"Interesting" behavior in std::locale

[Jonathan Wakely]

On 31/10/14 12:07 -0400, James Benze wrote:
>Hey all:
>
>I found some code in libstdc++ that, while I understand what it does,
>I'm not understand why it's programmed the way it is, and I was hoping
>someone could shed some light on the behavior...I was hoping someone
>would be bored at work today and wouldn't mind spending the time to
>satisfy my curiosity.
>
>The function is std::locale::classic() (in locale_init.cc:256).  It
>seems to me to be a singleton pattern, but implemented in a strange
>way.  First it initializes _S_classic, using a normal singleton
>initialization pattern.  No big deal there.  Then, it uses placement
>new to create the new c_locale object and then return it.  Every time.
>
>So my question is, why wasn't this programmed with an if check to see
>if the singleton is already initialized?  I made a quick test program:
>
>//with-if.cpp
>
>#include<locale>
>
>std::locale::_Impl*       _S_classic;
>
>typedef char fake_locale[sizeof(std::locale)]
>__attribute__ ((aligned(__alignof__(std::locale))));
>fake_locale c_locale;
>
>std::locale * global = NULL;
>
>int main()
>{
>   if(!global)
>      *(new (&c_locale) std::locale(_S_classic));
>   return 0;
>}

I don't know the reason, but you'd need more than just an 'if' as
presumably you'd also want to set 'global' to be non-null after the
first call, which would need to be synchronized to avoid data races.