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Our example class tt(Person) has three string data members and a tt(size_t
d_mass) data member. Access to these data members is controlled by
i(interface functions).
Whenever an object is defined the class's constructor(s) ensure that its data
members are given `sensible' values. Thus, objects never suffer from
uninitialized values. Data members may be given new values, but that should
never be directly allowed. It is a core principle (called
emi(data hiding)) of good class design that its data members are private. The
modification of data members is therefore fully controlled by member functions
and thus, indirectly, by the class-designer. The class em(encapsulates) all
actions performed on its data members and due to this
emi(encapsulation) the class object may assume the `responsibility' for its
own data-integrity. Here is a minimal definition of tt(Person)'s manipulating
members:
verbinclude(-a examples/setmembers.cc)
It's a minimal definition in that no checks are performed. But it should be
clear that checks are easy to implement. E.g., to ensure that a phone number
only contains digits one could define:
verb( void Person::setPhone(string const &phone)
{
if (phone.empty())
d_phone = " - not available -";
else if (phone.find_first_not_of("0123456789") == string::npos)
d_phone = phone;
else
cout << "A phone number may only contain digits\n";
})
Note the double negation in this implementation. Double negations are very
hard to read, and an encapsulating member tt(bool hasOnly) handles the test,
and improves tt(setPhone's) readability:
verb( bool Person::hasOnly(char const *characters, string const &object)
{
// object only contains 'characters'
return object.find_first_not_of(characters) == string::npos;
})
and tt(setPhone) becomes:
verb( void Person::setPhone(string const &phone)
{
if (phone.empty())
d_phone = " - not available -";
else if (hasOnly("0123456789", phone))
d_phone = phone;
else
cout << "A phone number may only contain digits\n";
})
Since tt(hasOnly) is an encapsulated member function we can ensure that
it's only used with non-empty string objects, so tt(hasOnly) itself doesn't
have to check for that.
Access to the data members is controlled by emi(accessor)
members. Accessors ensure that data members cannot suffer from uncontrolled
modifications. Since accessors conceptually do not modify the object's data
(but only retrieve the data) these member functions are given the predicate
tt(const). They are called hi(member: const)emi(const member) em(functions),
which, as they are guaranteed not to modify their object's data, are available
to both modifiable and constant objects (cf. section ref(ConstFunctions)).
To prevent emi(backdoors) we must also make sure that the data member is
not modifiable through an accessor's return value. For values of built-in
primitive types that's easy, as they are usually returned by value, which are
copies of the values found in variables. But since objects may be fairly
large making copies is usually prevented by returning objects by reference. A
backdoor is created by returning a data member by reference, as in the
following example, showing the allowed abuse below the function definition:
verb( string &Person::name() const
{
return d_name;
}
Person somebody;
somebody.setName("Nemo");
somebody.name() = "Eve"; // Oops, backdoor changing the name)
To prevent the backdoor objects are returned as em(const references) from
accessors. Here are the implementations of tt(Person)'s accessors:
verbinclude(-a examples/getmembers.cc)
The tt(Person) class interface remains the starting point for the class
design: its member functions define what can be asked of a tt(Person)
object. In the end the implementation of its members merely is a technicality
allowing tt(Person) objects to do their jobs.
The next example shows how the class tt(Person) may be used. An object is
initialized and passed to a function tt(printperson()), printing the person's
data. Note the reference operator in the parameter list of the function
tt(printperson). Only a reference to an existing tt(Person) object is passed
to the function, rather than a complete object. The fact that
tt(printperson) does not modify its argument is evident from the fact that
the parameter is declared tt(const).
verbinclude(-a examples/persondemo.cc)
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