Support std::optional in QDataStream

#include <QDataStream>

#include <optional>


struct MyStruct {
  qint32 idx;
  QString title;

  std::optional<QString> comment;
};

template<class T>
static QDataStream& operator<<(QDataStream& ds, std::optional<T> const& opt)
{
    auto const hasValue = opt.has_value();
    ds << hasValue;


    if (hasValue)
    {
        ds << opt.value();
    }

    return ds;
}


template<class T>
static QDataStream& operator>>(QDataStream& ds, std::optional<T>& opt)
{
    bool hasValue;
    ds >> hasValue;

    if (hasValue)
    {
        T aT;
        ds >> aT;
        opt = aT;
    }
    else
    {
        opt = std::nullopt;
    }

    return ds;
}