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Contrary to popular belief, this does work in NET 20+ (The popular misconceptions are largely due to other popular languages-namely, VC++-not guaranteeing that the pattern will work across platforms) For it to be absolutely correct, you must mark the m_v a l u e field volat i l e The reason this needs to be vo l a t i l e is similar to the reason that double checked locking doesn't work on some non-NET platforms The m_factory delegate probably refers to a method that creates, initial izes, and returns a new object, that is, as with the above example where it is n ew S i n g l eton ( ) Fields of the newly constructed object will be initialized in the process And this is the reason this pattern doesn't work on many memory models: on platforms where stores may be reordered, the write of the newly allocated object's reference to m_v a l u e could happen before the writes to the its fields A caller seeing that m_v a l u e is nonn u l l (and hence initialized) may proceed to using the object, and yet its fields will contain garbage, uninitial ized data TheNET 20 memory model disallows store reordering But a similar issue lurks with loads of the fields Because all of the proces sors mentioned above, in addition to the NET memory model, allow load to-load reordering in some circumstances, the load of m_v a l u e could move after the load of the object's fields The effect would be similar and marking m_v a l u e as volat i l e prevents it Marking the object's fields as volatile is not necessary because the read of the value is an acquire fence and prevents the subsequent loads from moving before, no matter whether they are vol a t i l e or not This might seem ridiculous to some: how could a field be read before a reference to the object itself This appears to violate data dependence, but it doesn't: some newer processors (like IA64) employ value speculation and will execute loads ahead of time If the processor happens to guess the cor rect value of the reference and field as it was before the reference was writ ten, the speculative read could retire and create a problem This kind of
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reordering is quite rare and may never happen in practice, but nevertheless it is a problem If you're watching closely, you probably noticed we restricted T to a ref erence type That's done so we can use m_v a l u e being n u l l instead of a sep arate initialization flag to determine whether we must initialize the value We can extend the above example to accommodate value types by intro ducing an initialization variable, similar to the opening code
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c l a s s L a z y I n it < T > { p r ivate private p r ivate p rivate T m_va l u e ; volat i l e bool m_i n i t i a l i zed ; obj ect m_sync new obj ect ( ) ; F u n c < T > m_factory ;
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p u b l i c L a zy I n it ( F u n c < T > factory ) { m_factory p u b l i c T Va l u e { get { if ( ! m_init i a l ized ) { l o c k ( m_sy n c ) { if ( ! m_i n it i a l i z ed ) m_v a l u e m_factory ( ) ; m_i n i t i a l i zed true;
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We must be careful because we need to ensure that loads of the initial ization flag never get reordered with respect to the value itself, in addition to any fields being initialized This is done by annotating m_i n i t i a l i z e d a s
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