Convert a QVariant of a custom type to a QString.

Question

I have a custom class called Money that I have declared with Q_DECLARE_METATYPE().

class Money {
public:
  Money(double d) {
    _value = d;
  }
  ~Money() {}
  QString toString() const {
    return QString(_value);
  }
private:
  double _value;
};
Q_DECLARE_METATYPE(Money);

Money m(23.32);

I store that in a QVariant and I want to convert it to a QString:

QVariant v = QVariant::fromValue(m);

QString s = v.toString();

Variable s ends up being a null string because QVariant doesn't know how to convert my custom type to the string. Is there any way to do this?

Answer 1

What happens if you try it this way?

class Money {
public:
  Money(double d) {
    _value = d;
  }
  ~Money() {}
  QString toString() const {
    return _value.toString();
  }
private:
  double _value;
};

Answer 2

Are you sure the following works?

return QString(_value);

I don't seem to find a QString ctor that takes a double. You will have to do the conversion here yourself. The Qt way is to:

QString toString() const {
 QVariant d(_value);
 return d.ToQString();
}

Answer 3

Ok I found one way to do this. I created a parent type called CustomType with a virtual method that I can implement to convert my custom type to a "normal" QVariant:

class CustomType {
public:
  virtual ~CustomType() {}
  virtual QVariant toVariant() const { return QVariant(); }
};

I then inherited my custom Money class from this.

class Money : public CustomType {
public:
  Money(double d) {
    _value = d;
  }
  ~Money() {}
  QVariant toVariant() {
    return QVariant(_value);
  }
private:
  double _value;
};

This allows me to pass my custom Money variables contained in QVariants so I can use them in the Qt property system, model/view framework, or the sql module.

But if i need to store my custom Money variable in the database (using QSqlQuery.addBindValue) it can't be a custom class, it has to be a known type (like double).

QVariant myMoneyVariant = myqobject.property("myMoneyProperty");
void *myData = myMoneyVariant.data();
CustomType *custType = static_cast<CustomType*>(myData);
QVariant myNewVariant = ct->toVariant();

myNewVariant now has the type of double, not Money so I can use it in a database:

myqsqlquery.addBindValue(myNewVariant);

or convert it to a string:

QString s = myNewVariant.toString();