|Ruby user's guide||Object initialization|
Our Fruit class from the previous chapter had two instance variables,
one to describe the kind of fruit and another to describe its
condition. It was only after writing a custom
method for the class that we realized it didn't make sense for a piece
of fruit to lack those characteristics. Fortunately, ruby provides a
way to ensure that instance variables always get initialized.
Whenever Ruby creates a new object, it looks for a method named
initialize and executes it. So one simple thing we can
do is use an
initialize method to put default values into
all the instance variables, so the
inspect method will
have something to say.
ruby> class Fruit | def initialize | @kind = "apple" | @condition = "ripe" | end | end nil ruby> f4 = Fruit.new "a ripe apple"
There will be times when a default value doesn't make a lot of sense.
Is there such a thing as a default kind of fruit? It may be
preferable to require that each piece of fruit have its kind specified
at the time of its creation. To do this, we would add a formal
argument to the
initialize method. For reasons we won't
get into here, arguments you supply to
new are actually
ruby> class Fruit | def initialize( k ) | @kind = k | @condition = "ripe" | end | end nil ruby> f5 = Fruit.new "mango" "a ripe mango" ruby> f6 = Fruit.new ERR: (eval):1:in `initialize': wrong # of arguments(0 for 1)
Above we see that once an argument is associated with the
initialize method, it can't be left off without
generating an error. If we want to be more considerate, we can use
the argument if it is given, or fall back to default values otherwise.
ruby> class Fruit | def initialize( k="apple" ) | @kind = k | @condition = "ripe" | end | end nil ruby> f5 = Fruit.new "mango" "a ripe mango" ruby> f6 = Fruit.new "a ripe apple"
You can use default argument values for any method, not just
initialize. The argument list must be arranged so that
those with default values come last.
Sometimes it is useful to provide several ways to initialize an object. Although it is outside the scope of this tutorial, ruby supports object reflection and variable-length argument lists, which together effectively allow method overloading.