QByteArray 的大小很难计算？

Question

最近我编写了用 Qt 进行文件传输的程序。认为它现在有效，我仍然对发生的事情感到好奇。请帮我找出原因。非常感谢。

为什么head的大小大于sizeof(qin32)、sizeof(qint32)和文件名长度之和？（我猜是函数-setVersion()的原因）

QFileInfo info(file_to_send.fileName());
QByteArray head;
QDataStream out(&head, QIODevice::WriteOnly);
out.setVersion(QDataStream::Qt_5_0);
out << qint32(file_to_send.size() + info.fileName().length() + sizeof(qint32)*2)
    << qint32(info.fileName().length())
    << info.fileName();
tcpClient.write(head);

Answer 1

info.filename()writes out its own length

如果你不想这样做，那么你可以这样做

QFileInfo info(file_to_send.fileName());
QByteArray head;
QDataStream out(&head, QIODevice::WriteOnly);
out.setVersion(QDataStream::Qt_5_0);
QByteArray filename = info.fileName().toLatin1();
out << qint32(file_to_send.size() + filename .length() + sizeof(qint32)*2);
    << qint32(filename .length())
out.writeRawData(fileName.constData(), filename.length());
tcpClient.write(head);

使用writeRawData 绕过任何内置编码

Answer 2

你把事情弄得很复杂。模式是这样的：

QFileInfo info(file_to_send.fileName());
QByteArray head;
QDataStream out(&head, QIODevice::WriteOnly);
out.setVersion(QDataStream::Qt_5_0);
out << qint32(0)
    << info.fileName(); // << YouCanAddMoreStuffHere;

out.device()->seek(0);
out << qint32(out.length());

tcpClient.write(head);

并阅读代码：

void SomeClass::slotReadClient() { // slot connected to readyRead signal of QTcpSocket
    QTcpSocket *tcpSocket = (QTcpSocket*)sender();
    QDataStream clientReadStream(tcpSocket);

    while(true) {
        if (!next_block_size) {
            if (tcpSocket->bytesAvailable() < sizeof(qint32)) { // are size data available
                break;
            }
            clientReadStream >> next_block_size;
        }

        if (tcpSocket->bytesAvailable() < next_block_size) {
            break;
        }
        QString fileName;
        clientReadStream >> fileName; // >> YouCanAddMoreStuffHere; // same as above

        next_block_size = 0;
    }
}