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Classes may be adapted in such a way that their objects may be inserted
into and extracted from, respectively, a tt(std::ostream) and
tt(std::istream).
The class tt(std::ostream) defines insertion operators for primitive
types, such as tt(int), tt(char *), etc.. In this section we learn how to
extend the existing functionality of classes (in particular tt(std::istream)
and tt(std::ostream)) in such a way that they can be used also in combination
with classes developed much later in history.
In particular we will show how the
i(insertion operator) can be overloaded allowing the insertion of any type of
object, say tt(Person) (see chapter ref(MEMORY)), into an tt(ostream). Having
defined such an overloaded operator we're able to use the following code:
verb(
Person kr("Kernighan and Ritchie", "unknown", "unknown");
cout << "Name, address and phone number of Person kr:\n" << kr << '\n';
)
The statement tt(cout) lshift() tt(kr) uses oplshift(). This member
function has two operands: an tt(ostream &) and a tt(Person &). The required
action is defined in an overloaded emi(free function) oplshift() expecting two
arguments:
verb(
// declared in `person.h'
std::ostream &operator<<(std::ostream &out, Person const &person);
// defined in some source file
ostream &operator<<(ostream &out, Person const &person)
{
return
out <<
"Name: " << person.name() << ", "
"Address: " << person.address() << ", "
"Phone: " << person.phone();
}
)
The free function oplshift() has the following noteworthy characteristics:
itemization(
it() The function returns a reference to an tt(ostream) object,
to enable `chaining' of the insertion operator.
it() The two operands of oplshift() are passed to the free function as its
arguments. In the example, the parameter tt(out) was initialized by tt(cout),
the parameter tt(person) by tt(kr).
)
In order to overload the em(extraction) operator for, e.g., the tt(Person)
class, members are needed modifying the class's private data members. Such
hi(modifier)em(modifiers) are normally offered by the class interface. For
the tt(Person) class these members could be the following:
verb(
void setName(char const *name);
void setAddress(char const *address);
void setPhone(char const *phone);
)
These members may easily be implemented: the memory
pointed to by the corresponding data member must be deleted, and the data
member should point to a copy of the text pointed to by the parameter. E.g.,
verb(
void Person::setAddress(char const *address)
{
delete[] d_address;
d_address = strdupnew(address);
}
)
A more elaborate function should check the reasonableness of the new
address (tt(address) also shouldn't be a 0-pointer). This
however, is not further pursued here. Instead, let's have a look at the final
hi(>>)hi(operator>>)oprshift(). A simple implementation is:
verb(
istream &operator>>(istream &in, Person &person)
{
string name;
string address;
string phone;
if (in >> name >> address >> phone) // extract three strings
{
person.setName(name.c_str());
person.setAddress(address.c_str());
person.setPhone(phone.c_str());
}
return in;
}
)
Note the stepwise approach that is followed here. First, the required
information is extracted using available extraction operators. Then, if that
succeeds, em(modifiers) are used to modify the data members of the object to
be extracted. Finally, the stream object itself is returned as a reference.
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