scope(exit)

[Sean Kelly]

MM wrote:
> I've been reading through Exception Safe Programming page and I had a question:
> 
> why not:
> scope(exit) unlock(m);	
> lock(m);	
> 
> iso the inverse?
> lock() might crash but might still have locked m 
> Or is this stpd reasoning?
> unlocking m when it is not locked should not ba a problem or the scope(exit)
> could check whether or not m is locked?

Personally, I find the basic guarantee to be of very limited use.  For 
most operations, the weak guarantee seems far more appropriate, and I 
believe it is reasonable to expect that of the lock function.  In other 
words, if the lock function fails then I would expect m to be in the 
state it was in prior to the lock call.  Further, it's possible that 
unlock might consider being asked to unlock a mutex that the calling 
thread doesn't own to be an error condition, which may result in an 
exception being generated.  Therefore, if lock fails and throws and 
exception and scope(exit) unlock(m) therefore fails and throws an 
exception as well, the application will terminate because it has two 
exceptions in flight simultaneously.  Therefore:

     lock(m);
     scope(exit) unlock(m);

is preferred because unlock will only be called if a lock was 
successfully obtained, and will be called regardless of whether the 
following code generates an error or completes successfully.


Sean