Re: code pattern for locking & unlocking

"" <>
Fri, 26 Nov 2010 18:22:25 CST
On Nov 25, 5:48 pm, Daniel Anderson <> wrote:


I often have to acquire locks and release them after I'm finished with
I use constructor/destructor to acquire & release locks. everything
works fine.
sometime I must acquire a lock for few instructions, then release it
in the middle of a big function. I use braces to get timely
destructors, but I find the "middle of nowhere" braces kind of
something like:

struct Lock
Lock(Mutex& mtx) mtx_(mtx) { mtx.lock(); }
~Lock() { mtx_.unlock(); }
operator bool() { return true;}


Since your Lock type depends on Mutex, why not just use the straight
Mutex and lock and unlock as you please?

void someFunc()
  // do some stuff
  // now time to update share data
     Lock myLock(data_mutex);
     //use locked data
  } // destructor called
  // do more stuff


Some don't even know or haven long forgotten that you can create
blocks arbitrarily like this. I agree that it doesn't look nice, but
it has a lot of uses and i think there's nothing wrong with using it.
It's an incredibly simple and elegant way of solving this problem.

I would like to have something like the using keyword in c#.
Is there a way to fake it in C++ ?

This is a C++ forum. C# is relevant to this question, but for those of
us who haven't worked with it, please post a short example.

for now I'm using an if to do it.

void someFunc()
  // do some stuff
  // now time to update share data
  if (Lock myLock = Lock(data_mutex))
     //use locked data
  } // Unlock done by destructor

  // do more stuff


You seem to be implying that this if statement will always evaluate to
true (since i assume this code is equivalent to your first example),
but when i see an "if" in any code, i always assume that it will
sometimes not evaluate to true. On the other hand, it reminds me of
 std::weak_ptr<int> wp;
 void f() {
   if( std::shared_ptr<int> sp = wp.lock() )
     ; // do something

Is it good programming ?

What a subjective question!

Is there a better way ?

I think creating an arbitrary scope is one of the simplest and least
error prone ways to do this. I'm sure at least one person (if not you)
will disagree with me, but i like simple. I suppose you could create a
class that inputs a function, locks, calls the function, then unlocks,
and you could use C++0x lambdas to integrate it into your current code
without too much refactoring, but that means you'll have to maintain
the locking class and every function/class/etc that uses it. I guess
i'm not the most articulate so i'll use someone else's words:
"Perfection is achieved not when you have nothing more to add, but
when you have nothing left to take away." ~ Antoine de Saint-Exupery.

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