Tutorials:Lua:ObjectOriented

Calling Notation

The first thing to understand is that you can create functions on your tables. Think of a simple table for a rectangle with a length and width and a function that calculates the area. You can go ahead and run this script in CE (go to Memory Viewer and hit `CTRL+L` to open the Lua Engine)

local rect = { length = 7, width = 9 }
function calculateArea(tbl)
  return tbl.length * tbl.width
end
print('Width: '..tostring(calculateArea(rect)))

Let's look at creating the function *on the table*. This helps avoid naming collisions and keeps the global namespace clean. Here's an example up front showing how that works:

local rect = { length = 7, width = 9 }

function rect.calculateArea(tbl) -- `.` specifies object as argument
  return tbl.length * tbl.width
end

function rect:calculateArea2() -- `:` creates assumed 'self' first argument
  return self.length * self.width
end

rect.calculateArea3 = function(tbl)
  return tbl.length * tbl.width
end


-- need to pass table with `.`
print('Area: '..tostring(rect.calculateArea(rect)))
print('Area2: '..tostring(rect.calculateArea2(rect)))
print('Area3: '..tostring(rect.calculateArea3(rect)))

-- `:` passes rect as first arg
print(':Area: '..tostring(rect:calculateArea()))
print(':Area2: '..tostring(rect:calculateArea2()))
print(':Area3: '..tostring(rect:calculateArea3()))

-- you can pass other tables as arguments even though the function is
-- on the `rect` table
print('Other Area:'..tostring(rect.calculateArea({ length = 4, width = 3 }))) -- p
print('Other Area2:'..tostring(rect.calculateArea2({ length = 4, width = 3 })))
print('Other Area3:'..tostring(rect.calculateArea3({ length = 4, width = 3 }

Using the `.` notation, you can see that the function exists as a property on the table like any other table value like `length` and `width`. Since it is a simple function, it's not really tied to operating on the table, it takes a parameter that we called `tbl` which should be the table to operate on. When calling it with a `.`, we also need to pass the object to operate on because it is just a simple function. Although it exists on our table with length and width specified, there is no real relation between it and the table.

Now look at `calculateArea2()`. Here we use a `:` when specifying that the function exists on our `rect` table. There are no arguments, but we use `self` inside of the function. By using a `:` in the function declaration, LUA assumes there is an unspecified argument named `self` before any others.

And with `calculateArea3` we directly assign the function as a property on the table. All three of these declarations are functionally the same. The functions are simple properties on the `rect` table, they each take one argument, and they return the value of multiplying the `length` and `width` properties on that argument.

Now look at the function calls where we use a `:`. These don't pass any arguments. That is because when using a `:` when *calling* a function property, the table the function resides on is passed as the first argument. This is the basis for creating methods on objects.

Metatables

Here are some useful links for reference:

• [[1]]
• [tutorial]