how to call QVariant::convert

[ODБOï]

hi,
I'm struggling to calling QVariant::convert

passing 'const QVariant' as 'this' argument discards qualifiers [-fpermissive]

callback method where i recive a QVariant and want to convert it to QVariantList

void Client::listUpdated(const QVariant &value){
  
    if(value.canConvert(QVariant::List) && value.convert(QVariant::List) ){
            setList(value.toList());
    }else{
        //error ..
    }
}

Im i calling convert with wrong argument ? what should i pass to it?

[J.Hilk]

@LeLev in principle, yes,
but if toList fails, you get an empty list. If you want to, you can check for that as well

void listUpdated(const QVariant &value){
    if(auto list = value.toList(); value.canConvert(QVariant::List) && !list.isEmpty()){
      setList(list);
    }
}

[JonB]

@LeLev
You can't QVariant::convert() a const QVariant & --- it changes it!! Make the parameter non-const or work on a copy or similar.

[ODБOï]

@JonB hi,
thanks for your answer,
should i create a local variable like this ?

void Client::listUpdated(const QVariant &value){
    QVariant tmpValue = value;
    if(value.canConvert(QVariant::List) && tmpValue .convert(QVariant::List) ){
      setList(value.toList());
    }
}

[J.Hilk]

@LeLev in your case, I would drop it all together

canConvert (QVariant::List) + toList() is in essence the same as convert(QVariant::List) with the difference that convert modifies itself and toList returns a modified Value

[ODБOï]

@J-Hilk hi
Thx for the suggestion.
If i understand correctly, this will do ?

void Client::listUpdated(const QVariant &value){
    QVariant tmpValue = value;
    if(tmpValue.canConvert(QVariant::List) && tmpValue.convert(QVariant::List) ){
      setList(tmpValue );
    }
}

[J.Hilk]

@LeLev it should, but I suggested:

void Client::listUpdated(const QVariant &value){
    if(value.canConvert(QVariant::List)  ){
      setList(value.toList());
    }
}

[ODБOï]

@J-Hilk but if i understand this part of doc correctly, they say i have to use canConvert And convert, because even if "canConvert" returns "true", "convert" can still return "false". Did i get it wrong?

[J.Hilk]

@LeLev in principle, yes,
but if toList fails, you get an empty list. If you want to, you can check for that as well

void listUpdated(const QVariant &value){
    if(auto list = value.toList(); value.canConvert(QVariant::List) && !list.isEmpty()){
      setList(list);
    }
}

[ODБOï]

@J-Hilk Thank you for clarifying that

[JonB]

@J-Hilk said in how to call QVariant::convert:

if(auto list = value.toList(); value.canConvert(QVariant::List) && !list.isEmpty()){

I know it works, but ... really? :) First call toList() on value, then afterward check whether it could convert. Are you in some backward time-warp? ;-)

[J.Hilk]

@JonB fair enough, is this particular case, 2nd if case inside the first one would probably be more sensible :D

[JonB]

@J-Hilk
It made me smile. I have never declared a variable inside an if ( condition. Witches' brew :) I can sort of see you'd have to do it your way round to achieve it. That's what you get for putting a ; inside an if ( :D Personally I'm not going to call toList() till after convert() has cleared me to do it.

[J.Hilk]

@JonB you mean:

void Client::listUpdated(QVariant value){
    if(value.canConvert(QVariant::List) && value.convert(QVariant::List) ){
      setList(value);
    }
}

I would agree, sometimes it's just simpler to pass by value and rely on the copy on write functionality of Qt-classes 😉

[JonB]

@J-Hilk
Well, no, you/someone said earlier we don't need to modify the QVariant value input parameter. You/someone said value.toList() was OK on a const. But we ought verify it's convertible before doing so/to distinguish from convertible-but-empty list. So I just thought (untested):

if (value.canConvert(QVariant::List))
    setList(value.toList());

?

Oh, that's what you started with? Yeah I see, I read that link now. Well, toList() is not going to fail here after canConvert() :)

Anyway, that wasn't really my amusement. It was calling toList() and then calling canConvert(), as far as I can see in order to fit it into a variable-defining if(. There's no big point here from me, I get now that the canConvert/convert() functions aren't the same as each other.