How can I extract the underlying value from QVariant?

jdent

I have a Model class of my own and need to extract the underlying value of the QVariant. That is, if the value in QVariant is a double I need to convert from QVariant to double, for instance.
Do I need to do a switch with types? is QVariant::metaType the way to find the kind of value inside the QVariant? Can you show an example?

ChrisW67

@jdent For basic types there are obvious QVariant methods like QVariant::toDouble(). The QVariant documentation is quite useful.

You can also register and handle custom types through QVariant. In that case you would use:

Q_DECLARE_METATYPE(MyCustomType);
MyCustomType foo;
QVariant v = QVariant::fromValue(foo);
MyCustomType bar = v.value<MyCustomType>();

or similar functions depending on Qt version.

SimonSchroeder

@jdent said in How can I extract the underlying value from QVariant?:

Do I need to do a switch with types? is QVariant::metaType the way to find the kind of value inside the QVariant? Can you show an example?

This would be one way to go. I have never had a need for this, but I'll give it a try how you could solve this with a switch statement:

switch(myVariant.metaType().id())
{
case QMetaType::fromType<int>().id():
    // handle int
    break;
case QMetaType::fromType<QString>().id():
    // handle QString
    break;
default:
    // maybe a message/log that this case is unhandled
    break;
}

(I have never used QMetaType this extensively. I hope that calling id() is the smartest choice to distinguish these cases.)

std::variant also has a solution with the overload pattern (using lambdas) using std::visit. It might be possible to use the overload pattern with std::visit on QVariants as well.