Source for this can be obtained from Github. Get a new copy with:
git clone https://github.com/EPCCed/2019-04-16-ModernCpp.git.git
or update your existing one with
git pull
then you can
cd 2019-04-16-ModernCpp/exercises/threads
area.cpp contains a C++ version of the Mandelbrot example which you used in Threaded Programming.
Parallelise the outer loop of the main computation using C++ threads. Try using either a function pointer or a lambda expression.
Try different mechanisms for synchronising the update to the shared counter: a mutex, and lock guard or an atomic int.