exception - Singleton constructor fails c++ - what can be? -

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i facing huge problem: have singleton class. program multi-threaded mfc, singleton's constructor can called different threads. surrounded singleton instance-getter function mutex avoid deadlocks , multiple constructions. call of below meant function fails directly @ first time.

the function looks (log_msg macro logs given string log file):

myclass& myclass::singleton () {     log_msg("myclass::singleton -> jump in");      static cmutex oinitmutex;     try     {         csinglelock osinglelock((csyncobject *) &oinitmutex, true);         log_msg("!!! singleton lock okay !!!");         static myclass omyclassinstance;         log_msg("!!! singleton construction okay !!!");         return omyclassinstance;     }     catch(...)     {         cstring excmsg("unexpected exception creating myclass singleton instance!");         log_msg(excmsg);         throw excmsg;     } }

i've figured out, construction of singleton object not fail (since "!!! singleton construction okay !!!" message).

log output says:

09.04.2013 ;07:14:51:832;"myclass::singleton -> jump in" 09.04.2013 ;07:14:51:841;"!!! singleton lock okay !!!" ... (constructor logs => nothing unexpected in it!!! runs fine, must!!!) 09.04.2013 ;07:14:52:125;"!!! singleton construction okay !!!" 09.04.2013 ;07:14:52:170;"unexpected exception creating myclass singleton instance!"

what means? when return statement throw exception(s)??? please me resolve matter...

while doesn't answer particular question, it's still solution overall problem: don't need mutex at all. c++11 standard [stmt.dcl]§4 specifies (when talking static variables local functions):

if control enters declaration concurrently while variable being initialized, concurrent execution shall wait completion of initialization.88 if control re-enters declaration recursively while variable being initialized, behavior undefined.

where note 88 is:

note 88: implementation must not introduce deadlock around execution of initializer.

in other words, compiler introduces synchronisation you; no need manually.


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