class Attribute
Member variable
In Perl 6 lingo, an attribute refers to a per-instance/object storage slot. An Attribute
is used to talk about classes' and roles' attributes on the meta level.
Normal usage of attributes does not require the user to use class Attribute explicitly.
The usual way to obtain an object of type Attribute
is by introspection:
my = Useless.^attributes(:local)[0];say .perl; # OUTPUT: «Attribute.new»say .name; # OUTPUT: «@!things»say .package; # OUTPUT: «(Useless)»say .has_accessor; # OUTPUT: «False»
Modifying a private attribute from the outside is usually not possible, but since Attribute is at the level of the meta class, all is fair game
my = Useless.new;.set_value(, [1, 2, 3]);say .get_value(); # OUTPUT: «[1 2 3]»
Traits
Trait is default
An attribute that is assigned Nil will revert to its default value set with the trait is default
.
my = C.new;say ;.a = Nil;say ;# OUTPUT: «C.new(a => 666)C.new(a => 42)»
Trait is required
The trait is required
will mark the attribute as to be filled with a value when the object is instantiated. Failing to do so will result in a runtime error.
my = C.new;CATCH# OUTPUT: «X::Attribute::Required: The attribute '$!a' is required, but you did not provide a value for it.»
Available as of 6.d language version (early implementation exists in Rakudo compiler 2018.08+):
You can specify a reason why the attribute is required:
my = D.new;CATCH# OUTPUT: «X::Attribute::Required: The attribute '$!a' is required because it is a good idea,but you did not provide a value for it.»
trait is DEPRECATED
multi sub trait_mod:<is>(Attribute , :!)
Marks an attribute as deprecated, optionally with a message what to use instead.
my = C.new( foo => 42 ); # doesn't trigger with initialization (yet)say .foo; # does trigger on usage
After the program is finished, this will show something like this on STDERR:
# Saw 1 occurrence of deprecated code.# =====================================# Method foo (from C) seen at:# script.p6, line 5# Please use 'bar' instead.
Methods
method name
Defined as:
method name(Attribute: --> Str)
Returns the name of the attribute. Note that this is always the private name, so if an attribute is declared as has $.a
, the name returned is $!a
.
my = Foo.^attributes(:local)[0];say .name; # OUTPUT: «@!bar»
method package
Defined as:
method package(Attribute: --> Mu)
Returns the package (class/grammar/role) to which this attribute belongs.
my = Boo.^attributes(:local)[0];say .package; # OUTPUT: «(Boo)»
method has_accessor
Defined as:
method has_accessor(Attribute: --> Bool)
Returns True
if the attribute has a public accessor method.
my = Container.^attributes(:local)[0];my = Container.^attributes(:local)[1];say .has_accessor; # OUTPUT: «False»say .has_accessor; # OUTPUT: «True»
method readonly
Defined as:
method readonly(Attribute: --> Bool)
Returns True
for readonly attributes, which is the default, or False
for attributes marked as is rw
.
my = Library.^attributes(:local)[0];my = Library.^attributes(:local)[1];say .readonly; # OUTPUT: «True»say .readonly; # OUTPUT: «False»
method type
Defined as:
method type(Attribute: --> Mu)
Returns the type constraint of the attribute.
my = TypeHouse.^attributes(:local)[0..2];for 0..2# OUTPUT: «(Positional[Int])# (Mu)# (Positional)»
method get_value
Defined as:
method get_value(Attribute: Mu )
Returns the value stored in this attribute of object $instance
.
my = Violated.^attributes(:local)[0];say .get_value(Violated.new); # OUTPUT: «5»
Note that this method violates encapsulation of the object, and should be used with care. Here be dragons.
method set_value
Defined as:
method set_value(Attribute: Mu , Mu \new_val)
Binds the value new_val
to this attribute of object $instance
.
my = A.^attributes(:local)[0];my = A.new;.speak; # OUTPUT: «5».set_value(, 42);.speak; # OUTPUT: «42»
Note that this method violates encapsulation of the object, and should be used with care. Here be dragons.
trait is required
multi sub trait_mod:<is> (Attribute , :!)
Marks an attribute as being required. If a value is not provided during object construction, an exception is thrown.
;Foo.new(bar => 42); # worksFoo.new(baz => 42);CATCH ;# OUTPUT: «X::Attribute::Required: The attribute '$!bar' is required, but you did not provide a value for it.»
is required
doesn't just affect the default constructor, it checks for the attribute at a lower level, so it will work for custom constructors written using bless.
trait is rw
multi sub trait_mod:<is> (Attribute , :!)
Marks an attribute as read/write as opposed to the default readonly
. The default accessor for the attribute will return a writable value.
;my = Boo.new;.bar = 42; # works.baz = 42;CATCH ;# OUTPUT: «X::Assignment::RO: Cannot modify an immutable Any»
Type Graph
Attribute