X<multiple inheritance>
Multiple inheritance often indicates a design problem, but Perl always
-give you enough rope to hang yourself with if you really need to.
+gives you enough rope to hang yourself with if you ask for it.
To declare multiple parents, you simply need to pass multiple class
names to C<use parent>:
So given the diagram above, Perl will search C<Child>, C<Father>,
C<PaternalGrandparent>, C<SharedGreatGrandParent>, C<Mother>, and
-finally C<MaternalGrandparent> This is a problem because now we're
+finally C<MaternalGrandparent>. This may be a problem because now we're
looking in C<SharedGreatGrandParent> I<before> we've checked all its
derived classes (i.e. before we tried C<Mother> and
C<MaternalGrandparent>).
use parent 'Father', 'Mother';
This pragma lets you switch to the "C3" resolution order. In simple
-terms, "C3" order ensures that parent classes are never searched before
-child classes, so Perl will now search: C<Child>, C<Father>,
+terms, "C3" order ensures that shared parent classes are never searched
+before child classes, so Perl will now search: C<Child>, C<Father>,
C<PaternalGrandparent>, C<Mother> C<MaternalGrandparent>, and finally
C<SharedGreatGrandParent>. Note however that this is not
"breadth-first" searching: All the C<Father> ancestors (except the
attribute is acceptable.
Finally, using accessors makes inheritance much simpler. Subclasses can
-use the accessors rather than having to know the inner details of the
-object.
+use the accessors rather than having to know how a parent class is
+implemented internally.
=head3 Writing Accessors
X<accessor>
As with constructors, Perl provides no special accessor declaration
-syntax, so classes must write them by hand. There are two common types
-of accessors, read-only and read-write.
+syntax, so classes must provide explicitly written accessor methods.
+There are two common types of accessors, read-only and read-write.
A simple read-only accessor simply gets the value of a single
attribute:
=head2 Method Call Variations
X<method>
-Perl supports several other ways to call methods besides the typical
-C<< $object->method() >> pattern we've seen so far.
+Perl supports several other ways to call methods besides the C<<
+$object->method() >> usage we've seen so far.
=head3 Method Names as Strings
$file->${ \'save' };
$file->${ returns_scalar_ref() };
$file->${ \( returns_scalar() ) };
+ $file->${ returns_sub_ref() };
This works if the dereference produces a string I<or> a subroutine
reference.
X<invocation>
Because Perl allows you to use barewords for package names and
-subroutine names, it can sometimes guess wrong about what you intend a
-bareword to be. For example, the construct C<< Class->new() >> can be
+subroutine names, it sometimes interprets a bareword's meaning
+incorrectly. For example, the construct C<< Class->new() >> can be
interpreted as either C<< 'Class'->new() >> or C<< Class()->new() >>.
In English, that second interpretation reads as "call a subroutine
-named Class(), then call new() as a method on the return value". If
-there is a subroutine named C<Class()> in the current namespace, Perl
-will always interpret C<Class->new()> as the second alterative: a call
-to C<new()> on the object returned by a call to C<Class()>
+named Class(), then call new() as a method on the return value of
+Class()". If there is a subroutine named C<Class()> in the current
+namespace, Perl will always interpret C<< Class->new() >> as the second
+alternative: a call to C<new()> on the object returned by a call to
+C<Class()>
You can force Perl to use the first interpretation (i.e. as a method
call on the class named "Class") in two ways. First, you can append a
We recommend that you avoid this syntax, for several reasons.
First, it can be confusing to read. In the above example, it's not
-clear if C<save> is a method or simply a subroutine that expects a file
-object as its first argument.
+clear if C<save> is a method provided by the C<File> class or simply a
+subroutine that expects a file object as its first argument.
When used with class methods, the problem is even worse. Because Perl
allows subroutine names to be written as barewords, Perl has to guess
use Scalar::Util 'blessed';
- if ( blessed($thing) ) { ... }
+ if ( defined blessed($thing) ) { ... }
If C<$thing> refers to an object, then this function returns the name
-of the package the object has been blessed into. Note that the example
-above will return false if C<$thing> has been blessed into a class
-named "0". If C<$thing> doesn't contain a reference to a blessed
-object, the C<blessed> function returns false (specifically: C<undef>).
+of the package the object has been blessed into. If C<$thing> doesn't
+contain a reference to a blessed object, the C<blessed> function
+returns C<undef>.
+
+Note that C<blessed($thing)> will also return false if C<$thing> has
+been blessed into a class named "0". This is a possible, but quite
+pathological. Don't create a class named "0" unless you know what
+you're doing.
Similarly, Perl's built-in C<ref> function treats a reference to a
blessed object specially. If you call C<ref($thing)> and C<$thing>
The C<isa> method returns I<true> if the object is a member of the
class in C<$class>, or a member of a subclass of C<$class>.
+If you override this method, it should never throw an exception.
+
=item DOES($role)
X<DOES>
provided for use by object system extensions that implement roles, like
C<Moose> and C<Role::Tiny>.
-You can also override C<DOES> directly in your own classes.
+You can also override C<DOES> directly in your own classes. If you
+override this method, it should never throw an exception.
=item can($method)
X<can>
overload C<can> to return a subroutine reference for methods which your
C<AUTOLOAD> method handles.
+If you override this method, it should never throw an exception.
+
=item VERSION($need)
X<VERSION>
X<AUTOLOAD>
If you call a method that doesn't exist in a class, Perl will throw an
-error. However, if that class or any of its parent classes defined an
-C<AUTOLOAD> method, that method will be called instead.
+error. However, if that class or any of its parent classes defines an
+C<AUTOLOAD> method, that C<AUTOLOAD> method is called instead.
-This is called as a regular method, and the caller will not know the
-difference. Whatever value your C<AUTOLOAD> method returns is given to
-the caller.
+C<AUTOLOAD> is called as a regular method, and the caller will not know
+the difference. Whatever value your C<AUTOLOAD> method returns is
+returned to the caller.
The fully qualified method name that was called is available in the
C<$AUTOLOAD> package global for your class. Since this is a global, if
Without the C<our $AUTOLOAD> declaration, this code will not compile
under the L<strict> pragma.
-As the comment says, this is not a good way to implement accessors.
-It's slow and too clever by far. See L<perlootut> for recommendations
-on OO coding in Perl.
+As the comment says, this is not a good way to implement accessors.
+It's slow and too clever by far. However, you may see this as a way to
+provide accessors in older Perl code. See L<perlootut> for
+recommendations on OO coding in Perl.
If your class does have an C<AUTOLOAD> method, we strongly recommend
that you override C<can> in your class as well. Your overridden C<can>
projects / platform / upstream / perl.git / commitdiff