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From: Louis Semprini on 2 Jul 2010 06:14
Hello,

I have read ISO/IEC JTC1 SC22 WG21 N2668 (a revision of N2647, which
is a revision of N2534).

"Concurrency Modifications to Basic String"
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2668.htm

N2668 corrects a few typos from the previous versions, but there is
still one paragraph left which has me stumped:

Can anyone explain the following claim from the paper:

|Thus the only real anomaly seems to be that creating the initial
|non-const reference invalidates previously generated const
|references. This anomaly has unfortunate consequences.
|Typically, when v offers "container thread safety", we can do
|
| #pragma omp parallel for
| for( size_t i = 0, n = v.size(); i < n; ++i ) {
| v[i] = 0;
| }
|
|However, for a basic_string v, we must insert v[0];
|before the pragma to render the code correct.

The code in the sample does not appear to include any const
references, only non-const references, so
1. in what way does the code demonstrate the anomaly?
2. why would inserting v[0] help anything, since it is just another
non-const reference?

Is the code in error? Or is there some tricky secret const reference
that I am missing?

Perhaps the authors meant to say "v[i] = v[0]" inside the loop?
Though this would be an odd example as one thread would be setting
v[0] to itself.

Or perhaps the code _does_ demonstrate a problem with the string
design, but not a problem relating to const references? I could see
that in this scenario, when n threads simultaneously try to do the
first 'unsharing' operation, the library would need to have proper
thread coordination to assure that the first thread completed the
unsharing operation before other threads attempted to generate the non-
const reference. But that seems like an ordinary requirement and not
one related to this paper.

Please enlighten. Many thanks!


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