Hi,
I have a junior question when using collection to store/get my own defined element. My elements is defined as below:
Object subclass: #MyItem instanceVariableNames: 'subItems' classVariableNames: '' poolDictionaries: '' category: 'MyTest'
Where the subItems instance var is also a collection.
But I don't know how to store/get such kind of MyItem by using collection, I think I need deep copy an element when add it to the collection.
I don't think the following code works.
|temp coll| temp := MyItem new. temp addSubItem: 'hello'; addSubItem 'world'; yourself. coll := OrderedCollection new. coll add: temp
How can I add the deep copied element to my collection and retrieve it later? shall I overload the #copy method?
Best regards and thanks -Xinyu
Hello Xinyu,
if you come from another programming language, a collection stores only pointers to objects. No chance (and no need) to copy an object into a collection.
XL> Object subclass: #MyItem XL> instanceVariableNames: 'subItems' XL> classVariableNames: '' XL> poolDictionaries: '' XL> category: 'MyTest'
XL> But I don't know how to store/get such kind of MyItem by XL> using collection, I think I need deep copy an element when add it XL> to the collection. No imho, see below.
XL> I don't think the following code works.
XL> |temp coll| XL> temp := MyItem new. XL> temp addSubItem: 'hello'; addSubItem 'world'; yourself. XL> coll := OrderedCollection new. XL> coll add: temp
assuming subItems is initialised as: OrderedCollection new and addSubItem: is defined as: subItems add: anObject the code should work.
XL> How can I add the deep copied element to my collection and XL> retrieve it later? shall I overload the #copy method?
The main question is: why would you want to copy the element? Each MyItem holds to its collection of subItems and coll holds to your temp.
There may be reasons you want to use a copy put usually you just put your objects into the collection and retrieve them with coll at:
If you really want to store a copy you could: temp1 := temp deepCopy. coll add: temp1.
BTW, do you know of the free Smalltalk books, Squeak by Example being the latest and specifically tailored to Squeak?
XL> Best regards and thanks XL> -Xinyu
Cheers
Herbert mailto:herbertkoenig@gmx.net
Hi, Herbert
Thank you very much. I got it. I referred to SBE(both smalltalk by example and squeak by example :-)).
the reason why I want to copy is because I don't want the element stored in collection to change when I modify temp var later. and because the element has other collections as instance vars, so i am not sure if I need deep copy.
I tried something like this:
coll add: (temp deepCopy).
...modify temp ex. temp:= MyItem new ...
temp := (coll at: 1) deepCopy.
The above code works when there are 2 levels of collection.
Thanks again and best regards. -Xinyu
On Jan 1, 2008 11:19 PM, Herbert König herbertkoenig@gmx.net wrote:
Hello Xinyu,
if you come from another programming language, a collection stores only pointers to objects. No chance (and no need) to copy an object into a collection.
XL> Object subclass: #MyItem XL> instanceVariableNames: 'subItems' XL> classVariableNames: '' XL> poolDictionaries: '' XL> category: 'MyTest'
XL> But I don't know how to store/get such kind of MyItem by XL> using collection, I think I need deep copy an element when add it XL> to the collection. No imho, see below.
XL> I don't think the following code works.
XL> |temp coll| XL> temp := MyItem new. XL> temp addSubItem: 'hello'; addSubItem 'world'; yourself. XL> coll := OrderedCollection new. XL> coll add: temp
assuming subItems is initialised as: OrderedCollection new and addSubItem: is defined as: subItems add: anObject the code should work.
XL> How can I add the deep copied element to my collection and XL> retrieve it later? shall I overload the #copy method?
The main question is: why would you want to copy the element? Each MyItem holds to its collection of subItems and coll holds to your temp.
There may be reasons you want to use a copy put usually you just put your objects into the collection and retrieve them with coll at:
If you really want to store a copy you could: temp1 := temp deepCopy. coll add: temp1.
BTW, do you know of the free Smalltalk books, Squeak by Example being the latest and specifically tailored to Squeak?
XL> Best regards and thanks XL> -Xinyu
Cheers
Herbert mailto:herbertkoenig@gmx.net
Beginners mailing list Beginners@lists.squeakfoundation.org http://lists.squeakfoundation.org/mailman/listinfo/beginners
To modify the behaviour of copy to be more appropriate for your class, the facility is provided via #postCopy.
The basic copy implementation is as follows.
copy
^ self shallowCopy postCopy
specializing #postCopy for your class, it can perfrom further copies of its inst vars if needed.
postCopy
instVarWithCollection := instVar withCollection copy.
So now the collection is a different collection although it contains the same elements.
best regards
Keith
beginners@lists.squeakfoundation.org
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Herbert König -
Keith Hodges -
Xinyu Liu