QByteArray problem

[Zizione]

@aha_1980
There is my full code , this is just test unit. I need exactly int value in it , and how i understand '\xFF' will store char in it ?

#include <QCoreApplication>
#include <QByteArray>
#include <QString>
#include <QTextStream>
int main(int argc, char *argv[])
{
    QTextStream cin(stdin); cin.setCodec("CP866");
    QTextStream cout(stdout); cout.setCodec("CP866");
    QCoreApplication a(argc, argv);
    QByteArray ba;
    ba.resize(4);
    ba[0] = 0xFF;
    ba[1] = 0xFF;
    ba[2] = 0xF;
    ba[3] = 0xF;
    char tdi[16];
    int b=0;
    for(int i=0;i<4;i++)
    {
        uint16_t mask = 0x8;
        for (int j = 0; j < 4; j++)
        {
            tdi[b] = (ba[i] & mask) ? '1' : '0';
            mask >>= 1;
            b++;
        }

    }
    for(int i=0;i<16;i++)
    {
        cout<<tdi[i]<<endl;
    }
    cout<<tdi<<endl;
    cout<<ba[0]<<endl;
    //cout<<sizeof(tdi)<<endl;
    return a.exec();
}

[VRonin]

welcome to the forum and you are jumping right into a controversial point of the API (see https://forum.qt.io/topic/89766/converting-qbytearray-to-unsigned-char/9). Basically QByteArray interface uses char while you are trying to use unsigned char and hence the warning.

You can either:

• ignore the warning (QByteArray is actually storing the correct value it is just casting it to signed)
• use manual casting: *reinterpret_cast<unsigned char*>(ba.data()[0])=0xFF;
• use memcpy: const unsigned char tempVal = 0xFF; std::memcpy(ba.data()+0,&tempVal ,1);

[Zizione]

@VRonin So it is ok with storing 0xFF value , if i need this value in my code from QByteArray it would be given ?

[VRonin]

Yes, the if you use [] that casts it automatiocally to signed but the value is still 0xff

[JonB]

@Zizione
In addition to the solutions offered by @VRonin....

I am not a C++-er, but I believe in your case your can use the U suffix to make your constant values unsigned int and thus convertible to unsigned char:

ba[0] = 0xFFU;

?

[J.Hilk]

@JonB
Wow, that totally slipped my mind, 👍

[aha_1980]

@JonB said in QByteArray problem:

@Zizione
In addition to the solutions offered by @VRonin....

I am not a C++-er, but I believe in your case your can use the U suffix to make your constant values unsigned int and thus convertible to unsigned char:

ba[0] = 0xFFU;

?

Yeah, but ba is signed char...

[J.Hilk]

@aha_1980
true, but the warning one gets with ba[0] = 0xFF is not about usigned and signed char but rather between int and char

0xFFU == unsigned char(0xFF) == uchar(0xFF) != 0xFF

the implicit conversation to ba[0] does not "drop" information

[aha_1980]

@J.Hilk have you tried?

    QByteArray ba;
    ba.resize(4);
    ba[0] = char(0xFF); // no warning
    ba[1] = 0xFFU; // warning: implicit conversion changes signedness: 'unsigned int' to 'char'
    ba[2] = 0xFF; // warning: implicit conversion changes signedness: 'int' to 'char'

Edit: screenshot

[J.Hilk]

@aha_1980 said in QByteArray problem:

@J.Hilk have you tried?

I always do, well 95%, before I post ;-)

to be honest, I'm surprised about the uint part.

[aha_1980]

@J.Hilk

then you can be surprised about the uchar() part too.

But the biggest surprise: your code will not even compile in gcc and clang because of line 65.

And one more hint: your code will crash as you access invalid index in ba ;)

[J.Hilk]

@aha_1980 said in QByteArray problem:

@J.Hilk
But the biggest surprise: your code will not even compile in gcc and clang because of line 65.

I saw you using linux in the screen shot and testet that there than, and noticed it too

And one more hint: your code will crash as you access invalid index in ba ;)

nope doesn't crash, ignore the project name its my test anything project x).
!

resize is apparently not a necessity?

Edit:
also ignore value 0, I commeted that line out, to run it with gcc, so theres a random value.
// ba[0] = unsigned char(0xFF);

[aha_1980]

@J.Hilk said in QByteArray problem:

resize is apparently not a necessity?

Ah, my fault: "If an assignment is made beyond the end of the byte array, the array is extended with resize() before the assignment takes place."

A big difference to QVector or QList here.