overriding parent's function in child class

bysreg · December 2012

straight to the code snippet :

--A.lua
A = Core.class()

function A:init()
self:foo()
end

function A:foo()
print("x")
end

--B.lua
B = Core.class(A)

function B:init()
end

function B:foo()
print("y")
end

local b = B.new()

The output is "x"

shouldn't it print "y" ?

i thought that it was because of the different self bug mentioned at my older thread here : http://www.giderosmobile.com/forum/discussion/1677/object-in-super-class039-init-function-is-not-the-same-as-in-the-inherited-class039-init-function-#Item_3

and here : http://www.giderosmobile.com/forum/discussion/1234/accessing-children-from-parent-class#Item_14

but now it seems it's already got the same self object but still it doesn't call the overridden function

thank you

ar2rsawseen · December 2012

@bysreg no it shouldn't because foo method of B class is not called anywhere.
Basically the order goes like this.

You initiate B class,
it initiates A class,
it calls foo method of A class, because it's in its constructor
then returns back to B init method,
does everything that needs to be done there
and returns instance.

And this jumping happens only in constructor, so now if you call

b:foo()

it will not call A:foo method

What effect are you trying to achieve? Maybe there's another way

bysreg · December 2012

@ar2rsawseen

so it is really designed to be like that ?
afaik, at least in other true OO language, the foo called in A will call the foo defined in B as the object b is the instance of class B which inherits A

ar2rsawseen · December 2012

@bysreg

Yes, I get you, in C#, for example, the B:foo method would have been called and if you haven't specifically said to call A:foo method it won't be called.

But since in Lua OOP is not native, but only programmed on top of it, it has it's own restrictions, but also a lot's of other new capabilities and is very, very, veeerrryyy flexible.

bysreg · December 2012

@ar2rsawseen

all right, thank you for your fast help.

Yes, then i guess Gideros class can make it more OO-like like the above suggested code output? but for now, i think i'll think of a different way to achieve the same effect

OZApps · December 2012

@bysreg,

Lua is quite fun and flexible once you understand the way it works.

However think of what you are trying to do like an Abstract Class in C++ so your new class will have all of the functions by default, you can override them with new functions, however when you use

B = Core.class(A)

A is like a super class and on init, the

A:init

is invoked first. If you want to invoke foo on class B, you will have to explicitly invoke it as

b:foo()

or in the

B:init()

function.

This is how it unfolds

 b = B.new()
 --> A:init()
 --> A:foo()
 --> prints x
 
 --> B:init()

Hope this would help you understand what happens and where you need to place your functions for what you need.

twisttap · December 2012

So as far as I understand from the discussion going on here, there is no way to skip the execution of code from parent class' init() ?

OZApps · December 2012

@twisttap,
if you have worked with C++/Java, when you inherit, there is always the super or the parent and if I recollect correct, the super is always called in most of the functions.

If you need that the parent class is not called, do not inherit from it. Unless I am missing something here.

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