Simple lock

Alf P. Steinbach alfps at start.no
Sat Mar 20 05:15:08 CET 2010


* MRAB:
> Gabriel Genellina wrote:
>> En Fri, 19 Mar 2010 23:31:23 -0300, MRAB <python at mrabarnett.plus.com> 
>> escribió:
>>> moerchendiser2k3 wrote:
>>
>>>>  class SetPointer
>>>> {
>>>> private:
>>>>     void *ptr;
>>>>      MY_LOCK lock;
>>>>   public:
>>>>     void SetPointer(void *p)
>>>>     {
>>>>         Lock(this->lock);
>>>>         this->ptr = p;
>>>>     }
>>
>>> 3. You're locking, but never unlocking. The sequence should be: lock, do
>>> stuff, unlock.
>>
>> Just FYI: C++ doesn't have try/finally, and such behavior is usually 
>> emulated using a local object. When it goes out of scope, it is 
>> automatically destroyed, meaning that the object destructor is called. 
>> Whatever you would write in a "finally" clause, in C++ goes into a 
>> destructor.
>>
>> Of course C++ guys would never say they're "emulating" try/finally, 
>> instead they declare RAII as *the* Only and Right Way :)
>>
> Lock() doesn't look like a declaration, but a function call (although
> it's been a while since I last did C++!).

Right. But the OP is clearly a beginner who has yet to learn conventions (like 
all uppercase reserved for macros) and good versus bad programming constructs 
(like using void*), and who is even confusing C++ with Python. So it might just 
be that 'Lock' is a macro, and then it can expand to anything.


> In the past I've written some C++ code where try..finally... would've
> been useful... *sigh*

Check out Marginean and Alexandrescu's ScopeGuard. Old DDJ-article. For MSVC you 
may want to change use of __LINE__ in that code, to MS-specific __COUNTER__.

Fwiw. I can't remember ever needing 'finally' in C++ code.

It indicates a lack of encapsulation of entitites that should clean up 
themselves. Simple smart pointers etc. can help. E.g., check out Boost library.


Cheers & hth., even if a little off-topic,

- Alf



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