Re: [Boost-users] Using MPL for conditional compilation

Ben Hutchings writes:

Alan M. Carroll wrote:

...

typedef typename boost::mpl::if_c::type testing;

That can't work. All the template arguments to boost::mpl::if_c must be syntactically valid after substiution of C's template parameters, and clearly that won't be the case. I think you need to write your own selection template:

template struct type_or_key_type { typedef T type; }; template<typename T> struct type_or_key_type { typedef typename T::key_type type; };

then use

typedef typename type_or_key_type::type testing;

I'd normally solve it this way: template <class T> struct key_type { typedef typename T::key_type type; }; typedef mpl::eval_if_c< IS_OUTER , key_type<PAYLOAD> , mpl::identity<PAYLOAD> >::type testing; HTH, -- Dave Abrahams Boost Consulting www.boost-consulting.com