How to convert QVector to QBytearray

Question

I would like to convert QVector<double> to QBytearray, and I have no idea on how to do this. I tried this but the program crashes:

QVector<double> vec;
QByteArray arr = QByteArray::fromRawData(reinterpret_cast<const char*>(vec),vec.size());
for(int i = 0; i< vec.size(); i++)
    arr.append(reinterpret_cast<const char*>(vec.data(&numberOfData),sizeof(double));

Can someone tell me how to do it properly?

You'll first have to specify what you want the `QByteArray` to contain. Simply the raw binary data from the vector? — Angew is no longer proud of SO, Nov 10 '14 at 10:43
http://stackoverflow.com/questions/22479986/convert-from-qbytearray-to-array-of-double — Marco A., Nov 10 '14 at 10:48
@Angew What do you mean? Next i will use Qbytearray to display picture. — Look_Im_Your_Root, Nov 10 '14 at 11:05
`QVector` is an array of floating-point numbers. `QByteArray` is an array of bytes. You need to specify what you mean with "convert." Store the binary data of the floating-point numbers (as e.g. `memcpy` would do)? Or something else? — Angew is no longer proud of SO, Nov 10 '14 at 11:10
The loop can't make any sense if you don't even use the loop variable to access the element in the vector... — leemes, Nov 10 '14 at 12:20

score 12 · Answer 1 · answered Nov 10 '14 at 12:18

You can use QVector::constData to get a pointer to the (const) raw data contained in the vector. However, you also need to multiply the size by the size of a single entry, i.e. sizeof(double). You don't need a loop afterwards like in your code.

QByteArray data = QByteArray::fromRawData(
        reinterpret_cast<const char*>(vec.constData()),
        sizeof(double) * vec.size()
    );

You could also use QDataStream to do the conversion, resulting in a much cleaner code, which also takes care of potential byte ordering issues.

QByteArray data;
QDataStream stream(&data, QIODevice::WriteOnly);
for (auto x : vec)
    stream << x;

HappyCactus · Accepted Answer · 2014-11-10T12:21:19.077

1

You must pass through a native array, so your QByteArray will receive a sequence of contiguous bytes.

double *bytes = new double[vec.size()];
for (int i = 0; i < vec.size(); ++i) {
   bytes[i] = vec[i];
}
QByteArray array = QByteArray::fromRawData(reinterpret_cast<void*>(bytes));
delete []bytes;

Disclaimer: untested code.

--- Update ---

As leemes correctly pointed out, you DON'T need to allocate and copy a byte array, QVector already provides two access functions to raw data. So you can simply use data() and constData(). Please see his response.

edited Nov 10 '14 at 12:21

answered Nov 10 '14 at 12:07

HappyCactus

1,935
16
24

2

Why do you copy the vector to a raw array if `vec.data()` already gives you a pointer to exactly that? – leemes Nov 10 '14 at 12:11
1

Right. I forgot about data() and constData(), I almost never use it. – HappyCactus Nov 10 '14 at 12:19

score 0 · Answer 3 · answered Apr 28 '16 at 22:02

0

Today this conversion is even simpler:

QVector<double> vec;
// populate vector
QByteArray data;
QDataStream stream(&data, QIODevice::WriteOnly);
stream << vec;

answered Apr 28 '16 at 22:02

ajeh

2,652
2
34
65

Will "<<" flush all the elements in "vec" to the stream? – diverger Mar 17 '18 at 04:45
I do not use Qt for a couple years, so dunno, but a simple test should give you the answer. – ajeh Mar 22 '18 at 20:49
Someone has just downvoted a simplified syntax answer w/o providing any motivation for downvoting. Sounds like this is retaliatory downvoting or trolling and therefore has no place on SO. This resource is becoming a cesspool day by day. – ajeh Sep 04 '18 at 19:24

bobbaluba · Answer 4 · 2018-10-26T08:10:48.800

I was a bit annoyed by this too. Building on other answers here, I came up with this:

template<typename data_type>
QByteArray toByteArray(data_type data) {
    QByteArray bytes;
    QDataStream stream(&bytes, QIODevice::WriteOnly);
    stream << data;
    return bytes;
}

This should let you write the following code:

QVector<double> vec{1, 2, 3, 4};
auto bytes = toByteArray(vec);

And will also work for other types that support streaming to a QDataStream.

How to convert QVector to QBytearray

4 Answers4

Linked