Object Prototypes

Feb 15, 2021 permanent    Lua

In Lua, object-oriented programming is achieved using prototypal inheritance, since Lua does not have the concept of a class. Prototypal inheritance simply means that objects can refer to a "prototype" object which is looked up when any field doesn't exist in the first object. In Lua, this is achieved by using the __index Lua Table-Access Metamethods. In the example, I'm creating a Person table which will serve as a metatable for all instances of a Person:

local Person = {firstName='', lastName='', age=nil}
function Person:new(o)
    local o = o or {}
    setmetatable(self, o)
    self.__index = self
    return o
end

function Person:getName()
    return self.firstName..' '..self.lastName
end

local me = Person({firstName='Lex', lastName='T', age=34})
print(me:getName())

I can then extend Person by creating a new object with modified parameters:

local PersonWithMiddleName = Person:new()
PersonWithMiddleName.middleName = ''

local me = PersonWithMiddleName(firstName='Lex', lastName='T', age=34, middleName='D')

In that example, the me object will be consulted for fields. If they aren't found, PersonWithMiddleName will be looked up. If not found, since it uses Person as the __index metatable, Person will then be consulted.

Javascript also uses prototypal inheritance at the core of its object-oriented paradigm.

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