What does this statement do?

ruby-talk
1

Hi Guys,

I have been wondering what does this statement do and what does it set.
It interferes with my methods_missing handler:

{code}
class MyClass

   something = "something else"

end
{code}

···

--
Posted via http://www.ruby-forum.com/.

2

Stanislaw Wozniak wrote:

Hi Guys,

I have been wondering what does this statement do and what does it set.
It interferes with my methods_missing handler:

{code}
class MyClass

   something = "something else"

end
{code}

I guess this is assigning a string to a variable that is created in the
MyClass scope. But my problem is that I have some setter methods that
I'm using through method_missing, but method_missing is never triggered
because it is assigning stuff just like in the example above.

···

--
Posted via http://www.ruby-forum.com/\.

3

Stanislaw Wozniak wrote:

Stanislaw Wozniak wrote:

Hi Guys,

I have been wondering what does this statement do and what does it set.
It interferes with my methods_missing handler:

{code}
class MyClass

   something = "something else"

end
{code}

I guess this is assigning a string to a variable that is created in the MyClass scope. But my problem is that I have some setter methods that I'm using through method_missing, but method_missing is never triggered because it is assigning stuff just like in the example above.

You're saying that "something" is supposed to be treated like a method instead of like a variable? Try

self.something = "something else"

···

--
RMagick: http://rmagick.rubyforge.org/

4

That statement is the same as calling MyClass.something=("something else")

If you want method_missing to catch it, you need to define a
method_missing on the class itself, like this:

class MyClass

  def self.method_missing(name, *args)
  end

end

Jason

···

On Mon, Mar 2, 2009 at 9:18 AM, Stanislaw Wozniak <stan@wozniak.com> wrote:

Stanislaw Wozniak wrote:

Hi Guys,

I have been wondering what does this statement do and what does it set.
It interferes with my methods_missing handler:

{code}
class MyClass

something = "something else"

end
{code}

I guess this is assigning a string to a variable that is created in the
MyClass scope. But my problem is that I have some setter methods that
I'm using through method_missing, but method_missing is never triggered
because it is assigning stuff just like in the example above.
--
Posted via http://www.ruby-forum.com/\.

5

Ok, this is the example I want to get to work. Method set_field is never
triggered because local variable set_field is being set.

class A

  def self.set_field=(value)
      puts "Trying to set field value"
  end

  def method_missing(name, *args)
  self.class.method(name).call args
  end
  def self.method_missing(name, *args)
  new.method(name).call args
  end

end

class B < A

   def my_method(value)
      set_field = value
   end

end

B.my_method

···

--
Posted via http://www.ruby-forum.com/.

6

Stanislaw Wozniak wrote:

Hi Guys,

I have been wondering what does this statement do and what does it set.
It interferes with my methods_missing handler:

{code}
class MyClass

   something = "something else"

end
{code}

I guess this is assigning a string to a variable that is created in the
MyClass scope. But my problem is that I have some setter methods that
I'm using through method_missing, but method_missing is never triggered
because it is assigning stuff just like in the example above.
--
Posted via http://www.ruby-forum.com/\.

That statement is the same as calling MyClass.something=("something else")

Actually, no. See Rick's excellent explanation. For test addicts:

[oracle@ora01 ~]$ ruby y.rb
test 1
foo called with 123
test 2
[oracle@ora01 ~]$ cat y.rb

class X
   def self.foo=(a)
     printf "foo called with %p\n", a
   end
end

puts "test 1"
X.foo = 123

class X
   puts "test 2"
   foo = 987
end

[oracle@ora01 ~]$

If you want method_missing to catch it, you need to define a
method_missing on the class itself, like this:

class MyClass

  def self.method_missing(name, *args)
  end

end

Won't help here, because the statement is interpreted as a local variable assignment.

Kind regards

  robert

···

On 02.03.2009 15:27, Jason Roelofs wrote:

On Mon, Mar 2, 2009 at 9:18 AM, Stanislaw Wozniak <stan@wozniak.com> wrote:

7

Yes this is a well-known ruby newbie gotcha.

Because the Ruby syntax allows simple names to refer to either a local
variable or a method invocation, there are some times when things get
ambiguous.

In the case of a name on the right hand side of an assignment, or equivalent
settings, the ruby compiler treats the name as a local variable if the name
has already been assigned a value, and as a method call with an implied
receiver of self otherwise.

In the case of a name on the right hand side, it treats the name as a local
variable, and assigns it a value.

You HAVE to explicitly give a receiver of self when invoking an attribute
setter method in ruby. So in my_method you have to use self.set_field =
value.

And this is why, for those who like to mark methods as private.

class Foo

private
def a=(value)
     ...
   end

   def b
     ....
   end

   public
   def c
      self.a= 2 # This is okay
      self.b # This triggers a NoMethodError "private method 'b'
called for ...
   end
end

···

On Mon, Mar 2, 2009 at 9:31 AM, Stanislaw Wozniak <stan@wozniak.com> wrote:

Ok, this is the example I want to get to work. Method set_field is never
triggered because local variable set_field is being set.

class A

def self.set_field=(value)
     puts "Trying to set field value"
end

def method_missing(name, *args)
self.class.method(name).call args
end
  def self.method_missing(name, *args)
  new.method(name).call args
end

end

class B < A

  def my_method(value)
     set_field = value
  end

end

B.my_method

--
Rick DeNatale

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