Calling a class method with local variable as parameter - ruby

I want to pass a string in a variable from a method to another class. I have this code:
class A
def method_a
variable = "some string"
B.method_b(variable)
end
end
class B
def self.method_b(parameter)
puts parameter
end
end
This code generates the following error:
Undefined local variable or method `variable`
What am I doing wrong?

What you've defined here is an instance method, one that can only operate on an instance of B:
class B
def self.class_only(v)
puts "Class: #{v}"
end
def instance_only(v)
puts "Instance: #{v}"
end
end
The class_only method does not require an instance:
B.class_only(variable)
The instance_only method must operate on an instance:
b = B.new
b.instance_only(variable)
Now anything the B method is given via arguments is valid, and any local or instance variables on the A side are things you can supply to the call. There's no scope issues here because you're explicitly passing them over.
For example:
class A
def test
variable = SecureRandom.hex(6)
B.class_only(variable)
end
end
A.new.test

Related

Accessing variable defined inside block

I have this snippet:
class MyClass
def self.callWithBlock (&block)
print block.blockVar
end
end
MyClass::callWithBlock do
blockVar = 'Hello'
end
which gives me an error:
in `callWithBlock': undefined method `blockVar' for #<Proc:0x000000017ed168#./block-test.rb:9> (NoMethodError)
from ./block-test.rb:9:in `<main>'
How to access this blockVar?
If you add binding at the end of the block, that would become the result of call-ing the block, and you can eval whatever local variables assigned in that block within the context of the binding.
class MyClass
def self.callWithBlock (&block)
print block.call.eval('blockVar')
end
end
MyClass::callWithBlock do
blockVar = 'Hello'
binding
end
# => Hello

How to pass a method to instance_eval?

I want to call instance_eval on this class:
class A
attr_reader :att
end
passing this method b:
class B
def b(*args)
att
end
end
but this is happening:
a = A.new
bb = B.new
a.instance_eval(&bb.method(:b)) # NameError: undefined local variable or method `att' for #<B:0x007fb39ad0d568>
When b is a block it works, but b as a method isn't working. How can I make it work?
It's not clear exactly what you goal is. You can easily share methods between classes by defining them in a module and including the module in each class
module ABCommon
def a
'a'
end
end
class A
include ABCommon
end
Anything = Hash
class B < Anything
include ABCommon
def b(*args)
a
end
def run
puts b
end
end
This answer does not use a real method as asked, but I didn't need to return a Proc or change A. This is a DSL, def_b should have a meaningful name to the domain, like configure, and it is more likely to be defined in a module or base class.
class B
class << self
def def_b(&block)
(#b_blocks ||= []) << block
end
def run
return if #b_blocks.nil?
a = A.new
#b_blocks.each { |block| a.instance_eval(&block) }
end
end
def_b do
a
end
end
And it accepts multiple definitions. It could be made accept only a single definition like this:
class B
class << self
def def_b(&block)
raise "b defined twice!" unless #b_block.nil?
#b_block = block
end
def run
A.new.instance_eval(&#b_block) unless #b_block.nil?
end
end
def_b do
a
end
end

Dynamic class creation, problem with function definition inside block: function body does not see local scope

I'm writing a function to dynamically create classes. I run into a problem with the *vars variable (below), where inside the block passed to Class::new, the "def initialize" method cannot see the value of *vars (and thus Ruby reports a unbound variable error on *vars).
What to do?
Thanks!
class MyParentClass
def do_something_with(*args)
end
def do_something_else_with(*vars)
end
end
def create_class(class_name,*vars)
new_class = Class::new(MyParentClass) do
def initialize(*args)
super
do_something_with(args)
do_something_else_with(vars)
end
end
Object::const_set(class_name.intern,new_class)
end
# Ruby: Error: *vars is unbound variable
# My notes: *vars is in scope inside the do..end block passed to Class::new, but cannot be seen inside def initialize (why?) . And, how to fix this?
I am not sure what are you trying to achieve with this contraption, but after some changes it works (well, depending on your definition of "to work"):
class MyParentClass
def do_something_with(*args)
puts "something #{args.inspect}"
end
def do_something_else_with(*vars)
puts "else #{vars.inspect}"
end
end
def create_class(class_name,*vars)
new_class = Class::new(MyParentClass) do
define_method :initialize do |*args|
super()
do_something_with(*args)
do_something_else_with(*vars)
end
end
Object::const_set(class_name.intern,new_class)
end
create_class :MyClass, 1, :foo, :bar
MyClass.new(2, :baz)
The trick to make vars visible is to define the constructor using a closure.

How can a class method (inside a module) update an instance variable?

How can a class method (inside a module) update an instance variable? Consider the code bellow:
module Test
def self.included(klass)
klass.extend ClassMethods
end
module ClassMethods
def update_instance_variable
#temp = "It won't work, bc we are calling this on the class, not on the instance."
puts "How can I update the instance variable from here??"
end
end
end
class MyClass
include Test
attr_accessor :temp
update_instance_variable
end
m = MyClass.new # => How can I update the instance variable from here??
puts m.temp # => nil
You'd have to pass your object instance to the class method as a parameter, and then return the updated object from the method.
That does nto quite make sense.
You use the initialize method to set default values.
class MyClass
attr_accessor :temp
def initialize
#temp = "initial value"
end
end
The initialize method is automatically run for you when you create a new object.
When your class declaration is run, there are no, and cannot be any, instances of the class yet.
If you want to be able to change the default values later you can do something like this:
class MyClass
attr_accessor :temp
##default_temp = "initial value"
def initialize
#temp = ##default_temp
end
def self.update_temp_default value
##default_temp = value
end
end
a = MyClass.new
puts a.temp
MyClass.update_temp_default "hej"
b = MyClass.new
puts b.temp
prints
initial value
hej
If you also want that to change already created instances' variables you need additional magic. Please explain exactly what you wish to accomplish. You are probably doing it wrong :)

Ruby - How to use the method parameter as the name of the variable?

How would I use the parameter value as the instance variable name of an object?
This is the object
Class MyClass
def initialize(ex,ey)
#myvar = ex
#myothervar = ey
end
end
I have the following method
def test(element)
instanceofMyClass.element #this obviously doesnt work
end
How can I have the test method return either myvar or myothervar value depending on the element parameter. I don't want to write an if condition though, I want to pass myvar or myother var via element to the object instance if possible.
def test(element)
instanceofMyClass.send(element.to_sym)
end
You'll get a missing method error if instanceofMyClass doesn't respond to element.
def test(element)
instanceofmyclass.instance_variable_get element
end
test :#myvar # => ex
test :#myothervar # => ey
I like the simplicity of send(), though one bad thing with it is that it can be used to access privates. The issue is still remains solution below, but at least then it's explicitly specified, and reader can see which methods are to be forwarded. The first one just uses delegation, while the second one uses more dynamic way to define methods on the fly.
require 'forwardable'
class A
extend Forwardable
def_delegators :#myinstance, :foo, :bar
class B
def foo
puts 'foo called'
end
def bar
puts 'bar called'
end
def quux
puts 'quux called'
end
def bif
puts 'bif called'
end
end
def initialize
#myinstance = B.new
end
%i(quux bif).each do |meth| # note that only A#quux and A#bif are defined dynamically
define_method meth do |*args_but_we_do_not_have_any|
#myinstance.send(meth)
end
end
end
a = A.new
a.foo
a.bar
a.quux
a.bif

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