[ANSWERED] What does "<Class>.super.new(self, {})" do?

[mr_3tianne]

Hey,

I'm new(ish) to LOVE, I have a bit of background knowedge of object-oriented programming but usually with "higher-level" engines that do a lot of the heavy lifting when it comes to creating/storing/manipulating objects.

I'm currently working on a project in LOVE using rxi's Classic library and I've got the basics of it down, I can create new objects, give properties to those objects, create methods for those objects etc. but I'm a bit stuck with what exactly <Class>.super.new(self, {}) is and what it does.

I know it is something to do with setting (or... getting?) the "super" ("parent class") of an object but I don't understand what the first and second arguments represent or why it is necessary to insert this particular piece of code when creating a new class; for example I had a piece of code that wasn't working until I inserted <Class>.super.new(self, {}) and then it worked no problem, but I don't understand what actually happened and why.

If anyone could explain this to me I'd be really grateful, I've had a quick search of the LOVE forums, Stack Overflow etc. but there's nothing that answered my question in a way I could understand and I'm now more confused than I initially was lol.


Thanks!

[marclurr]

This is calling the parent constructor. Because the OOP is just emulated there's no mechanism for Lua to call any methods in the parent class. For example:

Code: Select all

local ParentClass = Object:extend()

function ParentClass:new()
    print("ParentClass constructor")
end

function ParentClass:doSomething()
    print("ParentClass doSomething")
end

local ChildClass = ParentClass:extend()

function ChildClass:new()
    ChildClass.super.new(self)
    print("ChildClass constructor")
end

function ChildClass:doSomething()
    ChildClass.super.doSomething(self)
    print("ChildClass doSomething")
end

local child = ChildClass()
child:doSomething()

will print the following:

Code: Select all

ParentClass constructor
ChildClass constructor
ParentClass doSomething
ChildClass doSomething

It's up to you to forward constructor and method calls to the parent, unlike in Java/C# like OOP languages the default parent constructor is called automatically, unless you want to override it to use a different overloaded constructor. You still have to deliberately call parent methods though. My C++ is a little rusty so I'm not sure how that behaves, I suspect it's similar though.

Edit: Missed the self parameter.

[mr_3tianne]

Thank you for your explanation, I think I understand what's going on now. So just to confirm, in my example of <Class>.super.new(self, {}) the blank table (or whatever the second argument is) is the argument(s) for constructing the "parent" class?

[pgimeno]

It's the argument passed to the constructor of the parent class. See the README of Classic, there's a very similar example where instead of {}, it uses x, y.

It's used to construct an object; the class of the object will be the class passed as self, but the parent's constructor will initialize it as if it were an object of the parent class (which is possible because they are compatible). The constructor of the child class will then add its own specializations to that base object.

In the example in the README, the constructor of Rect will create an object of class 'self', which will be first initialized as if it were a Point, and then the constructor of Rect will add its own specializations (the width and height, to make it a rectangle instead of a point).

The 'self' that Rect receives may be the Rect class itself, or any class that is a child of Rect, but whatever it is, the constructor will initialize it as if it were a Rect.

[mr_3tianne]

That makes sense. Thank you!