Multithreaded code in C++ tends to become brittle over time. If you write your code well you’ll need almost no synchronization between different threads, but the price of that is that your code will be littered with undocumented conventions of when you can read or modify which state. In your average threaded C++ application there are countless potential race conditions, all of which never happen because people follow conventions about when to do what. Until someone doesn’t know about a convention that he has to follow or until you change the conventions and you forget to update one piece of code that you didn’t know about.
Enter the asserting mutex. The asserting mutex is a conditional breakpoint that triggers only if a potential race condition actually happens. I call it asserting mutex because you use it like a mutex to protect a critical section. It works very simply: If one thread locks the asserting mutex and a second thread attempts to lock the asserting mutex before the first thread unlocks it, you get an assert. And it guarantees that both threads will still be inside the critical section when you get the assert. The cost is one atomic increment and one atomic decrement, which is not free but cheap enough that you can place lots of asserting mutexes in your code before they cause problems. So you could use this to document many of your threading conventions. Used correctly this is a breakpoint that makes it very easy to find data races.
Here is the complete code: