我一直很难解决以下程序所示的问题:第51行的eval_if似乎无法正常工作。也就是说,当两种类型都不是安全类型时,将调用元功能get_promotion_policy。我已经做了两天了,看不到发生了什么。 TMP天才的任何帮助将不胜感激。
#include <type_traits>
#include <boost/mpl/and.hpp>
#include <boost/mpl/or.hpp>
#include <boost/mpl/if.hpp>
#include <boost/mpl/eval_if.hpp>
#include <boost/mpl/identity.hpp>
struct safe_tag {};
template<class T, class P = void>
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<safe_tag, T>
{};
template<class T, class U>
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::identity<boost::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<safe_type>::type promotion_policy;
};
common_policies<int, safe<int> > t;
int main(){
return 0;
}
最佳答案
typename std::is_same<
typename get_promotion_policy<T>::type,
typename get_promotion_policy<U>::type
>::type
触发
get_promotion_policy<T>
和get_promotion_policy<U>
的实例化,即使它在false分支中也是如此。您必须报告评估,例如:// 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>
>,
std::is_same<
typename boost::mpl::eval_if<is_safe<T>, get_promotion_policy<T>, boost::mpl::identity<void>>::type,
typename boost::mpl::eval_if<is_safe<U>, get_promotion_policy<U>, boost::mpl::identity<void>>::type
>,
boost::mpl::identity<boost::mpl::true_>
>::type::value,
"if both types are safe, the policies have to be the same!"
);
Demo
关于c++ - Boost/C++ eval_if失败,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/30689117/