Sending float over QDBus - qt

Question

We currently migrated from Qt 4.5.x to 4.7.3, and since then we get errors when trying to send floats over DBus ('float' not registered as DBus type).
For instance:
QVariantList getTemperatures()
{
QVariantList retVal;
retVal << getSensorValue(1) << getSensorValue(2);
return retVal;
}
getSensorValueis a method that returns a floatvalue.
Since a QVariant in version prior to 4.6 could only contain a double value, the return values were implicitly cast to a double, and a doublecan be sent along the DBus.
But, since version 4,6, a QVariant can contain a float; as a result, the DBus module complains that floats are not a known datatype - which is correct.
I tried to register the float datatype, and to implement the streaming operators:
qDBusRegisterMetaType<float>();
QDBusArgument &operator<<(QDBusArgument &argument, const float &myFloat)
{
return argument << static_cast<double>(myFloat);
}
const QDBusArgument &operator>>(const QDBusArgument &argument, float &myFloat)
{
double d;
argument >> d;
myFloat = static_cast<float>(d);
return argument
}
But, when I try to stream the double into the QDBusArgument(operator <<), I get an error that the float datatype is trying to override the double behavior.
This is also normal, because the underlying QDbus system already has streamed the datatype ('f') to the QDBusArgument, and then detects that a double is entering the stream.
And now my question: does anybody know how I could stream this float, without having to replace all float datatypes with doubles in the backend methods?

Answer 1

(I had created at first an answer where I suggested you to use beginStructure() and endStructure() in order to make QtDBus stop complaining but then I realized that it doesn't solve your problem: you probably don't want to pass your float as a "double in a structure" but simply as a double.)
When passing directly the float to the QDBusArgument it gets casted automatically into a double and there is no problem. But if you want to pass it through a QVariantList you have no other choice than casting it before putting it into the QVariantList.
However if you're not afraid by dirty solutions you can overload the insertion operator of QVariantList to let it do it for you:
// g++ -o main main.cpp -lQtCore -lQtDBus
#include <QtDBus/QDBusArgument>
QVariantList & operator<<(QVariantList & list, const float & f)
{
QVariant variant(static_cast<double>(f));
list << variant;
return list;
}
int main()
{
QDBusArgument test;
QVariantList list;
float f = 1.0;
list << f;
test << list; // doesn't trigger any error
return 0;
}