From: Henrik Horneber <ryco@...>
Date: 2004-10-06T03:55:12+09:00
Subject: Re: Extending existing classes

Edgardo Hames wrote:

> On Wed, 6 Oct 2004 03:29:35 +0900, Henrik Horneber <ryco@gmx.net> wrote:
> 
>>>>- extending String so you can tell it to remove cpp comments /* remove
>>>>me */
>>>
>>>Extending string with something like remove_cpp_comments seems awkward
>>>to me. In any usecase for this I'd rather have a class CppCode that I
>>>can ask to remove_comments. (Or even better, to give me the source
>>>without comments.)
>>>
>>>So my personal heuristic tells me not to extend in this case.
>>
>>Now, the reason I was immediatly reminded of that remove_cpp_comments
>>example is, that I wrote a small script some time ago, which used
>>exactly this case. First, I implemented remove_cpp_comments in its own
>>class (which was not called CppCode, but had the same intend), but out
>>of curiosity (and because in ruby I can, dang it! :) ) I moved the
>>remove_cpp_comments method to String. Turns out, the method got shorter
>> and somewhat clearer, at least to me. And I could write something like
>>
>>source_code = IO.readlines(file).join("\n")
>>source_code.strip_comments!
>># instead of source_code = CppCode.strip_comments( source_code)
>>
>>.... you get the idea.
>>
>>Even though the code became clearer on that level, I still have to agree
>>with Brian that it is somehow dirty/awkward on a different level to
>>extend String with such a method.
>>
> 
> 
> I usually try not to do that kind of things. Then, for each new
> lanaguage you are going to support, you should now add a
> remove_#{lang}_comments to String. But then, when reading a given
> source file you could remove comments that are not so in that
> language.
> 
> Regards,
> Ed

In my case there is never going to be a different language to support, 
but in general, that is a good reason not to do it. I'd like to get away 
from my example and to a little wider discussion of when to extend 
existing classes. I could have written my own subclass of String called 
CppSource, so I could have written

source_code = CppSource.new( IO.readlines(file).join("\n") )
source_code.strip_comments!

which might actually be a better solution than to put strip_comments! 
into String. On the other hand (again stealing from the other thread), 
writing

time_to_restart = TimeNumeric.new(5).hours + TimeNumeric.new(15).minutes

is just what we would like to avoid. So, as a guideline, should you only 
extend an existing class when you use lots of literals in your code?

regards,

Henrik