Hi,
I've just started teaching myself lua and things are going great except I'm not quite sure how to create objects like in other scripting languages like java and c#. I'll be mostly using lua for creating games using Love2d. Could anyone give me a run through on how exactly to do this (I think there is more then one).
Take a look through the Gwilty Programming Assignment #2 Thread, this is discussed extensively there.
[QUOTE=subenji99;33119188]Take a look through the Gwilty Programming Assignment #2 Thread, this is discussed extensively there.[/QUOTE]
Oh, thanks I'll do that!
Lua doesn't have objects in the traditional sense. Metatables can be used to accomplish the same goals.
[QUOTE=ROBO_DONUT;33122008]Lua doesn't have objects in the traditional sense. Metatables can be used to accomplish the same goals.[/QUOTE]
To, um... 'rephrase' this:
In Java and C++, you have [b]types[/b].
A [B]int[/B] is a type, so a is a [B]boolean[/B], and so is a [B]char[][/B] (array of characters).
You can also create your own types, using the [B]class[/B] keyword (as I'm sure you are aware).
The thing with Class types is that, two are not the same:
The type of class A, is never the same as the type of class B (even if they inherit a common class).
When C++ or Java is compiled, the compiler is aware of this.
If you went:
[cpp]
class A {
int thing;
}
class B {
int not_a_thing;
}
int main() {
A a();
a.not_a_thing = 7; // fail
std::cout << a.not_a_thing; // more fail
}
[/cpp]
The compiler cannot find "not_a_thing" in A at compile time.
Lua, on the contrary, is not a compiled language, and is very dynamic.
While it still has [b]types[/b], you cannot create your own: there are no 'type extensions'.
The only available types are: [I]nil, boolean, number, string, function, thread,[/I] and [I]table[/I].
You can only 'simulate' classes in Lua.
The way to do this is to change the [B]behaviour[/B] of a value. The only type you can do this with is [B]table[/B].
Any attempt to do it to other types will resulting in you changing the behaviour of EVERY other value of that type; whereas with tables, you can change the behaviour of each individual table.
The way to do this is with [B]metatables[/B]. The [B]meta-[/B] prefix means "something about something". So, [B]metadata[/B] means "data about data", and a [B]metatable[/B] is a "table about a table".
For example:
[lua]
metat = {
__add = function(a, b)
local new_t = {}
for i, v in ipairs(a) do
table.insert(new_t, v)
end
for i, v in ipairs(b) do
table.insert(new_t, v)
end
return new_t
end
}
t1 = {"apple", "banana"}
setmetatable(t1, metat)
t2 = {"mango", "pear"}
setmetatable(t2, metat)
t3 = {"pomegranate", "pineapple"}
t4 = {"peach", "orange"}
print(table.concat(t1, ", "))
print(table.concat(t1+t2, ", ")) -- This works, because it uses the first metatable it finds (t1's metatable here, not that it matters in this case, because they both use the same metatable).
print(table.concat(t1+t3, ", ")) -- This works as well! Why? Because t3 doesn't have a metatable, so it uses t1's
print(table.concat(t4+t1, ", ")) -- Same here!
print(table.concat(t3+t4, ", ")) -- This will error :( Why? Because Lua can't find a metatable to tell it what to do! There is no "default" behaviour for adding tables.
[/lua]
I'm hoping you'll see how you can use this to simulate classes.
The [I]__add[/I] [B]metamethod[/B] is the same as [B]operator+[/B] in C++.
If you wish to control [B]t.something[/B] and [B]t["something"][/B], you can define [I]__index[/I] and [I]__newindex[/I].
If you wish to make the table callable (only works in Lua 5.2!), as in [B]t(a, b, c)[/B] you can define [I]__call[/I].
I suggest you take a look at an introduction I wrote a bazillion years ago: [url=http://wiki.garrysmod.com/?title=User:Deco_Da_Man]What are metatables?[/url]
You seem like you're a few IQ levels about it's target audience... but it should still give you some insight.
I'll leave you with this quote:
"I’m So Meta, Even This Acronym"
Good luck in your future endeavours ;)
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