ЭЛЕКТРОННАЯ БИБЛИОТЕКА КОАПП |
Сборники Художественной, Технической, Справочной, Английской, Нормативной, Исторической, и др. литературы. |
13.1. Constructing an ObjectProblemYou want to create a way for your users to generate new objects. SolutionMake a constructor. In Perl, the constructor method must not only initialize its object, but must also first allocate memory for it, typically using an anonymous hash. C++ constructors, on the other hand, are called with memory already allocated. The rest of the object-oriented world would call C++'s constructors initializers. Here's the canonical object constructor in Perl: sub new { my $class = shift; my $self = { }; bless($self, $class); return $self; } This is the equivalent one-liner: sub new { bless( { }, shift ) } DiscussionAny method that allocates and initializes a new object acts as a constructor. The most important thing to remember is that a reference isn't an object until sub new { bless({}) } Let's add some initialization: sub new { my $self = { }; # allocate anonymous hash bless($self); # init two sample attributes/data members/fields $self->{START} = time(); $self->{AGE} = 0; return $self; } This constructor isn't very useful because it uses the single-argument form of To solve this, have the constructor heed its first argument. For a class method, this is the package name. Pass this class name as the second argument to sub new { my $classname = shift; # What class are we constructing? my $self = {}; # Allocate new memory bless($self, $classname); # Mark it of the right type $self->{START} = Now the constructor can be correctly inherited by a derived class. You might also want to separate the memory allocation and blessing step from the instance data initialization step. Simple classes won't need this, but it makes inheritance easier; see Recipe 13.10. sub new {
my $classname = shift; # What class are we constructing?
my $self = {}; # Allocate new memory
bless($self, $classname); # Mark it of the right type
$self->_init(@_); # Call _init with remaining args
return $self;
}
# "private" method to initialize fields. It always sets START to
# the current time, and AGE to 0. If called with arguments, _init
# interprets them as key+value pairs to initialize the object with.
sub _init {
my $self = shift;
$self->{START} = See Alsoperltoot (1) and perlobj (1); Chapter 5 of Programming Perl; Recipe 13.6; Recipe 13.9; Recipe 13.10 |