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As mentioned before (see section ref(Pretensions)), bf(C++) contains
specialized syntactic possibilities to implement i(data hiding). Data hiding
is the capability of sections of a program to hide its data from other
sections. This results in very clean data definitions. It also allows these
sections to enforce the integrity of their data.
bf(C++) has three keywords that are related to data hiding: ti(private),
ti(protected) and ti(public). These keywords can be used in the definition of
tt(struct)s. The keyword tt(public) allows all subsequent fields of a
structure to be accessed by all code; the keyword tt(private) only allows code
that is part of the tt(struct) itself to access subsequent fields. The keyword
tt(protected) is discussed in chapter ref(INHERITANCE), and is somewhat
outside of the scope of the current discussion.
In a tt(struct) all fields are tt(public), unless explicitly stated otherwise.
Using this knowledge we can expand the tt(struct) tt(Person):
verb( struct Person
{
private:
char d_name[80];
char d_address[80];
public:
void setName(char const *n);
void setAddress(char const *a);
void print();
char const *name();
char const *address();
};)
As the data fields tt(d_name) and tt(d_address) are in a tt(private)
section they are only accessible to the member functions which are defined in
the tt(struct): these are the functions tt(setName), tt(setAddress) etc.. As
an illustration consider the following code:
verb( Person fbb;
fbb.setName("Frank"); // OK, setName is public
strcpy(fbb.d_name, "Knarf"); // error, x.d_name is private)
Data integrity is implemented as follows: the actual data of a tt(struct)
tt(Person) are mentioned in the structure definition. The data are accessed by
the outside world using special functions that are also part of the
definition. These member functions control all traffic between the data fields
and other parts of the program and are therefore also called `interface'
functions. The thus implemented data hiding is illustrated in
fig(datahiding).
figure(first/datahiding)
(Private data and public interface functions of the class Person.)
(datahiding)
The members tt(setName) and tt(setAddress) are declared with tt(char const
*) parameters. This indicates that the functions will not alter the strings
which are supplied as their arguments. Analogously, the members tt(name)
and tt(address) return tt(char const *)s: the compiler prevents callers of
those members from modifying the information made accessible through the
return values of those members.
Two examples of member functions of the tt(struct) tt(Person) are shown
below:
verb( void Person::setName(char const *n)
{
strncpy(d_name, n, 79);
d_name[79] = 0;
}
char const *Person::name()
{
return d_name;
})
The power of member functions and of the concept of data hiding results
from the abilities of member functions to perform special tasks, e.g.,
checking the validity of the data. In the above example tt(setName) copies
only up to 79 characters from its argument to the data member tt(name),
thereby avoiding a i(buffer overflow).
Another illustration of the concept of data hiding is the following. As an
alternative to member functions that keep their data in memory a library could
be developed featuring member functions storing data on file. To convert a
program storing tt(Person) structures in memory to one that stores the
data on disk no special modifications are required. After recompilation
and linking the program to a new library it is converted from storage
in memory to storage on disk. This example illustrates a broader concept than
data hiding; it illustrates emi(encapsulation). Data hiding is a kind of
encapsulation. Encapsulation in general results in reduced coupling of
different sections of a program. This in turn greatly enhances reusability and
maintainability of the resulting software. By having the structure encapsulate
the actual storage medium the program using the structure becomes independent
of the actual storage medium that is used.
Though data hiding can be implemented using tt(struct)s, more often (almost
always) em(classes) are used instead. A class is a kind of struct, except that
a class uses private access by default, whereas structs use public access by
default. The definition of a tt(class) tt(Person) is therefore identical to
the one shown above, except that the keyword tt(class) has
replaced tt(struct) while the initial tt(private:) clause can be omitted. Our
typographic suggestion for class names (and other type names defined by the
programmer) is to start with a capital character to be followed by the
remainder of the type name using lower case letters (e.g., tt(Person)).
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