QByteArray数据移位注意事项

我们的目的是要将一个QByteArray数组中的四个字节合并成一个32位的无符号整数（quint32）或有符号整数（qint32）。假设将arr中的四个字节分别设置为0xF1, 0xF2, 0xF3,和 0xF4，我们要拼出一个无符号数0xF1F2F3F4。

代码1

void MainWindow::test()
{
    QByteArray arr;
    arr.resize(4);
    arr[0]=0xF1;
    arr[1]=0xF2;
    arr[2]=0xF3;
    arr[3]=0xF4;

    qint8 A1 = arr.at(0);
    qint8 A2 = arr.at(1);
    qint8 A3 = arr.at(2);
    qint8 A4 = arr.at(3);

    quint32 IA1 = quint32(A1<<24);
    quint32 IA2 = quint32(A2<<16);
    quint32 IA3 = quint32(A3<<8);
    quint32 IA4 = quint32(A4);

    quint32 value = (IA1 | IA2 | IA3 | IA4);

    qDebug()<<value; 
}

上面的代码是错误示范，得到的结果是0xFFFFFFF4。调试结果如下：

代码2

void MainWindow::test1()
{
    QByteArray arr;
    arr.resize(4);
    arr[0]=0xF1;
    arr[1]=0xF2;
    arr[2]=0xF3;
    arr[3]=0xF4;

    quint8 A1 = static_cast<quint8>(arr.at(0));
    quint8 A2 = static_cast<quint8>(arr.at(1));
    quint8 A3 = static_cast<quint8>(arr.at(2));
    quint8 A4 = static_cast<quint8>(arr.at(3));

    quint32 IA1 = quint32(A1<<24);
    quint32 IA2 = quint32(A2<<16);
    quint32 IA3 = quint32(A3<<8);
    quint32 IA4 = quint32(A4);

    quint32 value = (IA1 | IA2 | IA3 | IA4);

    qDebug()<<value;
}

上面的代码得到的结果是0xF1F2F3F4。调试结果如下：

总结

在字节移位操作时，要将其强制转换位unsigned char类型，即按无符号数据进行移位操作。

扩展

如果我们最后需要的是一个有符号数，则完成移位操作和拼接过程后再强制转为有符号类型就可以啦。

代码3

void MainWindow::test2()
{
    QByteArray arr;
    arr.resize(4);
    arr[0]=0xF1;
    arr[1]=0xF2;
    arr[2]=0xF3;
    arr[3]=0xF4;

    quint8 A1 = static_cast<quint8>(arr.at(0));
    quint8 A2 = static_cast<quint8>(arr.at(1));
    quint8 A3 = static_cast<quint8>(arr.at(2));
    quint8 A4 = static_cast<quint8>(arr.at(3));

    quint32 IA1 = quint32(A1<<24);
    quint32 IA2 = quint32(A2<<16);
    quint32 IA3 = quint32(A3<<8);
    quint32 IA4 = quint32(A4);

    quint32 value = (IA1 | IA2 | IA3 | IA4);

    qint32 value2 = static_cast<qint32>(value);//int

    qDebug()<<value;

    qDebug()<<value2;
}

调试结果如下：