[Qt-interest] QFile private member

[Pete Christensen]

Scott Aron Bloom wrote:
>  -----Original Message-----
>> From: qt-interest-bounces at trolltech.com [mailto:qt-interest-
>> bounces at trolltech.com] On Behalf Of Pete Christensen
>> Sent: Tuesday, February 10, 2009 1:54 PM
>> To: qt-interest at trolltech.com
>> Subject: Re: [Qt-interest] QFile private member
>>
>> Scott Aron Bloom wrote:
>>>> -----Original Message-----
>>>> From: qt-interest-bounces at trolltech.com [mailto:qt-interest-
>>>> bounces at trolltech.com] On Behalf Of Pete Christensen
>>>> Sent: 2009-02-10 13:32
>>>> To: qt-interest at trolltech.com
>>>> Subject: Re: [Qt-interest] QFile private member
>>>>
>>>> andrew.m.goth at l-3com.com wrote:
>>>>> Pete Christensen wrote:
>>>>>> OK, I give up, I can not do this. How do I declare a private
> member
>>>>>> variable of type QFile and initialize it in the constructor?
>>>>> Are you sure you want QFile and not QFile*?
>>>>>
>>>>>> Compiler complains:
>>>>>> /usr/include/qt4/QtCore/qfile.h:191: error:
>>>>>> 'QFile::QFile(const QFile&)'
>>>>>> is private
>>>>> The gripe is against the header file, not the implementation of
> the
>>>>> constructor.  Specifically the complaint is that the QFile copy
>>>>> constructor is private.  Why would that be a problem?  It looks to
>>> me
>>>>> that it's trying to make a default copy constructor for your
> class,
>>> but
>>>>> it can't since it would rely on copy constructors for all data
>>> members
>>>>> of your class.  You could explicitly define a copy constructor for
>>> your
>>>>> class, or you could just switch to QFile*, which can be copied
>>> without
>>>>> incident.
>>>>>
>>>> OK, it's starting to make a little sense. I looked up "copy
>>> constructor".
>>>> "A copy constructor is generally needed when an object owns
> pointers
>>> or
>>>> non-shareable references, such as to a file, in which case a
>>> destructor
>>>> and an assignment operator should also be written."
>>>>
>>>> So now I'm pretty sure QtCore does things like:
>>>>
>>>> 190 private:
>>>> 191     Q_DISABLE_COPY(QFile)
>>>> 192 };
>>>>
>>>> (this from qfile.h) to explicitly get the compiler to barf in
>>> situations.
>>>> I'll look into this destructor/assignment operator thing.
>>>>
>>>> Regarding subclassing, I'm not actually creating a new QFile type,
>>> this
>>>> was just the simplest I could get my code to communicate my
> problem.
>>>> Eventually I need to read the file in a thread.
>>>> _______________________________________________
>>>> Qt-interest mailing list
>>>> Qt-interest at trolltech.com
>>>> http://lists.trolltech.com/mailman/listinfo/qt-interest
>>> [Scott Aron Bloom]
>>> You are creating a new QFile, except by using QFile rather then
> QFile *.
>>> Simply making it a QFile *, and then in the constructor, creating a
>>> blank QFile...
>>>
>>> file = new QFile;
>>>
>>> You will solve all your issues...
>>>
>>> Scott
>>>
>>>
>> OK, cool. Andrew called it and Scott actually figured out that I know
>> nothing about C++ and then told me how to assign an actual object to a
>> pointer. (I think that's how to describe what's happening here.)
>>
>> The other suggestions, including Andreas', using reference "file
> QFile"
>> and not pointer "file * QFile" all fail to compile with the same
> error.
>> I'm pretty sure I'd need an explicit copy constructor to get that to
>> work. (And I'm finding there's nothing "plain" about C++, Andreas :))
>>
>> Thanks guys. Hopefully I can actually figure out how to use this
> pointer
>> I've now got.
>>
>> Re: destructor: will-do, Bob.
> 
> Good luck... However, I seriously recommend you take a basic C++ class
> first..  Even a C++ class using QT, but your fundamentals are weak and
> need work
> 
> Scott

Thanks again, Scott, for pointing out the obvious.