Qt-interest Archive, March 2008
extending QVariant to custom types
Message 1 in thread
I've got a simple class that pretty much holds a number, and string to
represent units (feet, meters and such). I'd like to use this class
with a widget I created which is compiled as a plug-in. I designed my
widget to work with QVariants hoping it would be flexible enough to let
me do what I wanted. Well QVariants can't convert custom types to
anything at all, so in order to call my custom type's toString function
I would need to use QVariant's templated value function. But that
requires me to link my widget against my custom class, which is a deal
breaker because I expect to need a lot more custom classes down the
road.
So, to put it simply, I need a way to have QVariant turn my custom class
in to a string? Would it be a good idea to modify the QVariant class?
I did some digging and found a method (qRegisterMetaTypeStreamOperators)
to register overloaded stream operators for QVariants, but these only
help if you wanted to save them in a binary format, is there a similar
method for character based QVariant stream operators?
Thanks for reading,
Joaquin Luna
Message 2 in thread
On Friday 14 March 2008 19:09:38 Joaquin Luna wrote:
> I did some digging and found a method (qRegisterMetaTypeStreamOperators)
> to register overloaded stream operators for QVariants, but these only
> help if you wanted to save them in a binary format, is there a similar
> method for character based QVariant stream operators?
what else do you need? why do you care how Qvariant stores them internaly?
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Message 3 in thread
I guess I could have been clearer when stating my problem. Ideally I would like a code snippet like the one below to work for user defined classes like the one I described earlier.
MyCustomClass x;
QVariant v = QVariant::fromValue(x);
QString s = v.toString();
I have overloaded the stream operators and registered them with the metatype system, but that servers a much different purpose than the one above.
-----Original Message-----
From: Arvid Ephraim Picciani [mailto:aep@xxxxxxxxxxxxxxx]
Sent: Friday, March 14, 2008 2:59 PM
To: qt-interest@xxxxxxxxxxxxx
Subject: Re: extending QVariant to custom types
On Friday 14 March 2008 19:09:38 Joaquin Luna wrote:
> I did some digging and found a method (qRegisterMetaTypeStreamOperators)
> to register overloaded stream operators for QVariants, but these only
> help if you wanted to save them in a binary format, is there a similar
> method for character based QVariant stream operators?
what else do you need? why do you care how Qvariant stores them internaly?
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[ signature omitted ]
Message 4 in thread
On Friday 14 March 2008 20:17:23 you wrote:
> I guess I could have been clearer when stating my problem. Ideally I would
> like a code snippet like the one below to work for user defined classes
> like the one I described earlier.
>
> MyCustomClass x;
> QVariant v = QVariant::fromValue(x);
>
> QString s = v.toString();
you need the string human readable?
if not just use QMetaType::save
if you do require human readable strings, well,... i'm still kinda hoping for
a builtin solution too, so we can serialise xml more easy. for now you need
to define your own operators (for QTextStream)
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[ signature omitted ]
Message 5 in thread
Yes, I am trying to display a human readable string for a QVariant holding a custom type. I have defined operators for my custom class as below.
friend QTextStream & operator<<(QTextStream &out, const SC3P_Measurement & myObj);
friend QTextStream & operator>>(QTextStream &in, SC3P_Measurement & myObj);
Is this supposed to allow me to stream out QVariants containing my custom types? I have tried it and it does not work for me, maybe you could provide more details on how this is supposed to work.
-----Original Message-----
From: Arvid Ephraim Picciani [mailto:aep@xxxxxxxxxxxxxxx]
Sent: Friday, March 14, 2008 3:24 PM
To: qt-interest@xxxxxxxxxxxxx
Subject: Re: extending QVariant to custom types
On Friday 14 March 2008 20:17:23 you wrote:
> I guess I could have been clearer when stating my problem. Ideally I would
> like a code snippet like the one below to work for user defined classes
> like the one I described earlier.
>
> MyCustomClass x;
> QVariant v = QVariant::fromValue(x);
>
> QString s = v.toString();
you need the string human readable?
if not just use QMetaType::save
if you do require human readable strings, well,... i'm still kinda hoping for
a builtin solution too, so we can serialise xml more easy. for now you need
to define your own operators (for QTextStream)
--
[ signature omitted ]
Message 6 in thread
On Friday 14 March 2008 21:05:06 Joaquin Luna wrote:
> Yes, I am trying to display a human readable string for a QVariant holding
> a custom type. I have defined operators for my custom class as below.
>
> friend QTextStream & operator<<(QTextStream &out, const SC3P_Measurement &
> myObj); friend QTextStream & operator>>(QTextStream &in, SC3P_Measurement &
> myObj);
>
>
> Is this supposed to allow me to stream out QVariants containing my custom
> types? I have tried it and it does not work for me, maybe you could
> provide more details on how this is supposed to work.
huh? err... maybe _you_ could provide more details. You don't exactly get
humanreadable strings out of QVariant. unless you qvariant_cast back to your
custom type and use the text operators then. you might as well just add a
toString() function to your class.
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[ signature omitted ]
