32 lines of goodness

This library has problems when a two classes inherit from the same base class. Check the thread for more details.

The Libary

local mt_class = {}

function mt_class:extends(parent)
   self.super = parent
   setmetatable(mt_class, {__index = parent})
   parent.__members__ = parent.__members__ or {}
   return self
end

local function define(class, members)
   class.__members__ = class.__members__ or {}
   for k, v in pairs(members) do
      class.__members__[k] = v
   end
   function class:new(...)
      local newvalue = {}
      for k, v in pairs(class.__members__) do
         newvalue[k] = v
      end
      setmetatable(newvalue, {__index = class})
      if newvalue.__init then
         newvalue:__init(...)
      end
      return newvalue
   end
end

function class(name)
    local newclass = {}
   _G[name] = newclass
   return setmetatable(newclass, {__index = mt_class, __call = define})
end

That's all there is to it! Just 32 lines!

Usage

Slap the above library in a file of your choose (maybe "32log.lua") and include it in your code using the require function.

The basic syntax is as follows:

class "ClassName" : extends(BaseClassName) {
    memberName = nonNilValue;
}

Once a class has been created you can create new instances of the class as follows:

local myInstance = ClassName:new()

If you create a method named __init it can be used to as a constructor with new.

class "Vector" {
    x = 0;
    y = 0;
    z = 0;
}
function Vector:__init(x, y, z)
    self.x = x
    self.y = y
    self.z = z
end
function Vector:print()
    print(self.x, self.y, self.z)
end
local vec = Vector:new(1, 0.5, 0.25)
vec:print()

Whatever value you set a member to in the definition, it will act as a default value for the member. This works well with values like numbers and strings where they are always copied by value but tables can get a little tricky:

class "Foo" {
   bar = {};
}
local foo = Foo:new()
foo.bar["foobar"] = 10;
local foo2 = Foo:new()
print(foo2.bar["foobar"])

Classes inherit their parent's default member values and meta-methods. You can also call a parent's method that was overloaded in the derived class using the super member:

class "Base" {}
function Base:foobar()
    print("foo")
end
class "Derived" : extends(Base) {} 
function Derived:foobar()
   self.super.foobar(self)
   print("bar")
end

See Also

Post any questions you might have in the original post.