Is there a shorter way to initialize a QByteArray?

Question

In my program I work a lot with serial communication so QByteArray is used very often.

I was wondering if there was a shorter way to initialize a QByteArray with specific bytes than:

const char test_data[] = {
    static_cast<char>(0xB1), static_cast<char>(0xB2),
    0x5, static_cast<char>(0xFF),
    static_cast<char>(0xEE), static_cast<char>(0xEE),
    static_cast<char>(0x0)}; // Note QByteArray should be able to hold 0 byte
const QCanBusFrame frame = QCanBusFrame(0xA1, QByteArray(test_data));

The static_cast<char> is necessary because otherwise C++11 gives an error about narrowing, because the range 0x7F to 0xFF is bigger than a char could fit--but a char is what the QByteArray constructor asks for.

This is the QByteArray constructor being used:

QByteArray::QByteArray(const char *data, int size = -1)

Astir

What happens when you declare the test_data as unsigned char array (unsigned char*) and then cast it to char array (char*)? – Katherinakatherine

@Katherinakatherine Are you sure the asterisk needs to be used? The line that inits test_data throws "invalid conversion from 'int' to 'const unsigned char*" – Astir

You can easily create a function which create a QByteArray with what you want as argument. – Bazil

can any of your byte values be 0x00 - if not you could use the const char* constructor with a string literal – Fairlead

Sorry about confusion, an array is actually a pointer that's what I meant. In your case it should be: const unsigned char test_data[] = {...} Then you should probably use: QByteArray((char*)test_data) – Katherinakatherine

@Bazil haha.... an obvious solution but I didn't think of it. That's what I'll do, thanks! – Astir

@SpaghettiCat You're welcome ;) – Bazil

@Fairlead but I wouldn't rely on the possibility that one of the bytes can/can't be 0x00 – Katherinakatherine

@Fairlead You're right, it's possible to pass "\x01\xB2\xB4" to the constructor, but it is much better to work with integers (you can use constant names, etc.). I should be able to have a "0" byte though... – Astir

Answer 1

Being inspired by the answers above this is what I finally came up with:

const quint8 testData[] {0xB1, 0x00, 0xB2, 0x00};
const QCanBusFrame cFrame = QCanBusFrame(
    0xA1, QByteArray(reinterpret_cast<const char*>(testData), sizeof(testData)));

I much prefer to have the bytes as byte numbers rather than literal characters when working with serial communication.

After having a discussion on ##c++ I was advised that reinterpret_cast is appropriately used in this situation.

Astir

Answer 2

Simple and effective:

QByteArray b = QByteArrayLiteral("\x12\x00\xa4\x42\x51\x00\x00\x99");

Vanvanadate

Hmm I don't see QByteArrayLiteral documented anywhere, but by God it works better than the exact same argument passed to QByteArray, because the "\x00" are not treated as a string terminator. The same can be achieved with QByteArray but you have to pass the correct size argument. Not a bad solution, thanks :) – Astir

@Astir It's just a macro: doc.qt.io/qt-5/qbytearray.html#QByteArrayLiteral – Dichroite

I wonder who documented that... – Vanvanadate

Only present starting from Qt5 – Synesthesia

Any other major version of Qt has reached EOL. – Vanvanadate

Nice! , It's worth mentioning that the QByteArrayLiteral will allow the x00 byte to be written, instead of meaning "End" – Commissure

Answer 3

As an alternative to QByteArrayLiteral, you can roll your own, if you wish:

#include <QByteArray>

template <int N> QByteArray arrayFromLiteral(const char (&data)[N]) {
   return QByteArray::fromRawData(data, N-1);
}

int main() {
   const auto arr = arrayFromLiteral("\xB1\xB2\0\1");
   Q_ASSERT(arr.size() == 4);
   Q_ASSERT(arr[0] == (char)0xB1);
   Q_ASSERT(arr[1] == (char)0xB2);
   Q_ASSERT(arr[2] == (char)0x00);
   Q_ASSERT(arr[3] == (char)0x01);
}

Anthotaxy

Answer 4

May be works slowly:

QByteArray ba = QByteArray::fromHex(QVariant("B1B2FFEEEE00").toByteArray());

Oliveira

Answer 5

like this:

const unsigned char str[] = {0xff, 0xed, 0xba, 0xd1};
QByteArray ba(reinterpret_cast<const char*>(&str[0]),std::extent<decltype(str)>::value);

now QByteArray constructor looks weird, but byte sequences are clear. You can also add terminating 0-byte to array instead of using std::extent, but in general you can have zero-bytes in the middle of sequence.

Addicted

Just because of curiosity, could you explain why the reinterpret_cast and std::extent necessary? – Katherinakatherine

I like the reinterpret_cast (C++ style, not C). Instead of extent, just use sizeof(str)... – Abadan

Can't you just cast str instead of &str[0] (looks ugly for me)? At least GCC accepts... – Abadan

Answer 6

Have you tried the following:

const unsigned char test_data[] = {
    static_cast<char>(0xB1), static_cast<char>(0xB2),
    0x5, static_cast<char>(0xFF),
    static_cast<char>(0xEE), static_cast<char>(0xEE),
    static_cast<char>(0xB3)};
const QCanBusFrame frame = QCanBusFrame(0xA1, QByteArray((char*)test_data));

You are using the constructor: QByteArray::QByteArray(const char *data, int size = -1).

If size is negative, data is assumed to point to a nul-terminated string and its length is determined dynamically. The terminating nul-character is not considered part of the byte array.

Katherinakatherine

I just tried it and it compiles successfully. It works without all the static_cast (the whole benefit of your solution is that those casts aren't needed). I'm not sure what I should be careful about with 0 bytes: I need to be able to use 0 bytes as well. – Astir

You absolutely need to call QByteArray((char*) test_data, sizeof(test_data));! The single parameter ctor (or with size == -1) will try to copy the data as normal c-string, which would have to be null-terminated! – Abadan

Probably if you have a 0x00 in the middle, it will detect it as the end of your char array. That's why I would pass the array size as argument – Katherinakatherine

This is my favourite solution, but the C-style casting is not good. I'm trying now to figure out a way to use a static_cast somehow. – Astir