Determing defined constants for a class

ruby-talk
1

From looking at the docs I can see that it is possible to determine
which Constants have been declared for a particular module, but I cannot
see a means of finding out what constants have been declared in a
particular class.

This leads to ask, if there a method that I can’t see for determining
declared constants in a class? and secondly, is it considered bad
practice to put constants in a class rather than a module?

Rob

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2

Robert McGovern wrote:

From looking at the docs I can see that it is possible to determine
which Constants have been declared for a particular module, but I cannot
see a means of finding out what constants have been declared in a
particular class.

This leads to ask, if there a method that I can’t see for determining
declared constants in a class?

A class is-a module (i.e. the “Class” class is derived from the “Module”
class). So to determine the constants for a class just call Class#constants.

… and secondly, is it considered bad practice to put constants
in a class rather than a module?

No.

3

A class is-a module (i.e. the “Class” class is derived from the “Module”
class). So to determine the constants for a class just call
Class#constants.

sigh Thanks Lyle, that was a Doh! moment.

I had tried doing self.constants and had it not work, checked the docs
and then posted the question.

Okay so constants returns an array of the constant names as a string in
short form (i.e. not including the path needed to use them). Hmm, so
with the following sample code, if I want to verify that an method is
passed a valid constant argument what would I want to do? (given the
constant will be passed into the method as a numeric)

example:

module ID3
module V2
class Frame
#constants
TRACK_LENGTH = 0
TRACK_TITLE = 1
# … and so on

 	    @frame_id = nil # this objects frame id

 	    def frame_id=(f_id)
        @frame_id = f_id if ID3::V2::Frame.constants.include? (f_id)
end # frame
 end # v2

end # id3

frame = ID3::V2::Frame.new

the following will fail because the constants returned

are in short from i.e. TRACK_LENGTH and f_id in the method is 0

f.frame_id = ID3::V2::Frame::TRACK_LENGTH

Rob

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4

Robert McGovern wrote:

Okay so constants returns an array of the constant names as a string in
short form (i.e. not including the path needed to use them). Hmm, so
with the following sample code, if I want to verify that an method is
passed a valid constant argument what would I want to do? (given the
constant will be passed into the method as a numeric)

Have you considered using constants with a symbolic rather than numeric
value?

     TRACK_LENGTH = :TRACK_LENGTH
     TRACK_TITLE = :TRACK_TITLE

Or, if you want to make sure that the only access to these values is
through the ID3::V2::Frame scope, you could create some objects:

     TRACK_LENGTH = Object.new
     TRACK_TITLE = Object.new

In this case, the array

Frame.constants.map{|c|Frame.class_eval(c)}

will let you check if a given objects is one of your allowed constants.
(You might consider a hash for this, if it’s going to be large.)

5

Have you considered using constants with a symbolic rather than numeric
value?

    TRACK_LENGTH = :TRACK_LENGTH
> > > will let you check if a given objects is one of your allowed constants. > (You might consider a hash for this, if it's going to be large.)

Many thanks for the suggestions Joel, exactly the sort of ideas I was after.

much appreicated!

Rob