Adding methods to an object

Posted by Blainer on Sat 13 Jun 2009 02:19 AM — 5 posts, 23,184 views.

Blainer Sat 13 Jun 2009 02:19 AM #0

If I have:


	assert (package.loadlib("sqlite3.dll","luaopen_luasql_sqlite3")) ()
	aard_db = assert(sqlite3.open_memory("aard"))

And I want to add:


	function aard_db:test(arg)
		self.nrows(arg)
		return something
	end

So I can do this:


	aard_db:test("something")

How do I do it? Can I create a class called aard_db and add the methods from sqlite3.dll?

Nick Gammon Australia Forum Administrator Sun 14 Jun 2009 09:17 PM #1

It's not going to be too easy. The variable aard_db is a userdata, not a table, and thus you can't add more things to it. A userdata is just a 4-byte pointer to an internal structure used by the database routines.

The userdata aard_db already has a metatable as well, so you can't just add one. I experimented with something like this:


mt = getmetatable (aard_db)


mt.__index.test = function (arg)
  print "in test"
  return self.nrows (arg)
end -- function

This indeed lets you add "test" as a function for the metatable in aard_db, however it doesn't know the self variable. Maybe you can get something along those lines to work, I'm not sure.

Erendir Germany Sun 14 Jun 2009 11:27 PM #2

just use this code:

mt.__index.test = function (self, arg)
  print "in test"
  return self.nrows (arg)
end -- function

or this:

function mt.__index:test(arg)
  print "in test"
  return self.nrows (arg)
end -- function

Nick Gammon Australia Forum Administrator Sun 14 Jun 2009 11:38 PM #3

That gave this error:


Run-time error
World: smaug
Immediate execution
[string "Immediate"]:9: bad argument #1 to 'nrows' (:sqlite3 expected, got string)
stack traceback:
        [C]: in function 'nrows'
        [string "Immediate"]:9: in function 'test'
        [string "Immediate"]:13: in main chunk

However a slight modification seemed to work:


db=sqlite3.open(GetInfo (82))  -- open MUSHclient prefs file

mt = getmetatable (db)

mt.__index.test = function (self, arg)
  print "in test"
  return self:nrows (arg)
end -- function

require "tprint"

for row in db:test("SELECT * FROM sqlite_master") do
  tprint (row)
end

db:close()  -- close

Nick Gammon Australia Forum Administrator Sun 14 Jun 2009 11:39 PM #4

Quote:

assert (package.loadlib("sqlite3.dll","luaopen_luasql_sqlite3")) ()

SQLite is built into MUSHclient these days, you don't need this line.