Question

When do we use each of this function calls in a threaded application.
given two functions fun1() and fun2() defined in the same class dealing with read/write of data into buffers(queue operation). to achieve multi-threading to these. we would have to run the two functions in a separate thread. now lets say the first function read is called at the start of its thread.

is it better to use moveTothread ( second thread)for
function write at the start of the first
functions thread

Or

define the second function in a new
thread class and call that thread at
the start of the first thread.

Answer 1

Using moveToThread we can change the thread affinity of an object. What the OP asks is how we can run two functions of the same class in different threads.

Let class A and two functions f1 and f2

class A
{
public:
    void f1();
    void f2(int i); 
    void run(); // shows how we can trigger f1 and f2 in different threads
}

Qt already provided a class for running functions in different threads and it is called QtConcurrentRun

The QtConcurrent::run() function runs a function in a separate thread.
The return value of the function is made available through the QFuture
API.

The function that is triggered can be either an external function or a member function. So in our case if we wanted from the object itself to start f1 and f2 in different threads we could do the following in run()

void run()
{
   // QFuture<void> because f1 is void 
   QFuture<void> future1 = QtConcurrent::run(this, &A::f1);
   int k = 5; // Concurrent run with arguments
   QFuture<void> future2 = QtConcurrent::run(this, &A::f2, k);
} 

similarly you could execute any public function of any class concurrently, eg

QImage image = ...;
QFuture<void> future = QtConcurrent::run(image, &QImage::invertPixels, QImage::InvertRgba);

A a;
QFuture<void> future1 = QtConcurrent::run(A, &A::f1);

Notice the difference between the two calls:

QtConcurrent::run() also accepts pointers to member functions. The
first argument must be either a const reference or a pointer to an
instance of the class. Passing by const reference is useful when
calling const member functions; passing by pointer is useful for
calling non-const member functions that modify the instance.

In order to check when a concurrently executed function has finished you should use a QFutureWatcher.