[Interest] Help, please !!!

[André Somers]

Op 23-4-2012 20:44, Nikos Chantziaras schreef:
> Then you're not doing what you think you're doing:
>
>       QList<  QList<int>  >  listOfLists;
>       QList<int>  listOfInts;
>       listOfInts.append(10);
>
>       listOfLists.append(listOfInts);
>       listOfLists[0][0] = 9;
>
>       qDebug()<<  listOfLists[0][0]<<  listOfInts[0];
>
> You are modifying a copy, so it prints "9 10" instead of "10 10".  This:
>
>     listOfLists[0][0] = 9;
>
> modifies a copy of listOfInts.  Also the reverse is true.  If you modify
> listOfInts, then the copy of it inside listOfLists is not updated.
>
> "Implicit sharing" means that data is copied when it's modified.  It's
> not a replacement for pointers.
>
Not true, in this case. Let's look at the signature of the operator that 
Scott is using:
T & QList<T>::operator[](int i);

Note the little & after the first T? That's right, it returns a 
*reference*. And a reference is something you can directly modify. The 
documentation for that method also explicitly states that:

 > Returns the item at index position/i/as a *modifiable* reference.

Your statement that "implicit sharing" means that a copy is made when 
data is modified, is way too general. That is only true if you actually 
*have* a (shared) copy (which you don't in this case).

And yes, I'm pretty sure Scott knows what he is doing, he is quite 
experienced...

André


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