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A s a first approximation o f a lazy initialization routine-and a s an example to motivate why the trickier pattern is required-let's look at a na'ive (and poorly performing) attempt
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c l a s s L a z y I n it < T > { p rivate p r ivate p r ivate p r ivate T m_v a l u e ; bool m_i n i t i a l i z ed ; o b j e c t m_sync = new obj e ct ( ) ; F u n c < T > m_fa ctory ;
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p u b l i c L a z y I n it ( F u n c < T > factory ) { m_factory = factory ; } p u b l i c T Value { get { 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 = t r u e ; } } ret u r n m_va l u e ; } } }
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Briefly, the data structure consists of four fields: the value that is lazy initialized (m_v a l ue), a flag specifying whether initialization has occurred (m_i n i t i a l i z e d ) , a synchronization object used for locking (m_s y n c ), and a delegate that, when invoked, lazily initializes the object in question, Inside the Va l u e accessor, we immediately acquire the lock and if the object hasn' t been initialized, we invoke the factory method, save its value, and set the initialization flag, We then return the value that got created , Now the S i n g l et o n data structure above could be written as such,
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c l a s s S i n g l eton { p rivate s t a t i c L a z y I n it < S i ngleton > s_i n s t = new L a z y I n it < S i ng l eton > ) = > new S i n g l eton ( ; p u b l i c s t a t i c S i n g leton I n s t a n c e
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get { ret u r n s_i n st Va l u e j } }
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All those examples of lazily initialized events, for example, can now simply be replaced with:
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new L a z y l n i t < Eve ntWa itHa n d le > ) => new Ma n u a l ResetEvent ( fa l s e
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This attempt is correct All initialization happens inside a lock, so there are no tricky memory model issues to consider We used a reference type, but, in this particular example, L a z y l n it < T > could have been a value type to avoid the overhead of allocating another heap object In many cases, lazy initialization is used to defer expensive resource allocation, which usually dwarfs the cost of having an extra object around The simplicity of this approach is also its downfall Since synchroniza tion is technically only needed while the value is initially created, it' s a shame we're taking the lock each time the value is subsequently accessed The popular solution to this problem is the double-checked locking pattern A check is first made outside of the lock to see whether the value was initialized yet; if it was, it can be retrieved with no synchronization; if it wasn't, the lock can be entered and the value initialized The subtle aspect to this pattern is that another check is done inside the lock to ensure another thread didn't concurrently initialize the value
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c l a s s L a z y l n it < T > where T : c l a s s { p rivate volat ile T m_va l u e j p r ivate obj ect m_sync = n e w obj e ct ( ) j p rivate F u n c < T > m_fa ctorY j p u b l i c Lazyl n it ( F u n c < T > factory ) { m_factory = f a c torY j } p u b l i c T Va l u e { get { if ( m_va l u e == n U l l ) {
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