}; Notice that we duplicate the HAND L E returned by the I n itOn c e E x e c ute in VS .NET

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}; Notice that we duplicate the HAND L E returned by the I n itOn c e E x e c ute
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be given out and freely closed without de-allocating the shared instance Notice that we don' t have a destructor and, thus, never get around to free ing the event The reason is subtle: if we were to get the HAN D L E value by call ing I n i tOn c e E xec uteOn c e inside a destructor, we'd be forcing allocation of an event just so that we could close it This is wasteful In addition to allow ing multiple initializations to race to publish a value (such as the lockless hand coded version earlier), the alternative I n i tOnc eBegi n I n i t i a l i z e func tion allows you to check the status of the initialization We'll soon see how to use this to free the HAND L E without forcing allocation In the other model, with the I n itOn c e B eg i n I n i t i a l i z e and I n itOn c e Com plete functions, multiple initialization callbacks may execute but only one will "win" and have its value published to the I N I T_ONC E data structure
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BOO l WINAPI I n itOn ceBegi n I n it i a l i z e ( l P I N I T_ONCE l p I n itOn c e , DWORD dwF lags , PBOOl fPend i n g , lPVOI D * lpContext ); BOOl WINAPI I n itOn ceComplet e (
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L P I N I T_ONCE l p l nitOn c e , DWORD dwF lag s , L PVOI D lpContext
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This model can be used for both "asynchronous" initialization-that is, where many threads attempt to initialize the value at once-in addition to the ordinary "synchronous" initialization mentioned above, where Win32 ensures the callback executes only once To specify asynchronous, you pass I N I T_ONC E_ASYNC to the function If this is not specified, other threads will be blocked on calling this until the first thread finishes initialization You may also pass I N I T_ONC E_C H E C K_ON LY as a flag that indicates that the lazily initialized value should be retrieved without actually forcing initialization If I n i tOn c e Beg i n I n i t i a l i z e returns T R U E , the f Pe n d i n g output parameter tells you what to do If I N I T_ONC E_C H E C K_ON LY was specified, the value tells you whether lazy initialization has occurred already, and the value will have been stored into I pContext Otherwise, if fPe n d i n g is TRUE, it means the calling thread must perform the initialization, and if it's F A L S E , the value is already initialized and will have been placed into I pContext If a thread is responsible for initializing the value, it then goes ahead after the call returns Notice there is no callback involved Once complete, it calls I n itOnc eCom p l e t e to supply the initialized value in the I pCont ext argu ment If I N I T_ONC E_ASYNC was passed to the begin initialization function, it must also be passed here in dwF l a g s It is also imperative that failed initial ization attempts signal the I N I T_ONC E data structure through I n itOn c eCom p l e t e by passing I N I T_ONC E_I N I TJAI L E D, otherwise with synchronous initialization threads could become deadlocked If the I n i tOn c eComplete function returns FALSE, it means that another thread raced and beat the call ing thread (with asynchronous initialization) and that the caller must retrieve the value now available by calling I n i tOn c e Begi n I n i t i a l i z e with the I N I T_ONC E_C H E C K_ON LY flag Here is a version of the L a zyI n it E v e n t class above that uses asynchro nous initialization
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