subclassing QIODevice for QAudioDecoder - c++

Question

Qt 5.7
According to the documentation, QAudioDecoder does not support streaming media. But is accepts either a filename or a QIODevice as source. Came the "bright idea": let's subclass QIODevice to create streaming media support.
But I always get an error:
Unable to start decoding process. Stream contains no data.
I have checked and double checked and debugged. Can't figure out what can be the problem.
I'm only testing it, so the code below is by no means final.
header:
class InputBuffer : public QIODevice
{
public:
InputBuffer(QString fileName);
~InputBuffer();
qint64 readData(char *data, qint64 maxlen);
qint64 writeData(const char *data, qint64 len);
bool isSequential();
qint64 bytesAvailable();
qint64 size();
private:
char* bufferRef;
qint64 length;
qint64 position;
};
implementation:
#include "inputbuffer.h"
InputBuffer::InputBuffer(QString fileName)
{
QFileInfo fileInfo(fileName);
length = fileInfo.size();
position = 0;
bufferRef = (char*)malloc(length);
QFile file(fileName);
file.open(QFile::ReadOnly);
file.read(bufferRef, length);
file.close();
emit readyRead();
}
InputBuffer::~InputBuffer()
{
free(bufferRef);
}
qint64 InputBuffer::readData(char *data, qint64 maxlen)
{
if (position >= length) {
return -1;
}
qint64 readSize = qMin(maxlen, length - position);
memcpy(data, bufferRef + position, readSize);
position += readSize;
return readSize;
}
qint64 InputBuffer::writeData(const char *data, qint64 len)
{
return -1;
}
bool InputBuffer::isSequential()
{
return true;
}
qint64 InputBuffer::bytesAvailable()
{
return length - position;
}
qint64 InputBuffer::size()
{
return length;
}
usage:
inputBufferRef = new InputBuffer("/home/pp/Zenék/Test.mp3");
inputBufferRef->open(QIODevice::ReadOnly);
audioDecoderRef = new QAudioDecoder();
audioDecoderRef->setAudioFormat(audioFormat);
audioDecoderRef->setSourceDevice(inputBufferRef);
connect(audioDecoderRef, SIGNAL(bufferReady()), this, SLOT(decoderBufferReady()));
connect(audioDecoderRef, SIGNAL(finished()), this, SLOT(decoderFinished()));
connect(audioDecoderRef, SIGNAL(error(QAudioDecoder::Error)), this, SLOT(decoderError(QAudioDecoder::Error)));
audioDecoderRef->start();

Answer 1

None of your isSequential(), bytesAvailable(), nor size() methods override the respective method in QIODevice, because the signatures don't match. They are all missing a const qualifier. Using the C++11 override keyword when overriding virtual methods helps.