What does this statement do?

Ruby
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}

2

Stanislaw W. 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.

3

Stanislaw W. wrote:

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”

4

On Mon, Mar 2, 2009 at 9:18 AM, Stanislaw W. [email protected]
wrote:

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

5

On Mon, Mar 2, 2009 at 9:31 AM, Stanislaw W. [email protected]
wrote:

self.class.method(name).call args
set_field = value
end

end

B.my_method

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

Rick DeNatale

Blog: http://talklikeaduck.denhaven2.com/
Twitter: http://twitter.com/RickDeNatale
WWR: http://www.workingwithrails.com/person/9021-rick-denatale
LinkedIn: http://www.linkedin.com/in/rickdenatale

6

Thanks Rick,

This is clear explanation. Makes sense…

7

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

8

On 02.03.2009 15:27, Jason R. wrote:

something = “something else”

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