Re: [boost] MSVC9 SFINAE quirks?

Subject: Re: [boost] MSVC9 SFINAE quirks?
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2009-01-10 11:38:05

AMDG

Simonson, Lucanus J wrote:
> Steven Watanabe wrote:
>   
>>  template <typename geometry_type_1, typename geometry_type_2>
>>  typename boost::enable_if<
>>      boost::mpl::and_<is_any_polygon_set_type<geometry_type_1>,
>>                       is_any_polygon_set_type<geometry_type_2>,
>>                       is_either_polygon_set_type<geometry_type_1, 
>> geometry_type_2>
>>      >,
>>      polygon_set_view<geometry_type_1, geometry_type_2, 3>
>>  >::type
>>  operator-(const geometry_type_1& lvalue, const geometry_type_2& rvalue) {
>>    return polygon_set_view<geometry_type_1, geometry_type_2, 3>
>>      (lvalue, rvalue);
>>  }
>>     
>
> This won't work.  In order to instantiate enable_if the compiler must instantiate both of its arguments.  That means the compiler will try to instantiate polygon_set_view<> for every time the generic operator - in the stl is used for iterator arithmetic.  Any error encountered trying to instantiate polygon_set_view will cause a syntax error and failed compilation even though the and_ evaluates to false.
>   

Not true.  Simply naming
polygon_set_view<geometry_type_1, geometry_type_2, 3>
doesn't force it to be instantiated.  If you're getting an error from
this, there must be some other reason.

In Christ,
Steven Watanabe