variables in custom API

[DeGariless 0]

I'll use short example programs to explain exactly what I am trying to achieve.

 

I have a a file test.lua that looks like this

testAPI = require("testAPI")

testAPI.talk()
testAPI.setWord("Goodbye")
testAPI.talk()

I also have a file /lib/testAPI.lua that looks like this

testAPI = {}

local word = "Hello"

function testAPI.talk()
  print(word)
end

function testAPI.setWord(s)
 word = s
end

return testAPI

when I run test.lua for the first time I get the output

Hello

Goodbye

 

but than I run it a second time and I get

Goodbye

Goodbye

 

How can I get the variable 'word' to reset anytime a new program uses that API?

[Fingercomp 37]

This happens because package library caches return values of libraries. This is simply workarounded, though.

local lib = {}
-- the following line is important
lib.__index = lib

-- the "word" variable will be defined later, so it will not be cached

-- define your functions (note: colons should be used)
function lib:talk()
  -- use "self" to access fields of library table
  print(self.word)
end

function lib:setWord(s)
  -- and here, too
  self.word = s
end

function lib:setWordAndTalk(s)
  -- if you need to access *methods* (functions) of your library, you should use the colon!
  self:setWord(s)
  self:talk()
end

return function()
  local object = {
    -- declare your variables here if you want them to be created with each new instance
    word = "Hello"
  }
  setmetatable(object, lib)
  return object
end

As I already said, the return value is cached. If you just return the table, it gets cached, and the result will be the same (a single library table for all programs that require'd the library.
In the code above, though, the thing that's cached is a function. It's sole purpose is to create a new instance of library. It defines some variables that won't be cached, and sets the library table as a metatable for the object. Then it just returns the object.
The library should be used differently than a regular one:

local lib = require("lib")()  -- get the function, and call it

-- use colon to access methods of library...
lib:talk()
lib:setWord("Test")
-- and dot to access properties (regular variables) of library:
print(lib.word)

lib:setWordAndTalk("Hello, world!")

Speaking of tables, they are not copied when passed as an argument to a function. Here's the code that demostrates this:

local function setValue(tbl, value)
  tbl.test = value
end

local someTable = {test="Hello"}
print(someTable.test)
setValue(someTable, "Hi")
print(someTable.test)

The output:

Hello
Hi

As you can see, the function didn't get a copy of table, instead, it modified a field of the same table! In the C (C++) programming language, this is called a reference to the value, and it's used to avoid creating a copy of big objects.
Moreover, this is a key concept of Lua's object-oriented programming: all methods of some class, when called, get a reference to the class which they can use. That's why methods should be generally called like this: class:method(args). This implicitly passes the class variable as the first argument for the method.

Edited August 22, 2016 by Fingercomp