[boost] Re: Re: A replacement to any_cast

From: Yuval Ronen (ronen_yuval_at_[hidden])
Date: 2004-12-13 05:08:42

I have to admit two things in shame:
1. My knowledge of the C++ standard is not that profound as to decide
whether the "template" is necessary or not.
2. I also didn't quite understand your reply. What was the conclusion? Is
the "template" needed to exist? Not needed to exist? Needed not to exist?

So, in light of these shortcomings of mine, I decied to give it a try. I
testes both versions (with and without the "template") on 3 compilers: GNU
GCC 3.2, GNU GCC 3.4.2 and MSVC 7.1. The results were that GCC 3.2 and MSVC
7.1 accepted both versions, while GCC 3.4.2 accepted only the version
without the "template".

So what's the conlusion now?
Are these suggested get() methods still such a bad idea?
Will they ever see the daylight?

> > > But you can't.  You have to write the following, as I showed
> > > above:
> > >
> > >    int a = some_any.template get<int>();
> >
> > ?? There are no dependent types in that utterance.  Unless some_any is
> > not known to be a boost::any (?)  Am I missing something?
>
> I tried to track down the relevant section in the standard to
> confirm what I wrote, but I couldn't.  I was relying on C++PL,
> 3rd Ed, App C, section C.13.6, "Template as a Qualifier."
>
> The example is (reduced):
>
>    struct Memory
>    {
>       template <typename T> T * get_new();
>    };
>
>    template <typename Allocator>
>    void
>    f(Allocator & m)
>    {
>       int * p1 = m.get_new<int>(); // error
>       int * p2 = m.template get_new<int>();
>    }
>
> This Memory::get_new() is used within a function template, but
> Stroustrup doesn't mention that it is precisely because get_new()
> is dependent that "template" is needed, so I didn't think that it
> was a dependent type.
>
> Thinking further, I realized that m.get_new<int>() actually means
> m.Allocator::get_new<int>(), so I now see that it is a dependent
> type according to 14.6.2.1/1.
>
> In the OP's case, the invoked member function would be any<T>'s,
> not T's, so it isn't dependent and "template" isn't needed, which
> is what you implied.
>
> -- 
> Rob Stewart                           stewart_at_[hidden]
> Software Engineer                     http://www.sig.com
> Susquehanna International Group, LLP  using std::disclaimer;
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