[Qt-interest] how to pass the reference to QMap to other class

[Mihail Naydenov]

----- Original Message -----
> From: franki <franki at franki.eu.org>
> To: qt-interest at qt.nokia.com
> Cc: 
> Sent: Friday, September 9, 2011 9:01 PM
> Subject: Re: [Qt-interest] how to pass the reference to QMap to other class
> 
> Friday 09 September 2011 14:21:58 BERAUD Alexandre napisał(a):
>>  QMap<int,DataStruct>& myMap;
>>  This is an uninitialized reference, which is invalid and is just
>>  tolerated by your compiler.
>> 
>>  myMap(firstClass->getMap())
>>  Here you use the copy constructor of the class and I guess some hidden
>>  dereference is involved.
>>  In my opinion it is better to access the QMap through the firstClass
>>  pointer because the use of references often leads to unreadable code
>>  with the compiler performing some magic behind the scene.
> 
> thanks for answer, there must be some connection between my variable myMap in 
> SecondClass and original QMap in first, because if I modify obcject in first 
> class I can read it in second, only when I add additional object in first, I 
> can't see it in second one.
> but just out of curiosity, is there a way to pass reference to QMap to other 
> class?
> I'm using 4.7.4 Qt and Linux platform

When you add new item to the map it is detached (cloned) and you hold a ref to the old map.
Something like that. The non-const ref in your second class is probably creating just a regular ref-counted copy.

MihailNaydenov

> 
> best regards
> Marek
> 
>> 
>>  Regards,
>> 
>>  Alex
>> 
>>  Le 09/09/2011 12:38, franki a écrit :
>>  > Hi all,
>>  >
>>  > I have one class where i have QMap which has some data, I want to be
>>  > able to easly access this data in second class, so what I want is to
>>  > pass this QMap from first class to second class as a reference
>>  > (without copying data).
>>  >
>>  > I googled and I've done it like this (snippet of code):
>>  >
>>  > first class header :
>>  >
>>  > public:
>>  >
>>  > FirstClass(QObject *parent)
>>  >
>>  > QMap<int,DataStruct> myMap;
>>  >
>>  > public slot:
>>  >
>>  > QMap<int,DataStruct>& getMap();
>>  >
>>  > first class body:
>>  >
>>  > QMap<int,DataStruct>& FirstClass::getMap() {
>>  >
>>  > return myMap;
>>  >
>>  > }
>>  >
>>  > second class header:
>>  >
>>  > public:
>>  >
>>  > SecondClass(Ui::MainPages m,FirstClass *firstClass);
>>  >
>>  > private:
>>  >
>>  > Ui::MainPages mp;
>>  >
>>  > FirstClass *firstClass;
>>  >
>>  > QMap<int,DataStruct>& myMap;
>>  >
>>  > second class body:
>>  >
>>  > SecondClass::SecondClass(Ui::MainPages m,FirstClas *firstClass):
>>  > QWidget(),myMap(firstClass->getMap())
>>  >
>>  > {
>>  >
>>  > mp=m;
>>  >
>>  > this->firstClass=firstClass;
>>  >
>>  > }
>>  >
>>  > So the problem is wierd, when I change data of the item in QMap in
>>  > firstClass I can see the change in secondClass by accessing private
>>  > variable myMap,
>>  >
>>  > but when I add new member to QMap in firstClass I can't see this 
> new
>>  > member in secondClass by using private variable myMap, although I can
>>  > access this new member when I'm using pointer to first class like 
> this:
>>  >
>>  > firstClass->myMap
>>  >
>>  > What is wrong with this, and how should I do that properly?
>>  >
>>  > I'm aware that in second class I can access QMap by pointer to 
> first
>>  > class:
>>  >
>>  > firstClass->myMap
>>  >
>>  > but I wanted to have reference to myMap in second class without using
>>  > pointer to first class.
>>  >
>>  > please advise me.
>>  >
>>  > best regards
>>  >
>>  > Marek
>>  >
>>  >
>>  >
>>  > _______________________________________________
>>  > Qt-interest mailing list
>>  > Qt-interest at qt.nokia.com
>>  > http://lists.qt.nokia.com/mailman/listinfo/qt-interest
> 
> 
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