In this example code where I have child classes (Bar, Baz) inheriting the class methods of the parent (Foo), how might I ensure @foo is only created once across all children?
class Foo
def self.foo
# only want @foo to be set once across any child classes
# that may call this inherited method.
@foo ||= expensive_operation
end
end
class Bar < Foo
def self.bar
self.foo + 'bar'
end
end
class Baz < Foo
def self.baz
self.foo + 'baz'
end
end
Instead of depending on a class-specific instance variable being present, just reference it directly:
class Baz < Foo
def self.baz
Foo.foo + 'baz'
end
end
@foo in self.foo, but the callers need only call Foo.foo? And that will ensure @foo is only set once?
Foo.foo then it's going to ensure that all calls are directed to exactly the same instance. If you call self they're on their own unless you start to share things between class instances, and then it gets messy. This is the most direct approach.
Class, named Foo.This is exactly what class instance variable is intended for - language mechanism to have one variable shared between classes which inherit from a class. It's generally not recommended to use because it behaves like you want to - and that's confusing for people coming from other languages.
class Foo
def self.foo
# notice @@
@@foo ||= expensive_operation
end
def self.expensive_operation
puts "Expensive operation"
"cached value "
end
end
class Bar < Foo
def self.bar
self.foo + 'bar'
end
end
class Baz < Foo
def self.baz
self.foo + 'baz'
end
end
Foo.foo
Bar.bar
Baz.baz
This prints Expensive operation only once.
@@foo = "WTF?" immediately before Foo.foo after all the classes are defined.warning: class variable access from toplevel. I'd expect that not to be possible, syntax error. I have no idea how it knows where @@foo is defined, but it's a warning that stops execution of program (nothing gets printed) so it's pretty close.puts to print anything to $stdout.
self.foo + 'bar'can be writtenfoo + barasselfisBarwhen that is executed. Same forBaz.