Re: C++0x static local initialization thread safety

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <>
Tue, 26 Apr 2011 14:25:45 CST
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

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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