In general, how can I get a reference to an object whose name I have in a string?
More specifically, I have a list of the parameter names (the member variables - built dynamically so I can't refer to them directly).
Each parameter is an object that also has an from_s
method.
I want to do something like the following (which of course doesn't work...):
define_method(:from_s) do | arg |
@ordered_parameter_names.each do | param |
instance_eval "field_ref = @#{param}"
field_ref.from_s(param)
end
end
The most idiomatic way to achieve this is:
some_object.instance_variable_get("@#{name}")
There is no need to use +
or intern
; Ruby will handle this just fine. However, if you find yourself reaching into another object and pulling out its ivar, there's a reasonably good chance that you have broken encapsulation.
If you explicitly want to access an ivar, the right thing to do is to make it an accessor. Consider the following:
class Computer
def new(cpus)
@cpus = cpus
end
end
In this case, if you did Computer.new
, you would be forced to use instance_variable_get
to get at @cpus
. But if you're doing this, you probably mean for @cpus
to be public. What you should do is:
class Computer
attr_reader :cpus
end
Now you can do Computer.new(4).cpus
.
Note that you can reopen any existing class and make a private ivar into a reader. Since an accessor is just a method, you can do Computer.new(4).send(var_that_evaluates_to_cpus)