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Double-Checked Locking pattern issue

Hello gurus,
For the wellknown Double-Checked Locking pattern,

http://www.ddj.com/184405726?pgno=1

Step 1: Allocate memory to hold a Singleton object.
Step 2: Construct a Singleton object in the allocated memory.
Step 3: Make pInstance point to the allocated memory.

After reading for a couple of times, I still do not understand why
some compiler will exchange step 2 and step 3 code? If there are any
exception in step 2, the swap code will make pInstance point to an
invalid memory address. Any ideas why compiler do the swap?

Expand|Select|Wrap|Line Numbers
  1.  
  2. Singleton* Singleton::instance() {
  3. if (pInstance == 0) {
  4. Lock lock;
  5. if (pInstance == 0) {
  6. pInstance = // Step 3
  7. operator new(sizeof(Singleton)); // Step 1
  8. new (pInstance) Singleton; // Step 2
  9. }
  10. }
  11. return pInstance;
  12. }
  13.  
  14.  

thanks in advance,
George
Dec 30 '07 #1
0 1178

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