Convert a QVariant of a custom type to a QString.

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?

From stackoverflow

• 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;
  };
  

  dirkgently : You sure you can call a method on a native datatype?

  darkadept : Nope. double does not have a .toString() method. That wouldn't work.

• 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();
  }
  

  darkadept : You're right, there is no ctor that takes a double. I should have written that line as "return QString::number(_value);"

  dirkgently : Either should fix your problem.

• 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();
  

  darkadept : I think the void *QVariant::data() method isn't in the Qt documentation.