Re: [boost] [mpl] BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF

Subject: Re: [boost] [mpl] BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF
From: Edward Diener (eldiener_at_[hidden])
Date: 2010-07-18 20:39:43

On 7/18/2010 8:02 PM, Daniel Walker wrote:
> On Sun, Jul 18, 2010 at 7:11 PM, Edward Diener<eldiener_at_[hidden]>  wrote:
>> On 7/18/2010 12:26 PM, Steven Watanabe wrote:
>>>
>>> AMDG
>>>
>>> Edward Diener wrote:
>>>>
>>>> I found the BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF macro in the latest
>>>> has_xxx.hpp of mpl on the trunk.
>>>>
>>>> Is BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF ( and its corresponding
>>>> BOOST_MPL_HAS_XXX_TEMPLATE_DEF ) scheduled to be added to Boost in an
>>>> upcoming release ? I find it useful for TMP introspection in much the
>>>> same way as BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF ( and its corresponding
>>>> BOOST_MPL_HAS_XXX_TRAIT_DEF ).
>>>
>>> It's currently in the release branch and should be released in 1.44.
>>
>> That is good to hear but...
>>
>> My testing with VC9 and VC10 shows that if I pass too many tempate arguments
>> I get a compiler failure:
>
> Do an svn update on your local copy of trunk. You no longer need to
> pass arguments to detect member templates. This change was checked in
> a couple of weeks ago, so you might not have gotten it yet. See the
> updated documentation.

I did not find anything in the updated documentation about these macros. 
Was I supposed to generate updated docs for MPL in the trunk through 
some jamfile ?

>
>>
>> ----------------------------------------------------
>>
>> #include<boost/mpl/has_xxx.hpp>
>>
>> struct AAType
>>   {
>>   template<class X>  struct AAMemberTemplate { };
>>   };
>>
>> BOOST_MPL_HAS_XXX_TEMPLATE_DEF(AAMemberTemplate)
>>
>> has_AAMemberTemplate<AAType,int,int>  yz; // error
>
> After updating from svn, change the line to
> has_AAMemberTemplate<AAType>  yz;
>
> You should no longer see an error.

You are correct.

 > You can detect member templets with
> any number of arguments up to the configurable
> BOOST_MPL_LIMIT_METAFUNCTION_ARITY, which defaults to 5.

OK. Thanks !

>
> <snip>
>> The code in the MPL is highly complicated, else I would spend the hours
>> trying to understand it and perhaps fix it. But I hope that at least
>> reporting my failure with VC9 and VC10 will help to get this working.
>
> Thanks for reporting! Actually, when I ran your example code I noticed
> a header dependency in boost/mpl/has_xxx.hpp that I had accidentally
> missed. So, your report has helped improve things already.

I am glad.

Thanks for your code. I will look at it and see if I can't do something 
further on my own which would allow one to find out the number and type 
of the template parameters of a class template at compile time, given 
the template name. Something instinctively tells me that this might be 
accessible, if it has not already been done.