Question

I am doing a project with Qt where in invoke method runs on a seperate thread and called from main thread.
If I pass a QByteArray as const it builds and runs.
However if I remove the const it builds but terminates when I try to run it,
and throws an assert Q_ASSERT(b).

Why is it getting terminated?

I have to append some data into QByteArray.

const bool b = QMetaObject::invokeMethod(m_thread, "calculateSpectrum",
                              Qt::AutoConnection,
                              Q_ARG(QByteArray, buffer),
                              Q_ARG(int, format.frequency()),
                              Q_ARG(int, bytesPerSample),
                              Q_ARG(qint64, dataLength));
    Q_ASSERT(b);
    Q_UNUSED(b) // suppress warnings in release builds

Answer 1

basically the signature of QMetaobject::invokemethod is…

bool    invokeMethod ( QObject * obj, const char * member, Qt::ConnectionType type, QGenericReturnArgument ret, QGenericArgument val0 = QGenericArgument( 0 ), QGenericArgument val1 = QGenericArgument(), QGenericArgument val2 = QGenericArgument(), QGenericArgument val3 = QGenericArgument(), QGenericArgument val4 = QGenericArgument(), QGenericArgument val5 = QGenericArgument(), QGenericArgument val6 = QGenericArgument(), QGenericArgument val7 = QGenericArgument(), QGenericArgument val8 = QGenericArgument(), QGenericArgument val9 = QGenericArgument() );

now in this we are using the Q_ARG macro whose syntax is..

QGenericArgument    Q_ARG ( Type, const Type & value )

which requires a const parameter.

Now when u remove this const parameter the condition becomes false and it will throw an assert at runtime.