Best way to achieve private functions in lua object?-CodePudding

I'm learnig Lua and how OOP in Lua works and I've seen this code in the reference

  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 extra = function ()
        if self.balance > self.LIM then
          return self.balance*0.10
        else
          return 0
        end
      end

      local getBalance = function () return self.balance   self.extra() end

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

This seems to work fine but when I print the address of a the private function before the return like this

...
print(extra)      
return {
        withdraw = withdraw,
        deposit = deposit,
        getBalance = getBalance
      }

and then create two accounts

account1 = newAccount(1)
account2 = newAccount(1)

I get two different address values

function: 00D2B290
function: 00A22040

Is this OK? I understand this as the same function being instanciated once per object constructor, wouldn't be more efficient if the same function was instanciated once and then refencenced? Or is this something lua handles this way?(I've been coding in C recently where this could be a problem I guess)

I know that I could make this

local function private_extra(self)
    if self.balance > self.LIM then
        return self.balance*0.10
    else
        return 0
    end
end

local 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   private_extra(self) end

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

and it could work the same way and the address of the function would be the same when the constructor is called more than once. Would this be a better approach talking about efficiency? Although I think that it makes the code uglier.

CodePudding user response：

In Lua, a function object must refer to both the closure environment and the function's code. In your example, the two extra functions have different addresses, because they are two different closures. In other words, each one refers to a different self variable. However, they do both refer to the same function code, so there shouldn't be a significant impact on performance.

I'm not even sure why PiL uses a self variable in newAccount, since it returns a whole different table. Private data can all be stored in local variables, and anything you want public can be in the returned table.