Creating classes

Creating a class

Let us create a Window class.

local class = require "30log"  -- adds 30log to your code
local Window = class("Window") -- creates a class

That's it. The string argument Window is the name of the class. Later on, we can query that name calling class.name.

print(Window.name) -- outputs "Window"

Giving a class a name is optional though.

local unnamedWindow = class() 
print(unnamedWindow.name) -- outputs nil

We can define attributes for any class.
Let us define a width and a height attributes for our Window class and assign them values.

Window.width = 150
Window.height = 100

30log provides a shortcut, for convenience : passing an optional params table (with named keys) as a second argument to class().

local Window = class("Window", {width = 150, height = 100})
print(Window.width) -- outputs 150
print(Window.height) -- outputs 100

Named and unnamed classes

When a class has a name attribute, it is considered to be a named class. In that case, calling tostring or print on that class (or any other function which triggers __tostring metamethod) returns a string. This string specifies the passed-in object is a class, gives its name and memory address.

print(Window) -- outputs "class 'Window' (table: 0x0002cdc0)"

In case the class has not any named defined, it is considered to be an unnamed class, and this will be reported by a question mark symbol.

local Window = class(nil, {width = 150, height = 100})
print(Window) -- outputs "class '?' (table: 0x0002cdb8)"

This feature is mostly meant for debugging purposes. Yet, one can define a custom __tostring implementation for any class metatable.

local Window = class('Window')
getmetatable(Window).__tostring = function(t)
  return "I am class "..(t.name)
end
print(Window) -- outputs "I am class Window"