Grand central dispatch examples courtesy of @HalfdanJ

gcd.cpp

// group of for loops with waiting
int m, n;
dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH,0), ^{
    // before all parallel m
    dispatch_apply(m, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^(size_t i) {
        // parallel m code with i for index
    });
    // after all parallel m
});
dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH,0), ^{
    // before all parallel n
    dispatch_apply(n, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^(size_t i) {
        // parallel n code with i for index
    });
    // after all parallel n
});
dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

// big calculation async
int k;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH,0), ^{
    // initial async calculations
    dispatch_async(dispatch_get_main_queue(), ^{
        // jump to main thread
    });
    // jump back to asynchronous thread
});

// single for loop
int q = 100;
dispatch_apply(q, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^(size_t i) {
    // parallel q loop with i for index
    // no guarantees about what this prints
    cout << i;
});