I've been having a very difficult time with a problem illustrated by the
following program: It looks like the eval_if in line 51 isn't working
correctly. That is, the metafuction get_promotion_policy is being
invoked when both types are not safe types. I've been working on this
for two days and can't see what's going on. Any help from a TMP genious
would be much appreciated.
#include
#include
#include
#include
#include
#include
struct safe_tag {};
template
struct safe : public safe_tag {
T m_t;
typedef P PromotionPolicy;
};
template<class T>
struct is_safe : public
//std::is_arithmetic<T>
std::is_base_of
{};
template
struct common_policies {
static_assert(
boost::mpl::or_<
is_safe<T>,
is_safe<U>
>::value,
"at least one type must be a safe type"
);
template<typename Z>
struct get_promotion_policy {
static_assert(
is_safe<Z>::value,
"only safe types have promotion policies"
);
typedef typename Z::PromotionPolicy type;
};
// if both types are safe, the policies have to be the same!
static_assert(
boost::mpl::eval_if<
boost::mpl::and_<
is_safe<T>,
is_safe<U>
>,
typename std::is_same<
typename get_promotion_policy<T>::type,
typename get_promotion_policy<U>::type
>::type,
boost::mpl::identityboost::mpl::true_
>::type::value,
"if both types are safe, the policies have to be the same!"
);
// now we've verified that there is no conflict between policies
// return the one from the first safe type
typedef typename boost::mpl::if_<
is_safe<T>,
T,
typename boost::mpl::if_<
is_safe<U>,
U,
//
boost::mpl::void_
>::type
>::type safe_type;
typedef typename get_promotion_policy::type
promotion_policy;
};
common_policies t;
int main(){
return 0;
}
