref(CLASSHEADER) and ref(NAMESPACEHDR) the tt(using)
directive should follow the preprocessor directive(s) including the header
files, using a setup like the following:
        verb(    #include <iostream>
    #include "person.h"

    using namespace std;

    int main()
    {
        ...
    })

