Imagine I have this:
const string& get_name()
{
static auto* ptr_name=new string("Ron");
return *ptr_name;
}
If multiple threads are calling get_name is that UB or not?
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NoSenseEtAl
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2 Answers
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This is thread safe in C++11 and forward.
VS-2013 has not yet implemented this part of C++11. VS-14 does:
Howard Hinnant
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I have known about magic statics, I just did not know if publishing that static by returning it from a function is race free. – NoSenseEtAl Nov 28 '14 at 05:48
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race free was a wrong terminology... I was thinking about sequential consistency... Aka can users of a function see garbage or null as returned value, or if they see the real address of string can they see garbage where address points to(instead of "Ron") – NoSenseEtAl Nov 28 '14 at 05:56
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Also see [Runtime crash with static std::mutex](https://connect.microsoft.com/VisualStudio/feedback/details/808030/runtime-crash-with-static-std-mutex) on Microsoft Connect, [Visual C++ 2015 runtime broken on Windows Server 2003 (C++11 magic statics)](https://connect.microsoft.com/VisualStudio/feedback/details/1789709/visual-c-2015-runtime-broken-on-windows-server-2003-c-11-magic-statics) on Microsoft Connect, and any number of posts about crashes on Stack Overflow. Microsoft screwed this up bigtime. – jww Feb 02 '17 at 13:17
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Since C++11, initialization of function scope static variables is thread safe : the first tread calling get_name() will initialize ptr_name, blocking other threads until the initialization is completed. All subsequent calls will use the initialized value.
With prior C++ implementations, there is no such guarantee and all bets are off
quantdev
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