How to assert if a std::mutex is locked?

Question

With GCC 4.8.2 (on Linux/Debian/Sid 64 bits) -or GCC 4.9 when available - in C++11- I have some mutex

std::mutex gmtx;

^{actually, it is}

Answer 1

Well, if the expense of the assertion really isn't an issue then you can just call try_lock() from another thread where its behavior is guaranteed to be well defined:

void beta(void) {
  assert(std::async(std::launch::async, [] { return gmtx.try_lock(); })
                 .get() == false &&
         "error, beta called without locking gmtx");
  // some real work
}