Re: C++0x static local initialization thread safety
Am 26.04.2011 09:41, schrieb Brendan:
In C++ static local initialization was not thread safe. I was
wondering whether this was addressed at all in C++0x?
The use case I'm imagining, is to use static to only construct
regex's, or other constant datastructures once:
bool string_matches_pattern (std::string str) {
static const std::regex my_regex(R"(some_regex_pattern)");
return regex_match(str, my_regex);
}
For this to be thread safe, the language would have to implicitly
generate a mutex and lock it around initialization of static locals. I
know there was some work done on the C++0x memory model, and getting
the language ready for thread safety, and I was wondering if this was
ever done?
Yes, there has been added extra wording that provides special guarantees
in 6.7 [stmt.dcl] p. 4:
"If control enters the declaration concurrently while the variable is being initialized, the concurrent execution shall wait for completion of the initialization.(88)"
with footnote 88:
"The implementation must not introduce any deadlock around execution of the initializer."
Alternatively, is it possible to mark local variables, thread_local in
C++0x?
bool string_matches_pattern (std::string str) {
thread_local const std::regex my_regex(R"(some_regex_pattern)");
return regex_match(str, my_regex);
}
This would construct the regex once for every thread.
Yes, this is also possible.
HTH & Greetings from Bremen,
Daniel Kr?gler
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