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c l a s s Boxed i n t e r n a l T m_va l u e j i n t e r n a l Boxed ( T v a l u e ) { m_v a l u e } private volat i l e Boxed m_v a l u e j private F u n c < T > m_factorY j p u b l i c L a z y l n it ( F u n c < T > factory ) { m_factory p u b l i c T Va l u e { get { if ( m_va l u e nUll) I nt e rlocked Compa r e E x c h a nge ( ref m_va l u e , new Boxed ( m_fa ctory ( , n U l l ) j ret u r n m_va l u e j } }
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Because VC++ doesn't strengthen the model of the underlying machine, it can be problematic to write portable lazy initialization in native code Tech nically speaking, you can do it, as we'll see But we will conclude this sec tion by looking at new Windows Vista APIs that allow you to write portable lazy initialization code without needing to worry about the memory model The code is more verbose, albeit the various portability concerns are han dled by the OS for you: which you prefer is purely a tradeoff in complex ity versus flexibility
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Double-Checked Locking: The Basic Pattern Many of the above ideas apply equally to native code You have to be very careful, however, in your placement of v o l at i l e keywords and memory fences to prevent the plethora of reordering problems on all platforms Because VC++ volat i l e s don't imply fences in the emitted assembly code a t the processor level, you need to add some fences in precarious places
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template< typename T > c l a s s L a z y l n it { volat i l e T * m pVa l u e j C R I T ICAL_S ECTION m_c rst j T ( m_p F a ctory * ) ( ) j public : L a z y I n i t ( T ( p F a ctory * ) ( { m_pVa l u e NU L L j m_p F a ctory p F a c t or Yj I n i t i a l izeCrit i c a lSection ( &m_c r st ) j
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} - L a z y I n it ( ) { // Pos s i b ly delete/ c le a n u p m_pVa l u e De leteC rit i c a lSection ( &m_c r st ) j
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T getVa l u e ( ) { if ( ! m_pVa l u e ) { EnterCrit i c a lSection ( &m_c r st ) j if ( ! m_pVa l u e ) T pVa l u e
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} LeaveC r i t i c a lSection ( &m_c r st ) j } _Rea d B a r r i e r ( ) j ret u r n m_v a l u e j } }j
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This looks a lot like the C# version earlier, except for two interesting fences A _W r i t e B a r r i e r is found after instantiating the object, but before writing a pointer to it in the m_pVa l u e field That's required to ensure that writes in the initialization of the object never get delayed past the write to m_pVa l u e itself As noted earlier, the NET memory model disallows such movement; but VC++ does not, unless explicit fences are used Similarly, we need a _Re a d B a r r i e r just before returning m_v a l u e so that loads after the call to getVa l u e are not reordered to occur before the call This is surprisingly needed for processors like IA64 that do pointer and value speculation It's unfortunate that we need this last barrier because the only danger ous period of time is immediately after construction Because there's no fixed length on this window of time, it is generally not possible to remove the barrier However, I will also point out that neither fence is required on X86, Intel64, and AMD64 processors It's unfortunate that weak processors like IA64 have muddied the waters, but if you are willing to write entirely processor specific code, you can consider emitting the fences or writing #i fdef IA64 around them
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Windows Vista One-Time Initialization The one-time initialization fea ture that was introduced in Windows Vista is a bit like the L a z y I n it < T > shown earlier in that you must create a n instance o f a n I N I T_ON C E and ini tialize it before it can be used Initialization only prepares the data structure for subsequent use and doesn't associate a callback as the L a z y I n i t < T > data structure above did
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