std::lock_guard() for a locked std::mutex

Question

I am new to C++11 threading.

The following piece of code should be executed only by the first thread.

The other threads (which might race with the first thread) should not enter the locked code area (that's why the std::try_lock() is there).

std::mutex mutex;

// now ensure this will get called only once per event
if (std::try_lock(_mutex) != -1)
{ 
    return;
}

{
    std::lock_guard<std::mutex> guard(_mutex);
    // critical section

} // mutex will be unlocked here        

(Outside from writing my own lock_guard) Is there a way to use a similar & standard std::lock_guard variant, but which will take my !locked! mutex (effect of std::try_lock() above) and simply unlock it when the d-tor of that guard will be called?

Answer 1

Use this:

// now ensure this will get called only once per event
if (_mutex.try_lock())
{ 
    std::lock_guard<std::mutex> guard(_mutex, std::adopt_lock);

    // critical section
} // mutex will be unlocked here

Update And don't use variable names starting with underscore (as _mutex ).

Answer 2

Have a look at this

and this

From this info you can see that if you specify a second parameter like this std::lock_guard<std::mutex> guard(_mutex, std::try_to_lock) the behaviour is changed to act like std::try_lock rather than std::lock