How to use QVariant::fromValue with QString?

Question

I have the following code:

QString* data = new QString("data to QML");
engine.rootContext()->setContextProperty(QStringLiteral("consoleText"), QVariant::fromValue(data));

and this one does not work, the error message in QTCreator is the following:

...\qglobal.h:693: error: static assertion failed: Type is not registered, please use the Q_DECLARE_METATYPE macro to make it known to Qt's meta-object system #define Q_STATIC_ASSERT_X(Condition, Message) static_assert(bool(Condition), Message)

I do no think that I should use Q_DECLARE_METATYPE for QString because if I do something like this:

engine.rootContext()->setContextProperty(QStringLiteral("consoleText"), QVariant::fromValue(QString("data to QML")));

it works fine.

I am interested in that how can I use the QVariant::fromValue() with a pre-declared QString.

Don't allocate `QString` on the heap, it is entirely pointless. — dtech, Dec 14 '15 at 23:59
I was just testing how to connect QML with c++ side, did not pay attention to other things. — Bence Kaulics, Dec 15 '15 at 00:11

score 4 · Accepted Answer · edited Dec 14 '15 at 23:12

4

QVariant::fromValue() expects a QString, not a pointer to a QString.

Additionally, allocating a QString object on the heap doesn't make much sense. Under the hood, QString uses copy-on-write (COW) as an optimization; the actual data stored in the QString will always be on the heap anyway.

edited Dec 14 '15 at 23:12

BaCaRoZzo

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answered Dec 14 '15 at 22:54

MrEricSir

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I am just testing this is not a valid code, the `QString` will be filled by a `QSerialPort`. – Bence Kaulics Dec 14 '15 at 23:00
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Aaaah that mask at on your cat is awesome. :D (Ok, mark this as "too chatty") – BaCaRoZzo Dec 14 '15 at 23:12

score 1 · Answer 2 · answered Dec 14 '15 at 22:54

data is a pointer to a QString, not a QString itself. To use the QVariant::fromValue() method, you must dereference the pointer:

engine.rootContext()->setContextProperty(QStringLiteral("consoleText"), QVariant::fromValue(*data));
                                                                                            ^

This is because a QString * is not a registered meta-type (by default).

How to use QVariant::fromValue with QString?

2 Answers2