c++qt

Qt hex representation of a negative 8bit int?

Similar question however, I'm looking for a qt solution.

I'm looking for a way convert a 8 bit negative int into a hex representation. The example will explain it better.

qint8 width = - 100;
qDebug() << QString::number(width, 16);

// "ffffffffffffff9c"

The output is much more than 2 bytes.

However, when i change it to unsigned, it works fine.

quint8 width = - 100;
qDebug() << QString::number(width, 16);

// "9c"

The documentations states:

typedef quint8

Typedef for unsigned char. This type is guaranteed to be 8-bit on all platforms supported by Qt.

typedef qint8

Typedef for signed char. This type is guaranteed to be 8-bit on all platforms supported by Qt.

Shouldn't unsigned not be able to deal with negative numbers?

Solution

  • The problem is that QString::number can accept int or uint types. It doesn't have versions for 8-bit integers, so they are implicitly casted to larger integers. It works fine with signed integer because leading zeros are removed.

    You can however use QDataStream, it provides operator << for large variety of types:

    QByteArray buffer;
QDataStream stream(&buffer, QIODevice::WriteOnly);
qint8 width = - 100;
stream << width;
qDebug() << buffer.toHex(); 
// output: "9c"