Re: How to write an is_nothrow_swapable<T>?

[3rd attempt after 2 days]

On 2012-11-16 10:13, Zhihao Yuan wrote:
  > Non-throw swapable is quite useful, and it can be more useful if the
  > concept can be tested. If we already have an object of type T, I
  > think the following one is enough:
  >
  > template <typename T>
  > constexpr bool is_nothrow_swapable(T& t) {
  > using std::swap;
  > return noexcept(swap(t, t)) or
  > (std::is_nothrow_move_constructible<T>::value and
  > std::is_nothrow_move_assignable<T>::value);
  > }
  >
  > However, I want it to be able to work on just a type, to work as an
  > integral_constant. Is that doable? Comments on how to implement an
  > is_swapable<T> are also welcome. Thanks.

I'm not sure whether I understood your question correctly, because you
mention (a) the ability to work on just a type and (b) you refer to
is_swapable (instead of is_nothrow_swapable). I guess that (b) is just a
thinko on your side. What about a variation of your constexpr function
so that it does not depend on any argument anymore:

template <typename T>
constexpr bool is_nothrow_swappable() {
    using std::swap;
    return noexcept(swap(std::declval<T&>(), std::declval<T&>())) or
          (std::is_nothrow_move_constructible<T>::value and
           std::is_nothrow_move_assignable<T>::value);
}

template<typename T>
struct is_nothrow_swappable_trait :
    std::integral_constant<bool, is_nothrow_swappable<T>()>
{
};

?

Further, I think that the additional parts

   or
        (std::is_nothrow_move_constructible<T>::value and
         std::is_nothrow_move_assignable<T>::value)

are not really helpful, because they should automatically be deduced
correctly from

#include <type_traits>
#include <utility> // std::swap templates

template <typename T>
constexpr bool is_nothrow_swappable() {
    using std::swap;
    return noexcept(swap(std::declval<T&>(), std::declval<T&>()));
}

template<typename T>
struct is_nothrow_swappable_trait :
    std::integral_constant<bool, is_nothrow_swappable<T>()>
{
};

HTH & Greetings from Bremen,

Daniel Kr?gler

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