Get a class from its name

Ruby
1

This is how i’m currently getting a class object from a name:

     cname = "A::B::C::D"
     klass = cname.split("::").inject(Kernel) {|a,b| a.const_get(b)}

I think it’s pretty elegant, but I’m always happy to hear other ways
to do it :wink:
Anyone does this in some other way? (I don’t want to eval code…)

regards,

2

On Aug 20, 7:16 pm, Rolando A. [email protected] wrote:

–
Rolando A.
Phone: +56-9 97851962

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I’ve seen little helper methods for this, but never so elegant. Nice!

3

Hi –

On Tue, 21 Aug 2007, Rolando A. wrote:

This is how i’m currently getting a class object from a name:

  cname = "A::B::C::D"
  klass = cname.split("::").inject(Kernel) {|a,b| a.const_get(b)}

I think it’s pretty elegant, but I’m always happy to hear other ways to do it
:wink:
Anyone does this in some other way? (I don’t want to eval code…)

I think it’s usually done the way you’ve got it there. It’s one of
those things that people write over and over (like map_with_index and
singleton_method), hopefully some day to enter the core as
const_get_deep or something.

David

4

Hi –

On Tue, 21 Aug 2007, Tim P. wrote:

I think it’s pretty elegant, but I’m always happy to hear other ways to do it

It could just be a wrapper for the core method rb_path2class – which
does the same thing in Ruby C API land.

It’s a more general-purpose thing, though; it’s for any constant, not
just classes. It’s really just a variation on const_get, which already
will give you any constant from any existing class/module nesting.

David

5

2007/8/21, David A. Black [email protected]:

will give you any constant from any existing class/module nesting.
Definitively.

Folks, please keep in mind that the proposed solution from the OP’s
posting works only in the global context:

irb(main):001:0> module A
irb(main):002:1> module B
irb(main):003:2> module C
irb(main):004:3> end
irb(main):005:2> end
irb(main):006:1> end
=> nil
irb(main):007:0> “A::b::C”.split(“::”).inject(Kernel) {|a,b|
a.const_get(b)}
=> A::b::C
irb(main):008:0> module A
irb(main):009:1> p “B::C”.split(“::”).inject(Kernel) {|a,b|
a.const_get(b)}
irb(main):010:1> end
NameError: uninitialized constant Kernel::B
from (irb):9:in const_get' from (irb):9 from (irb):10:in inject’
from (irb):9:in each' from (irb):9:in inject’
from (irb):9
from :0
irb(main):011:0> module A
irb(main):012:1> p “B::C”.split(“::”).inject(self) {|a,b|
a.const_get(b)}
irb(main):013:1> end
A::b::C
=> nil

There needs to be at least a bit more logic to take care of nested
scopes and constants with leading “::”. Having said that it’s probably
wise to put the method in class Module, like:

class Module
def deref(name)
parts = name.split “::”
start = self

if parts.first == ""
  # start with global scope
  start = Kernel
  parts.shift
end

parts.inject(start) {|a,b| a.const_get(b)}

end
end

class Object
def deref(name)
self.class.deref(name)
end
end

irb(main):021:0> module A
irb(main):022:1> module B
irb(main):023:2> module C
irb(main):024:3> end
irb(main):025:2> end
irb(main):026:1> end
=> nil
irb(main):027:0> deref “A::b::C”
=> A::b::C
irb(main):028:0> module A
irb(main):029:1> p deref(“B::C”)
irb(main):030:1> p deref(“::a::B”)
irb(main):031:1> end
A::b::C
A::B
=> nil

Kind regards

robert

6

On 8/20/07, David A. Black [email protected] wrote:

:wink:
Anyone does this in some other way? (I don’t want to eval code…)

I think it’s usually done the way you’ve got it there. It’s one of
those things that people write over and over (like map_with_index and
singleton_method), hopefully some day to enter the core as
const_get_deep or something.

“A::b::C::D”.to_class

It could just be a wrapper for the core method rb_path2class – which
does the same thing in Ruby C API land.

Blessings,
TwP

7

On Aug 21, 2007, at 10:41 AM, Robert K. wrote:

Definitively.

Folks, please keep in mind that the proposed solution from the OP’s
posting works only in the global context:

Ok… I get your point. It’s not a general dereferencer, i mean, it
doesn’t take the current scope and figure it outs the name from
there, it always assume the string is the full path… Maybe what you
propose could be the general solution to the deref problem :slight_smile:

robert

regards,

8

On Aug 20, 2007, at 20:16 , Rolando A. wrote:

This is how i’m currently getting a class object from a name:

    cname = "A::B::C::D"
    klass = cname.split("::").inject(Kernel) {|a,b| a.const_get

(b)}

I think it’s pretty elegant, but I’m always happy to hear other
ways to do it :wink:
Anyone does this in some other way? (I don’t want to eval code…)

I use pretty much the same thing:

class String
def const_get
self.split( “::” ).inject( Kernel ) { | parent, child |
parent.const_get( child ) }
end
end

Example

module A
class B
C = ‘c’
end
end

“A”.const_get
=> A

“A::B”.const_get
=> A::B

“A::b::C”.const_get
=> “c”

~Wayne

s///g
Wayne E. Seguin
Sr. Systems Architect & Systems Administrator