Qt C ++从QJsonValue转换为QByteArray

Question

I have a large (megabytes) string in a QJsonValue , that I need to convert to QByteArray , as I am sending the string as data with a QNetworkRequest .

Currently I am doing this:

myQJsonObject["myQJsonValue"].toString().toUtf8()

Would this incur copying the same data to memory many times for some reason? If so, how would you go about implementing this without unnecessary copyings?

Answer 1

and why you do not use QJsonDocument ? This should be used for reading and writing. There is a method QJsonDocument::toBinaryData . This API should do everything with most effective way.

---

Update to comment:

Single JSon value is must be one of other JSon types: object, string or some number. I'm pretty sure you have JSon object. So your code should look like this::

JSonValue val = someJsond["someKey"];
if (val.isObject()) {
    QJSonDocument doc(val.toObject());
    SendToServer(doc.toBinaryData());
} else {
    // error or:
    SendToServer(val.toString().toUtf8());
}

Answer 2

The call to myQJsonObject["myQJsonValue"].toString() does not involve data copy thanks to copy-on-write semantics of Qt.

The toUtf8 call is costly. QString stores the data as Unicode (16-bit QChar s), and encoding it in UTF-8 involves more than data copy.

QString::constData() returns a pointer to the underlying character array. But then, each character is represented by 2 bytes instead of 1 or 2 bytes in case of Utf-8. This might mean sending two times more data over the network.

So if your data consists of mostly ASCII characters, then UTF-8 is probably a better option. If it contains lots of non-Ascii characters, and the other side can handle UTF-16, then UTF-16 is worth considering.