c++11 thread/mutex implementation in VS2012 - assertion fired

Question

I want to move my code from boost- to std-threads. While I thought it should be quite straight forward I'm running into weird problems. The code below is a minimal example which fires an assertion "f:\dd\vctools\crt_bld\self_x86\crt\src\thr\mutex.c(131):unlock of unowned mutex" with VS2012. Searching for this brings up older bug reports which (i think) should be already fixed.

int result = 0;
std::mutex m;
m.lock();
std::thread t1([&](){
    result = 42;
    m.unlock();
});
m.lock();
std::cout << result << std::endl;
t1.join();

Can someone explain to me why this doesn't work?

Thanks

Answer 1

m.unlock() requires that the calling thread owns the mutex. Your code does not meet that requirement (as the unlock()ing thread never calls m.lock()) and so the program's behavior is undefined.