Setting instance variable "dynamically"

ruby-talk
1

The following will work with arbitrary objects …

obj.instance_eval(“lambda { |v| @a = v }”).call(value)

Well, you can do it simpler, even if you want to use
instance_eval. No need for a lambda:

irb(main):001:0> class Foo
irb(main):002:1> def test(obj)
irb(main):003:2> instance_eval(“@x=obj”)
irb(main):004:2> end
irb(main):005:1> end

I stand corrected. For some reason I thought that the value of ‘obj’
wouldn’t be available inside of an instance_eval.

At this point you must be thinking: “Why don’t I just use
instance_variable_set?”

I’d love to see the answer to this question you’d suggest. :-))

The question was meant to be rhetorical, although with the simplification
the choice is less clear.

···

–
– Jim Weirich / Compuware
– FWP Capture Services
– Phone: 859-386-8855

2

“Weirich, James” James.Weirich@FMR.COM schrieb im Newsbeitrag
news:1C8557C418C561429998C1F8FBB283A728BA95@MSGDALCLB2WIN.DMN1.FMR.COM…

The following will work with arbitrary objects …

obj.instance_eval(“lambda { |v| @a = v }”).call(value)

Well, you can do it simpler, even if you want to use
instance_eval. No need for a lambda:

irb(main):001:0> class Foo
irb(main):002:1> def test(obj)
irb(main):003:2> instance_eval(“@x=obj”)
irb(main):004:2> end
irb(main):005:1> end

I stand corrected. For some reason I thought that the value of ‘obj’
wouldn’t be available inside of an instance_eval.

As far as I can see instance_eval just changes the value of ‘self’.

At this point you must be thinking: “Why don’t I just use
instance_variable_set?”

I’d love to see the answer to this question you’d suggest. :-))

The question was meant to be rhetorical,

That’s what I assumed. I thought you might come up with a funny answer…
:slight_smile:

although with the simplification
the choice is less clear.

Yes, that’s true. And, as we have seen, instance_eval is more backward
compatible.

Kind regards

robert
3

As far as I can see instance_eval just changes the value of 'self'.

it change also the class

svg% ruby -e 'b = ; b.instance_eval { def a() puts "array"; end}; b.a'
array
svg%

Guy Decoux

4

“ts” decoux@moulon.inra.fr schrieb im Newsbeitrag
news:200401191613.i0JGDRQ14600@moulon.inra.fr…

As far as I can see instance_eval just changes the value of ‘self’.

it change also the class

svg% ruby -e ‘b = ; b.instance_eval { def a() puts “array”; end}; b.a’
array
svg%

Well, but that is just a consequence of self pointing to another instance.
You could have achieved the same by doing:

17:50:29 [w]: ruby -e ‘b = ; b.instance_eval { def self.a() puts
“array”; end}; b.a’
array

To verify it’s really an instance method:

17:50:23 [w]: ruby -e ‘b = ; b.instance_eval { def a() puts “array”;
end}; b.a; .a’
array
-e:1: undefined method `a’ for :Array (NoMethodError)

Am I overlooking something?

Kind regards

robert
5

17:50:29 [w]: ruby -e 'b = ; b.instance_eval { def self.a() puts
"array"; end}; b.a'
array

def a() end is not the same than def self.a() end

Am I overlooking something?

nothing important

Guy Decoux

6

it change also the class

svg% ruby -e ‘b = ; b.instance_eval { def a() puts “array”; end}; b.a’
array
svg%

Well, but that is just a consequence of self pointing to another instance.
You could have achieved the same by doing:

17:50:29 [w]: ruby -e ‘b = ; b.instance_eval { def self.a() puts
“array”; end}; b.a’
array
[…]
Am I overlooking something?

Guy is emphasizing the fact that this modifies the singleton class.

···

On Tue, Jan 20, 2004 at 01:55:00AM +0900, Robert Klemme wrote:

–
_ _

__ __ | | ___ _ __ ___ __ _ _ __
'_ \ / | __/ __| '_ _ \ / ` | ’ \
) | (| | |__ \ | | | | | (| | | | |
.__/ _,|_|/| || ||_,|| |_|
Running Debian GNU/Linux Sid (unstable)
batsman dot geo at yahoo dot com

Linux poses a real challenge for those with a taste for late-night
hacking (and/or conversations with God).
– Matt Welsh