QMetaType::type with QString

goocreations

I want to return the id of a previously registered custom class by using a QString. I've tried the following, but all of them return QMetaType::UnknownType

@QString name = "MyClass";

QMetaType::type(name.toLatin1().data()); // fail
QMetaType::type(name.toUtf8().data()); // fail
QMetaType::type(name.toLocal8Bit() .data()); // fail

char *chars = new char[name.length()];
strcpy(chars, name.toStdString().c_str());
QMetaType::type(chars); // fail@

I thought there was a bug with QMetaType or Q_DECLARE_METATYPE. This is not the case, since the following succeeded:

@QMetaType::type("MyClass"); // success@

Since no QString object is passed, the parameter is (I assume) directly used as a char*. Does anyone know how to correctly decode a QString to a char* so that QMetaType::type works correctly?

EDIT: Does this maybe have something to do with the terminating-character?

SGaist

Hi,

Two questions for you:

Did you use qRegisterMetaType<>() ?
What combination of Qt/Platform are you using ?

QMetaType::type will only work if your type has been registered.

Hope it helps

ChrisW67

Works fine here:
@
#include <QCoreApplication>
#include <QDebug>

class MyClass { };
Q_DECLARE_METATYPE(MyClass)

int main(int argc, char **argv)
{
QCoreApplication app(argc, argv);

int type = qRegisterMetaType<MyClass>("MyClass");
qDebug() << "MyClass" << type;

QString name("MyClass");
qDebug() << "QString" << QMetaType::type(name.toAscii());
qDebug() << "String literal" << QMetaType::type("MyClass");

MyClass *c =  static_cast<MyClass*>(QMetaType::construct(type));
qDebug() << c;
delete c;

return 0;

}
@

I suspect you have not called qRegisterMetaType<>().

goocreations

I'm using Qt5 and qRegisterMetaType is not documented anymore, so I'm not sure if it's deprecated. I also don't want to use qRegisterMetaType, since it is run-time bound. Q_DECLARE_METATYPE on the other hand is (as far as I know) preprocessor bound.

I've registered my class with:

@Q_DECLARE_METATYPE(MyClass);@

and since

@QMetaType::type("MyClass");@

worked and I was able to create an object with this, means that the registration was successful. My problem is with retrieving it by using a QString. The toAscii() function is also gone in the new Qt5, so I'm not sure if toLatin1() does exactly the same, but for some reason the passed char* is not working.

goocreations

Ok, I'm not sure what I did, but can't replicate my problem. So I don't think it has anything to do with the QString encoding. Here is my class decleration:

@class MyClass
{
...
};

Q_DECLARE_METATYPE(MyClass);

int main()
{
if(QMetaType::type("MyClass") == QMetaType::UnknownType)
{
int id = qMetaTypeId<ViSigmoidActivationFunction>();
cout << id <<endl;
cout << QMetaType::typeName(id) << endl;
}
}@

The output of main is as follows:

@1126
MyClass@

So my registration of my class was successfull, but there is a problem retireving the id using QMetaType::type("MyClass"). When I add the following to the first line of main:

@qRegisterMetaType<MyClass>("MyClass");@

the 2 output staments are never execute, meaning my type was found. But I don't want to use this, since it is a runtime registration. Additionally Qt5 always refers to Q_DECLARE_METATYPE, saying that qRegisterMetaType is only when you want to use signals/slots parameters.