QDataStream with wchar_t array

jars121

Hi,

I'm working on some IPC functionality, whereby a structure (defined within both applications) is sent from one application to another.

The source of the data is a memory-mapped file within Windows, which is accessed using the various Windows APIs. I won't dwell on that as I've verified that this is working as expected, and it doesn't have any relevance to my question/issue.

The structure looks like this:

struct myStruct {
    uint8_t val1;
    uint16_t val2;
    wchar_t val3[15];
    wchar_t val4[35];
    int32_t val5;
    float val6[4];
};

An instance of the structure is populated by casting from the memory-mapped file:

myStruct *myStructInstance = (myStruct *)memoryMappedFile;

I'm using QDataStream (with a QByteArray) to serialise the structure for transmission to the remote application:

QByteArray ba;
QDataStream ds(&ba, QIODevice::WriteOnly);
ds.setVersion(QDataStream::Qt_5_15);

ds << val1 << val2 << val3 << val4 << val5 << val6;

The packed QByteArray is then transmitted to the remote application (in this instance, via a QTcpSocket):

m_remoteClientSocket->write(ba);
m_remoteClientSocket->flush();

My questions are as follows:

Is this the advised method of sending a known structure between two applications?
How do I then de-serialise the received QByteArray at the remote application? I understand how to do it for uint8_t, uint16_t, etc. types, but I'm not sure how to do it for arrays, particularly wchar_t arrays.

Any guidance would be greatly appreciated!

Christian Ehrlicher

@jars121 said in QDataStream with wchar_t array:

ds << val1 << val2 << val3 << val4 << val5 << val6;

This does not do what you think it does because QDataStream has no overloads for c-arrays. You have to iterate over the arrays by yourself.

SGaist

Hi,

In addition to what @Christian-Ehrlicher wrote, you should write the QDataStream operators for your structure. This will allow your code to be simpler.

jars121

Thank you both for your input, it's greatly appreciated.

I now understand the need for overloading the QDataStream operators. My next question is, what should the wchar_t be converted to to package it into the QDataStream? I.e. do I use a QString overload like this:

QDataStream &operator << (QDataStream &ds, const QString &val)
{
    ds << (const char *) val.toLocal8Bit();

    return ds;
}

Which is called like this:

ds << val1 << val2 << QString("%1").arg(QString::fromWCharArray(myStructInstance->val3), (sizeof(myStructInstance->val3) / sizeof(wchar_t)), QChar(' ')) << etc.

The above allows me to fix the width of the resultant char array, so the data can be successfully unpacked by the remote application. I don't see why this wouldn't work, but I'm not sure whether this is the best way to deal with the wchar_t arrays.

EDIT: I've just realised that the size of the string is automatically packaged with the string, so the above 'fixed-width string conversion' isn't necessary. I've just tested the below, and am able to receive the receive the resultant data into a QByteArray and then convert it back into a QString in the remote application:

ds << val1 << val2 << QString::fromWCharArray(myStructInstance->val3) << etc.

Christian Ehrlicher

This only works when your wchar_t doesn't contain '\0'.
Also it will ont work for val6