Re: Guarantee of side-effect free assignment

"Alf P. Steinbach" <>
Tue, 9 Oct 2007 21:26:06 CST
* Jiri Palecek:

I think one of the problems with your solution with a randomly throwing

Non-provably not-throwing, not randomly throwing. ;-)

constructor could be a compiler can still rewrite it like this:

   Singleton* Singleton::instance()
       if (pInstance == 0)
           Lock lock;
           if (pInstance == 0)
             // we know pInstance is 0 here
               pInstance = // Step 3
               operator new(sizeof(Singleton)); // Step 1
             try {
               new (pInstance) Singleton; // Step 2
             } catch(...) {
               delete (void*)pInstance;
       return pInstance;

And this transformation is legal (at least I think) as long as
Singleton::Singleton() (even in case of an exception) doesn't call
Singleton::instance() (and the allocation and deallocation function
likewise). You could only see the pointer to the non-constructed from an
async signal (but that's OK, because even the pointer is not sig_atomic_t,
so can be anything in an async signal) or from a different thread, but the
standard doesn't say anything about threads.

If pInstance is a non-local static it can be inspected from within the
Singleton constructor. The rewrite will then have changed the effect of
a normal single-threaded program. So at least in that case, it's not
allowed (assuming Greg's logic holds, which I think it does).

Which means that the conclusion that the assignment has to happen after
the constructor, is incompatible with the Meyers/Alexandrescu conclusion
that "there is no way to express this constraint in C or C++".


- Alf

A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
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