Example of Encapsulation Object Oriented Privacy v. Chapter 16.4 Example

[Dustin Juliano <velusdarkbind@...>]

This is a contrast for the example provided in (16.4 - Privacy) of  
"Programming in Lua".

The given example from the original text was:


 function newAccount (initialBalance)
      local self = {balance = initialBalance}

      local withdraw = function (v)
                         self.balance = self.balance - v
                       end

      local deposit = function (v)
                        self.balance = self.balance + v
                      end

      local getBalance = function () return self.balance end

      return {
        withdraw = withdraw,
        deposit = deposit,
        getBalance = getBalance
      }
    end


This works, but could become cumbersome and obfuscated if you are  
making a "class" in a Lua file that has several dozen "member"  
functions, "member data", and mixes of "private" and "public."

My suggested version is clearer, but requires you to make a module (a  
separate Lua file). This shouldn't be a problem, though.

[bank.lua]

--Bank "class"
P = {}
	-- You can declare your class (instantiate it) here, in addition to  
this
	-- module which defines it. This is optional. See note at end.
	-- This will create a global variable in the calling environment.
	bank = P

	-----------------------------------------
	-- "private" member variable declarations
	-----------------------------------------	
	local m_balance = 0
	
	-----------------------------------------	
	--"public" member function definitions
	-----------------------------------------
	function P.withdraw(v)
		m_balance = m_balance - v
	end
	
	function P.deposit(v)
		m_balance = m_balance + v
	end

	function P.getBalance() return m_balance end

--If you omit 'bank = P' from above, this will be absolutely needed
--to use your instantiated "class".
return P

[EOF bank.lua]

To use it, from inside Lua interpreter or another script, load it up.  
If you did not declare 'bank  = P' just assign the require or dofile  
function to a variable and you can access it that way.

m_balance is never accessible from outside the table. If you do a  
check, it is not even listed as part of the table; it is fully  
encapsulated by those member functions.

Hope this is helpful to anyone looking to do OOP with Lua. I had read  
through many examples and in the book and saw a, perhaps, easier way  
to accomplish what the example was offering.

Cheers,
D.