Re: [Boost-users] inspecting templates and boost::is_pointer

From: David Abrahams (dave_at_[hidden])
Date: 2006-06-24 20:07:47

"Ovanes Markarian" <om_boost_at_[hidden]> writes:

> Dave, 
>
> thanks for your answer. I think this should be definitely a part of
> FAQ, otherwise someone could spent endless hours on matters why some
> of boost classes do not work as expected.
>
> I have one more question. I tried the following code with VC 8.0:
>
> #include <boost/mpl/has_xxx.hpp>
> struct test2 { typedef int xxx; };
>
> BOOST_MPL_HAS_XXX_TRAIT_DEF(has_xxx)
> typedef has_xxx my_test;
>
> And got a compiler error:
>
> main.cpp(22) : error C2146: syntax error : missing ';' before identifier
> 'my_test'
>
> Somehow the macro was not expanded. I replaced xxx with some other name
> before but end up in compiler errors, so that I decided to check if the
> standard example is compilable at all by using the typedef.
> Do you have any suggestions?

Oh, the example in the docs is clearly wrong (a job for LiTRe,
Aleksey?)

Read the docs but ignore the example.  You want

     BOOST_MPL_HAS_XXX_TRAIT_DEF(xxx)

But then has_xxx will be a unary metafunction, not a type, so the
typedef you have there is illegal.

   typedef has_xxx<test2> my_test;
   BOOST_MPL_ASSERT((my_test));

>
>
> Thanks,
> Ovanes
>
> -----Original Message-----
> From: David Abrahams [mailto:dave_at_[hidden]] 
> Sent: Saturday, June 24, 2006 04:51
> To: boost-users_at_[hidden]
> Subject: Re: [Boost-users] inspecting templates and boost::is_pointer
>
> "Ovanes Markarian" <om_boost_at_[hidden]> writes:
>
>> Hello all,
>>
>> I am trying to specialize boost::is_pointer so, that it can determine 
>> if a type is a pointer wrapper (shared_ptr, auto_ptr, weak_ptr).
>
> First of all, don't do that.  It's a really bad idea, and it will probably
> break some (Boost) library code.  is_pointer is supposed to detect real
> pointers, and if you start changing what it does with other types, you'll
> break the expectations of every bit of code that currently uses is_pointer.
> This has come up before, so maybe it should be a FAQ (John?)
>
>> Does anybody has ideas how this can be done? May be it would be 
>> possible to handle if e.g. one can determine if a class/struct has 
>> some typedef inside:
>
> http://www.boost.org/libs/mpl/doc/refmanual/has-xxx-trait-def.html
>
> --
> Dave Abrahams
> Boost Consulting
> www.boost-consulting.com

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com