Lua: Basic OO

Lua
Basics
About Syntax Datatypes Hello World
Intermediate
Using Metatables Writing files Basic Object-orientation example Vararg statements MMF Interface
Advanced
Iterators How to take over Eastern Europe Basic Object engine
XLua
MMF Interface WIN Interface OpenGL C Libraries
Add to this template

This article assumes the use of Lua 5.1. Information may not be accurate or may need revision if you are using a different version.

This article or section is in need of expansion. Please add further information.

Though Lua has no true class support, it is still possible to emulate object-oriented programming through various methods, usually metatables. Lua uses a special colon syntax to indicate methods, but regular dot syntax can be used instead; however, in the latter case you must remember to include the object to be affected as the first parameter!

Let's say we wanted to check a string for letter case, and return a table containing 'upper', 'lower', or 'n/a' for each character. Additionally, we want to add the function to the basic 'string' library, because it makes sense to do that seeing as it's a string operation. We could always use some function like string.case(str), which is fine, but we should take advantage of the fact that the 'string' library sets a metatable for strings; this is something Lua assumes we can't do (we can do it but it requires the 'debug' library... not something you want to supply with your program!).

By using colon syntax, we can shorten the original function and make it somewhat more readable.

string.case = function (self)
    local casetable = { }
    local casestr = ""
    for chr in self:gmatch("(.)") do
        if select(2, chr:gsub("(%u)", "%1")) == 1 then
            casestr = 'upper'
        elseif select(2, chr:gsub("(%l)", "%1")) == 1 then
            casestr = 'lower'
        else
            casestr = 'n/a'
        end
        table.insert(casetable, casestr)
    end
    return casetable
end

This can be tested like so:

print(table.concat(("This is a test. ABC 123 !!!"):case(), ", "))

--prints: upper, lower, lower, lower, n/a, lower, lower, n/a, lower, n/a, lower, lower, lower, lower, n/a, n/a, upper, upper, upper, n/a, n/a, n/a, n/a, n/a, n/a, n/a, n/a