QList <qstring> and QDataStream</qstring>

[baileigh]

Trying QDataStream to record QList and then retrieve it.

QList <QString> l = {"a", "bb", "ccc"};
qDebug() << l;
QByteArray arr;
QDataStream in(&arr, QIODevice::WriteOnly);
in.setVersion(QDataStream::Qt_DefaultCompiledVersion);
in << l;
QList <QString> l2;
QDataStream out(&arr, QIODevice::ReadOnly);
out >> l2;
qDebug() << l2;

It's empty out. Why?

[inna]

User data types need to be sent to the flow: 1. Determine the operator of entry and withdrawal. It's simple:

friend QDataStream &operator <<(QDataStream &in, const MyClass &_obj)
{
    in << _obj.m_field1;
    in << _obj.m_field2;
    in << _obj.m_field3;
    in << _obj.m_field4;
    return in;
}
friend QDataStream &operator >>(QDataStream &out, MyClass &_obj)

{

out >> _obj.m_field1;

out >> _obj.m_field2;

out >> _obj.m_field3;

out >> _obj.m_field4;

return out;

}

There's a minus here, if we've decided to add another field, the previously recorded data cannot be read. I mean, there's no backup. To avoid this, a more complex option can be used:

friend QDataStream &operator <<(QDataStream &in, const MyClass &_obj)

{

QVariantMap m;

m["field1"] = QVariant::fromValue(_obj.m_field1);

m["field2"] = QVariant::fromValue(_obj.m_field2);

m["field3"] = QVariant::fromValue(_obj.m_field3);

m["field4"] = QVariant::fromValue(_obj.m_field4);

in << m;

return in;

}

friend QDataStream &operator >>(QDataStream &out, MyClass &_obj)

{

QVariantMap m;

out >> m;

_obj.m_field1 = m["field1"].toInt();

_obj.m_field2 = m["field2"].toString();

_obj.m_field3 = m["field3"].toDouble();

_obj.m_field4 = m["field4"].toBool();

return out;

}

Or even a little easier:

template <class T>

static void convert(const QVariant &_val, T &result)

{

result = _val.value<T>();

}

friend QDataStream &operator >>(QDataStream &out, MyClass &_obj)

{

QVariantMap m;

out >> m;

convert(m["field1"], _obj.m_field1);

convert(m["field2"], _obj.m_field2);

convert(m["field3"], _obj.m_field3);

convert(m["field4"], _obj.m_field4);
return out;

}

In such a case, the fields may be added/delayed and all previously recorded data may be considered.

2. Specify the qt meta-system that a user type of data can be streamed. To this end, prior to the flow recording process, call the function:

qRegisterMetaTypeStreamOperators<MyClass>("MyClass");

All registrations are usually made immediately after the programme is launched.
If we're gonna write a list, we'll also call it:

qRegisterMetaTypeStreamOperators<QList<MyClass>>("QList<MyClass>");

qRegisterMetaTypeStreamOperators<QVector<MyClass>>("QVector<MyClass>");

If a member of the class is also a user type, it also needs to define the flow recording function and cause qRegisterMetaTypeStreamOperators.

For basic Qt types qRegisterMetaTypeStreamOperators, usually completed, so it's weird that you don't have QList Runway.