Re: [boost] Unnamed namespaces?

From: Giovanni Piero Deretta (gpderetta_at_[hidden])
Date: 2007-06-15 11:38:49

On 6/15/07, Eric Niebler <eric_at_[hidden]> wrote:
> Giovanni Piero Deretta wrote:
> > On 6/15/07, David Abrahams <dave_at_[hidden]> wrote:
> >> [...]
> >> I would put the reference itself in an unnamed namespace, and I'm pretty
> >> sure that handles it.  As long as the reference refers to something
> >> not in an unnamed namespace:
> >>
> >>    template<typename T> struct static_const
> >>    {
> >>      static T const value;
> >>    };
> >>
> >>    template<typename T>
> >>    T const static_const<T>::value = {};
> >>
> >>    struct placeholder {};
> >>
> >>    namespace // <<============== HERE
> >>    {
> >>      placeholder const &_1 = static_const<placeholder>::value;
> >>    }
> >>
> >>    template<class T> void foo(T const &)
> >>    {
> >>      // use _1 here. OK, _1 refers to the same object
> >>      // in all translation units.  It doesn't matter that the
> >>      // reference is has a different identity; the object is
> >>      // the same: static_const<placeholder>::value
> >>    }
> >>
> >>
> >
> > Instead of using a class statics, I've been using function static locals,
> > the same trick used by the BOOST_PARAMETER_KEYWORD macro.
> > What are the advantages and the disadvantages of the two aproach?
>
>
> Function static locals are not initialized until a thread of execution
> passes over their declaration. That means no static initialization, IIUC.
>

IIRC there are some cases where even function static locals are
guaranteed to be still initialized statically, but I do not remember
exactly in what cases (probably pods initialized with constants),
anyways, your observation below makes this useless in my case.

>
> > Consider that I'm using the trick to name stateless function objects
> > in header files
> > These function objects sometime have a constructors, but I could
> > easily make all of them POD if this would force the compiler to use
> > static initialization.
>
> 'Fraid not. And the Boost.Parameter trick looks like this, IIRC:
>
>    type const &name = get_static_local<type>();

That exactly what I'm doing.

>
> Then this *really* doesn't do static initialization, because you're
> calling a function.
>

You are definitely right.

>
> > Anyways, why is it a necessity to have those objects statically
> > initialized even if they are stateless?
>
> If you are using the above formulation, then the reference is not
> initialized until runtime. Due to order of initialization issues, you
> could end up indirecting through an uninitialized reference and crash
> even before you hit main().

I feared something like that. I'll change my code to use a class
template static. Hopfully most of the usages are protected by a macro
and I only need to change that.

>
> I think BOOST_PARAMETER_KEYWORD is bad voodoo and should be changed, FWIW.
>

thanks!

gpd