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From: Daniel Anderson <woni...@gmail.com>
Newsgroups: comp.lang.c++.moderated,comp.lang.c++
Subject: Re: code pattern for locking & unlocking
Date: Sun, 28 Nov 2010 08:39:01 CST
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On Nov 26, 8:22 pm, "Hak...@gmail.com" <hak...@gmail.com> wrote:
> On Nov 25, 5:48 pm, Daniel Anderson <woni...@gmail.com> wrote:
> 
> 
> 
> 
> 
> 
> 
> 
> 
>> Hi!
> 
>> I often have to acquire locks and release them after I'm finished with
>> them.
>> 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
>> embarrassing.
>> 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?

Because of exception!

> 
>> 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.
Simple, yes. I've doing it for a long time
elegant, I use to think so, until someone remove some braces in my
code that where there for RAII, since I do not think it is elegant
> 
>> 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
> this:
>  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!

according to Plato (or was it Socrates) good is not subjective :)

> 
>> 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

I do not disagree, I'm just looking for advice

> 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.

this is a good guy this Antoine.
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