[Interest] Proper use of SharedPointer/WeakPointer

[Jason H]

Hrm, my objects *do* have parents, and when the parent goes, the children need to go too. How is that resolved?




________________________________
 From: André Somers <andre at familiesomers.nl>
To: interest at qt-project.org 
Sent: Thursday, February 23, 2012 9:37 AM
Subject: Re: [Interest] Proper use of SharedPointer/WeakPointer
 
Op 23-2-2012 15:29, Thiago Macieira schreef:
> On quinta-feira, 23 de fevereiro de 2012 06.26.19, Jason H wrote:
>> So in a return-everything query function, it would look like:
>>
>> QList<QSharedPointer<QObject*>  >  ObjectDatabase::returnAll()
>> {
>>      QList<QSharedPointer<QObject*>  >  results;
>>      foreach (const obj, m_pObjects)
>>      {
>>
>>          results<<  QSharedPointer(obj);
>>      }
>>
>>      return results;
>> }
>>
>> Yes?
> No.
>
> It is something like this:
>
> QList<QSharedPointer<QObject*>  >  ObjectDatabase::returnAll()
> {
>     return m_pObjects;
> }
>
> You must already have those objects in QSharedPointer somewhere. Otherwise
> QSharedPointer *will* delete them.
Also, to make the whole thing a bit more readable, I prefer to define 
typedefs for all classes that I use as a shared pointer. I postfix SPtr 
to the name of the class for the shared pointer version, but that's a 
matter of preference. I guess some convention would be good.

Thiago's advice is good though: if you use smart pointers, make sure you 
use them everwhere, especially for shared pointers. Any place where you 
see something else besides a new inside the constructor of a shared 
pointer, you know you have a problem. So:

MyClassSPtr obj = MyClassSPtr(new MyClass); //ok
MyClassSPtr obj2 = MyClassSptr(someRawMyClassPtr); // ALERT! Trouble 
about to happen!

Also, beware of QSharedPointer lifetime management interfering with 
QObject lifetime management. IMO, shared pointers of QObjects better 
*not* have parents:

MyClassSPtr obj3 = MyClassSPtr(new MyClass(parentQObject); //could spell 
big trouble!

André

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