Re: [boost] [parameter] aux_/void.hpp: potential undefined behavior

Subject: Re: [boost] [parameter] aux_/void.hpp: potential undefined behavior
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2011-11-04 07:47:01

Krzysztof Czainski wrote:
> 2011/11/3 Stewart, Robert <Robert.Stewart_at_[hidden]>
> > Krzysztof Czainski wrote:
> > >
> > > struct void_ {};
> > >
> > > namespace aux
> > > {
> > >   inline void_& void_reference()
> > >   {
> > >       static void_ instance; // (1)
> > >       return instance;
> > >   }
>
> don't you think it's better to use static initialization? Or
> maybe it doesn't matter?
>
> I mean, if the (empty autogenerated) default constructor is
> called, then the compiler needs to generate some flag and an if
> condition, so the default constructor is only called during the
> first invocation of void_reference(). Of course, that can be
> optimized away, but isn't it just better to use static
> initialization here? (something like prefere pass-by-const-ref
> to pass-by-value)

I presume you mean the following:

namespace aux
{
   inline void_ & void_reference()
   {
      static void_ = {};
      return instance;
   }
}

My reading of 6.7/4 in N3092 suggests that would permit initialization without the need to introduce a first call test since there is no declared default constructor and using an initializer eliminates the default constructor invocation that would lead to its definition.

If we're interpreting things correctly, then, yes, this is a worthwhile tip to capture, especially in light of C++11's requirement for thread-safe initialization of function local statics.

_____
Rob Stewart                           robert.stewart_at_[hidden]
Software Engineer                     using std::disclaimer;
Dev Tools & Components
Susquehanna International Group, LLP  http://www.sig.com

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