Cancelling a thread running a long operation

Question

I'm trying to work out a design predicament I have.

ClassWithLongOperation
{
    Run()
    {
        RecrusiveOperation();
    }

    RecrusiveOperation()
    {
        /* RECURSION */
    }
}

MyThread
{
    ClassWithLongOperation Op1(10);
    Op1.Run();  // Takes several minutes.

    ClassWithLongOperation Op2(20);
    Op2.Run();

    SomeOtherClassWithLongOperation Op3;
    Op3.Run();

    // Do some other stuff
}

The GUI starts MyThread, which runs for a good 5-6 minutes. I want to be able to have a big fat Cancel button on my GUI, so the user can cancel the operation.

I could create a global boolean variable bCancelled, and check if its been set in RecursiveOperation, but I want to be a good C++ & OO programmer and avoid global variables. Especially if they would have to spread across multiple files.

So how would I (following good design) safely cancel MyThread? What could I change in my setup to allow this?

I'm also using _beginthreadex to start the thread, but I could use boost if it would allow for an easier solution.

Answer 1

Your flag not need to be global to your entire program, but it needs to be visible to your class code. Create the flag to be a private instance member and a public function to change it to false/true. In your recursive function, test its value to verify if the task should continue. When you want, set its value to false (through the function of course) to stop the recursive calls, i.e., when the user clicks the button you call the function in the desired instance. This way you will not break any OO principle, since you have a private flag and a public member function to safely change it.