Re: [boost] Re: Re: A replacement to any_cast

From: Roland Weiss (weissr_at_[hidden])
Date: 2004-12-13 05:51:57

Yuval Ronen wrote:
> 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?

They are not needed when /using/ any objects. Have a look at section 5.1 of

C++ Templates, The Complete Guide, Vandevoorde & Josuttis

for a quick overview on the topic when to use the keywords "typename"
and "template" to disambiguate template code. Short rule: never do this
outside of template code, and only for constructs depending on the
template parameters.

greetings,
Roland

> 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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