I have this line in a class method to define an initializer:
define_method(:initialize){|value| @value = value}
But I want something more like this:
define_method(:initialize){|value=nil| @value = value if value}
which doesn’t seem to be possible because of the “value=nil” part. Is
there some way to do this that isn’t eval? Eval would work fine since
this is pretty static code and I have unit tests for it but I try to
avoid it if I can. I’ve only used it before for performance to turn
some code with runtime tests into eval-time tests.
Pedro.
Pedro Côrte-Real wrote:
I have this line in a class method to define an initializer:
define_method(:initialize){|value| @value = value}
But I want something more like this:
define_method(:initialize){|value=nil| @value = value if value}
First of all, you don’t need the `if value’ part – when Ruby sees
@value, it is immediately created, with the value nil, so there’s no
problem with giving it the value nil explicitly. Second, I believe you
can just do this:
define_method(:initialize){|value| @value = value}
If #initialize is called with no arguments, `value’ will just be nil,
although IRB may issue a warning.
Cheers,
Daniel
You could do the following:
Foo.class_eval do
def bar(tar=0)
tar * 2
end
end
Foo.new.tar
=> 0
Foo.new.tar 4
=> 8
On 7/25/06, Dr Nic [email protected] wrote:
You could do the following:
Foo.class_eval do
def bar(tar=0)
tar * 2
end
end
Yep, this works great, thanks. It’s an eval but it’s the one with a
block instead of a string. Didn’t know “def” worked inside a block.
Must free myself of silly mental restrictions brought over from lesser
languages… 
Thanks,
Pedro.
On 7/25/06, Daniel S. [email protected] wrote:
First of all, you don’t need the `if value’ part – when Ruby sees
@value, it is immediately created, with the value nil, so there’s no
problem with giving it the value nil explicitly.
Yes, I knew that.
Second, I believe you
can just do this:
define_method(:initialize){|value| @value = value}
If #initialize is called with no arguments, `value’ will just be nil,
although IRB may issue a warning.
That’s what I have but it throws a warning and I wanted to shut it up.
Guess I’ll have to do the eval.
Thanks,
Pedro.
Oooh. And another cool use of class_eval is:
code = %q{
def bar(tar=0)
tar * 2
end
}
=> " def bar(tar=0)\n tar * 2\nend"
Foo.class_eval code
=> nil
Foo.new.bar 10
=> 20
So, this means you can take the text to eval against the class from
anywhere. 
Must free myself of silly mental restrictions brought over from lesser
languages…
If you’re feeling really lazy or just want to test something in IRB
before posting on a forum then this is what you might type:
Foo.class_eval { def bar(tar=0); tar * 2; end}
Of course then you should reformat it for the forum
[Warning: Anal retentive refactoring ahead]
define_method(:initialize) do |*values|
case values.size
when 0…1
@value = values.first
else
raise ArgumentError, “wrong number of arguments (#{values.size} for
0)”
end
end
or perhaps
define_method(:initialize) do |*values|
@value = values.shift
unless @value.length == 0
raise ArgumentError, “wrong number of arguments (#{values.size} for
0)”
end
end
Too… many… options… ahh!
Hi,
At Tue, 25 Jul 2006 19:33:38 +0900,
=?ISO-8859-1?Q?Pedro_C=F4rte-Real?= wrote in [ruby-talk:203711]:
define_method(:initialize){|value=nil| @value = value if value}
define_method(:initialize) {|*values|
case values.size
when 0
value = nil
when 1
value = values.first
else
raise ArgumentError, “wrong number of arguments (#{values.size} for
0)”
end
@value = value
}
Daniel DeLorme wrote:
=> nil
irb(main):003:0> @c = nil
=> nil
irb(main):004:0> defined? @a
=> nil
irb(main):005:0> defined? b
=> “local-variable”
irb(main):006:0> defined? @c
=> “instance-variable”
I know, but I was trying to simplify the problem
Cheers,
Daniel
Daniel S. wrote:
First of all, you don’t need the `if value’ part – when Ruby sees
@value, it is immediately created, with the value nil
Sorry for being pedantic but this is not quite true (although in this
case it doesn’t make a difference):
irb(main):001:0> @a = 1 if false
=> nil
irb(main):002:0> b = 2 if false
=> nil
irb(main):003:0> @c = nil
=> nil
irb(main):004:0> defined? @a
=> nil
irb(main):005:0> defined? b
=> “local-variable”
irb(main):006:0> defined? @c
=> “instance-variable”
Daniel
As long as we’re throwing around options…
define_method(:initialize) do |*values|
@value = values.shift
unless @value.length == 0
raise ArgumentError, “wrong number of arguments (#{values.size} for
0)”
end
end
define_method(:initialize) do |*values|
raise ArgumentError, “wrong number of arguments (#{values.size} for
0)” if values.length > 1
@value = values.first
end
Cheers!
Patrick