Re: [Boost-users] [MPL] has_xxx always returns false!

26 Aug 2010

      AMDG

Eric wrote:
...
This code:
BOOST_MPL_HAS_XXX_TRAIT_DEF(normal);
<snip>
if I understand correctly,  this code is suppose to do call a different 
"getNormal" method if the method "normal" exists (or not) in the template
parameter class.
Basically mimicking the infamous "__if_exists" macro on the Microsoft c++
compiler.... right?
but, has_normal<VT> always return false (visual studio 2008) so we don't get the
right expected behavior!
has_normal checks for a nested *type* called normal,
not for a member function.

#include 
#include 

BOOST_MPL_HAS_XXX_TRAIT_DEF(normal);

struct test1 {};
struct test2 { typedef void normal; };
struct test3 { int normal(); };

int main() {
    BOOST_MPL_ASSERT_NOT((has_normal<test1>));
    BOOST_MPL_ASSERT((has_normal<test2>));
    BOOST_MPL_ASSERT_NOT((has_normal<test3>));
}

In Christ,
Steven Watanabe