[ruby-talk:8442] Re: string like istringstream (was: A bug inruby)

Jean Michel <[e-mail]> wrote:
>Kevin Smith  <[e-mail]> wrote:

>>Since Ruby is dynamically typed, Strings and 
>>Files can behave the same (and thus be 
>>interchangable) even if they do not inherit from 
>>each other.
>>
>>For eample:
>>
>>  stream.each_line do
>>	| line |
>>	puts ">" + line
>>  end
>>
>>This will work whether stream is a File or a 
>>String. If there are other operations that you 
>>would like String to support (like lineno or 
>>seek) you would just have to create a subclass of 
>>String that implements those methods.

>I do not think this solves my  problem. I will give the specific example
>where I  had the problem  to show  it: I have  as an exercise  written a
>program to  get an ID3V2 tag  from a .mp3  file. I have defined  a class
>ID3V2tag and  a method  read_ID3V2tag in  class IO  which returns  a new
>object of class ID3V2tag (or nil if no such tag was found in the file).
>
> Now, I want to re-use my code to parse an ID3V3 tag sitting in a string
>in memory. In C++, I can just  make the string into an istringstream and
>share the same code.  I don't see how I can share  the code between both
>situations in  ruby (of  course there would  be a way:  put the  code in
>class String and read the file in memory; but this is not desirable with
>multi-megabyte  files!). It  seems that  I need  a class  in which  both
>strings and  files can belong, but  I cannot do it  since neither String
>nor IO is a module. What did I miss here?

I think you haven't adequately explained to the Ruby developers
what it is that you really want.

What is missing from the discussion is a description
of the specific API that is to be shared and what amount of code 
resuse one has in mind.  In C++, there are really three kinds of 
code reuse: literal (object code) reuse -- via inheritance, 
semi-literal reuse  -- via inheritance plus redefined virtual functions, 
and generic reuse of algorithms plus names -- via templates 
(freely generates different object code for different types).  The C++ 
library code for iostreams is really the second type above, but 
the overriden virtual functions are hidden away from the programmer
(underneath the covers, every iostream is basically a strstream
with different virtual methods for what to do when the
char or wchar buffer needs to be filled or emptied;  
obviously be shared; it's a great paradigm, because the code for 
serializing to and from this buffer can be shared and
building strings out of character sequences can
be literally shared.

In Ruby, every method is dynamically dispatched, redefinable, 
looked up by name, and has weakly typed arguments, so the 
three types of reuse essentially collapse to one.  But the 
fact that methods are weakly typed, doesn't mean that 
suitable implementations for each type are conjured from 
thin air, nor does it mean that every interesting point
for customization inside of an interface has been
exposed.

All of that is really just to say that this a library
issue and not a language issue.

In this case, the methods for the Marshal module are
pretty opaque, but it seems likely that you could 
define _dump and _load for your ID3V2tag class and
achieve a suitable kind of code reuse through the
fact that Marshal can work on IO objects and strings
(at least that is my theoretical understanding
based on the documentation).


-= Josh