Lua Override Assignment Operator

This first edition was written for Lua 5.0. While still largely relevant for later versions, there are some differences.
The fourth edition targets Lua 5.3 and is available at Amazon and other bookstores.
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16 – Object-Oriented Programming

A table in Lua is an object in more than one sense. Like objects, tables have a state. Like objects, tables have an identity (a selfness) that is independent of their values; specifically, two objects (tables) with the same value are different objects, whereas an object can have different values at different times, but it is always the same object. Like objects, tables have a life cycle that is independent of who created them or where they were created.

Objects have their own operations. Tables also can have operations:

Account = {balance = 0} function Account.withdraw (v) Account.balance = Account.balance - v end This definition creates a new function and stores it in field of the object. Then, we can call it as Account.withdraw(100.00)

This kind of function is almost what we call a method. However, the use of the global name inside the function is a bad programming practice. First, this function will work only for this particular object. Second, even for this particular object the function will work only as long as the object is stored in that particular global variable; if we change the name of this object, does not work any more:

a = Account; Account = nil a.withdraw(100.00) -- ERROR! Such behavior violates the previous principle that objects have independent life cycles.

A more flexible approach is to operate on the receiver of the operation. For that, we would have to define our method with an extra parameter, which tells the method on which object it has to operate. This parameter usually has the name self or this:

function Account.withdraw (self, v) self.balance = self.balance - v end Now, when we call the method we have to specify on which object it has to operate: a1 = Account; Account = nil ... a1.withdraw(a1, 100.00) -- OK With the use of a self parameter, we can use the same method for many objects: a2 = {balance=0, withdraw = Account.withdraw} ... a2.withdraw(a2, 260.00)

This use of a self parameter is a central point in any object-oriented language. Most OO languages have this mechanism partly hidden from the programmer, so that she does not have to declare this parameter (although she still can use the name self or this inside a method). Lua can also hide this parameter, using the colon operator. We can rewrite the previous method definition as

function Account:withdraw (v) self.balance = self.balance - v end and the method call as a:withdraw(100.00) The effect of the colon is to add an extra hidden parameter in a method definition and to add an extra argument in a method call. The colon is only a syntactic facility, although a convenient one; there is nothing really new here. We can define a function with the dot syntax and call it with the colon syntax, or vice-versa, as long as we handle the extra parameter correctly: Account = { balance=0, withdraw = function (self, v) self.balance = self.balance - v end } function Account:deposit (v) self.balance = self.balance + v end Account.deposit(Account, 200.00) Account:withdraw(100.00)

Now our objects have an identity, a state, and operations over this state. They still lack a class system, inheritance, and privacy. Let us tackle the first problem: How can we create several objects with similar behavior? Specifically, how can we create several accounts?


Copyright © 2003–2004 Roberto Ierusalimschy. All rights reserved.

Heuristic 1:When faced with more than one possible way of doing something while designing a language, pick the most common, most intuitive one, or else you will end up with Perl+.

Now, how is it more natural (at least to an English speaker)? Let's look at how we write/say things in English:

Steven is now 10 years old (as opposed to 10 years old Steven now is). I weigh more than 190 pounds (as opposed to more than 190 pounds I weigh).

In code:

The following also sounds more natural:

"If Mary is 18 yo, then she can have a candy". "If I am younger than 21 yo, then I will ask my brother to by me tequila".

than:

"If 18 yo Mary is ..." "If 21 is greater than my age ... "

Now the code:

Note that this is not natural to either programmers nor English speakers. The sentences sound like yoda-speak, and the code is nicknamed yoda-conditions. These might be helpful in C++, but I am sure most people would agree: if a compiler could do the heavy lifting and alleviate the need for yoda-conditions, life would be a bit easier.

Of course, one could get used to anything. For examples, number 81 is written as:

Eighty One (English) Eighty and one (Spanish) One and Eighty (German).

Finally, there are 4! = 24 valid ways of saying "green apple lies on table" in Russian - the order (almost) does not matter, except that 'on' must come together with 'table'. So, if you are a native Russian speaker (for example), then you might not care whether one writes or because both seem equally natural.

While linguistics is a fascinating subject, I never formally studied it and do not know that many languages. Hopefully I have provided enough counter-examples though.

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