Question

I’m trying to test a class that uses a QIODevice. In actuality, the object will probably use a QFile, but for my unit test I’d prefer to use a QBuffer for speed. Dependency injection and polymorphism combine to get me what I want.

I have a problem, however. My class constructor looks like this:

Object::Object(QIODevice& source)
{
   if(!source.open(QIODevice::ReadOnly))
   {
      qDebug("Object: Could not open source.");
   }
}

Then in my test I check for the message:

void TestObject::printsErrorOnOpenFailure()
{
   QTest::ignoreMessage(QtDebugMsg, "Object: Could not open source.");
   QBuffer buffer;
   Object obj(buffer);
}

Unfortunately, open still seems to succeed even without a QByteArray to operate on. What’s the best way to give my object a QIODevice that I know it can’t open?

Answer 1

You can’t make QBuffer::open() to return false (*). So you can’t use QBuffer in your scenario.

But what about sub-classing and just overwriting open() to always return false?

class UnopenableDevice : public QBuffer {
public:
    bool open(QIODevice::OpenMode m) { return false; }
};

---

(*) At least not using the flags WriteOnly and/or ReadOnly. Passing invalid flags is the only possibility to make it return false. Quoting Qt 4.8.0 sources:

corelib/io/qbuffer.cpp:

332 bool QBuffer::open(OpenMode flags)              
333 {
334     Q_D(QBuffer);
335 
336     if ((flags & (Append | Truncate)) != 0)
337         flags |= WriteOnly;
338     if ((flags & (ReadOnly | WriteOnly)) == 0) {
339         qWarning("QBuffer::open: Buffer access not specified");
340         return false;    //  <----- only possibility to return false!
341     }
342 
343     if ((flags & Truncate) == Truncate)
344         d->buf->resize(0);
345     d->ioIndex = (flags & Append) == Append ? d->buf->size() : 0;
346 
347     return QIODevice::open(flags);
348 }

corelib/io/qiodevice.cpp:

540 bool QIODevice::open(OpenMode mode)
541 {
542     Q_D(QIODevice);
543     d->openMode = mode;
544     d->pos = (mode & Append) ? size() : qint64(0);
545     d->buffer.clear();
546     d->accessMode = QIODevicePrivate::Unset;
547     d->firstRead = true;
548 #if defined QIODEVICE_DEBUG
549     printf("%p QIODevice::open(0x%x)\n", this, quint32(mode));
550 #endif
551     return true;
552 }