Self.new and initialize ( was Re: Creating sub-classes using

Ruby
1

On 5/6/06, Logan C. [email protected] wrote:

  @records = []
  read_each_record_in_file(file) do |record_data|
       record = Database::Record.new
       extract_record_data_into(record_data, record)
       @records << record
  end

end
end

I think I still have a long way to go before I can write a decent,
Ruby-ish example :). This one is a little beyond my ability to
understand at a glance…

What happens when I call Database.new(some_file) ? In which order to
the “new” and “initialize” methods get called?

2

On May 6, 2006, at 4:10 PM, Bira wrote:

I think I still have a long way to go before I can write a decent,
Ruby-ish example :). This one is a little beyond my ability to
understand at a glance…

What happens when I call Database.new(some_file) ? In which order to
the “new” and “initialize” methods get called?

Basically new calls initialize, and then returns the instance of the
object. The normal implementation of new would look something like:

class Class
def new(*args, &block)
obj = self.allocate
obj.initialize(*args, &block)
obj
end
end

My new does some extra work by defining the Record class based on
some info in the file. There’s a big problem with my code though and
it’s called Only one database per program!. Or at least only one at a
time

e.g.

db1 = Database.new(file1)
db2 = Database.new(file2)

if file2 has a different record format than file1, it’s going to
seriously mess up any interactions with db1. An alternative method
would be of course to do something like

class Database
def initialize(file)
@Record = Struct.new(…)
…
end
end

3

Logan C. wrote:

On May 6, 2006, at 4:10 PM, Bira wrote:

I think I still have a long way to go before I can write a decent,
Ruby-ish example :). This one is a little beyond my ability to
understand at a glance…

What happens when I call Database.new(some_file) ? In which order to
the “new” and “initialize” methods get called?

Basically new calls initialize, and then returns the instance of the
object. The normal implementation of new would look something like:

class Class
def new(*args, &block)
obj = self.allocate
obj.initialize(*args, &block)
obj
end
end

My new does some extra work by defining the Record class based on
some info in the file. There’s a big problem with my code though and
it’s called Only one database per program!. Or at least only one at a
time

e.g.

db1 = Database.new(file1)
db2 = Database.new(file2)

if file2 has a different record format than file1, it’s going to
seriously mess up any interactions with db1. An alternative method
would be of course to do something like

class Database
def initialize(file)
@Record = Struct.new(…)
…
end
end

This “only one database per program” problem you refer to is actually a
problem with my design. I wonder how to go around it.

I do want to be able to have two different Databases that use different
Record formats, but still be able to access the fields via Struct-like
accessors.

I realize that with this design it’s not possible, since
Database::Record refers only to a single class, not to different
classes.

4

On May 6, 2006, at 4:31 PM, Eli B. wrote:

if file2 has a different record format than file1, it’s going to
This “only one database per program” problem you refer to is
classes.

–
Posted via http://www.ruby-forum.com/.

Just go up one level, e.g.

module Database # Not really a class, more of a class factory
def self.new(file)
db_klass = Class.new
db_klass.class_eval do
record_format = parse_format(file)
self.const_set(‘Record’, Struct.new(*record_format))
@@db_file = file
def initialize
@file = @@db_file
…
end
end
db_klass
end
end

SongDatabase = Database.new(‘songs.txt’)

song_db = SongDatabase.new

do stuff with song_db