Why is std::same_as implemented in such a weird way?

Question

cppref gives a possible implementation of std::same_as:

namespace detail {
    template<class T, class U>
    concept SameHelper = std::is_same_v<T, U>;
}

template<class T, class U>
concept same_as = detail::SameHelper<T, U> && detail::SameHelper<U, T>;

Why is it not implemented just as follows:

template<class T, class U>
concept same_as = std::is_same_v<T, U> && std::is_same_v<U, T>;

or even shorter:

template<class T, class U>
concept same_as = std::is_same_v<T, U>;

Jarod42 · Accepted Answer · 2020-04-06T09:43:17.163

2

It is t handle subsumption which only happens with concepts.

With your proposal,

same_as<T, U> doesn't subsume same_as<U, T>.

Further reading in cppreference.

edited Apr 06 '20 at 09:43

answered Apr 06 '20 at 09:29

Jarod42

203,559
14
181
302

for the last, but the first alternative is equivalent, no? – 463035818_is_not_an_ai Apr 06 '20 at 09:30
`std::is_same_v` is not concept, so `same_as` would be atomic constraint with expression `std::is_same_v && std::is_same_v`. – Jarod42 Apr 06 '20 at 09:35
thanks I already found [this](https://stackoverflow.com/a/52062387/4117728) for the details. It is more to read than I have time now, so I put it on my list :) – 463035818_is_not_an_ai Apr 06 '20 at 09:39

Why is std::same_as implemented in such a weird way?

1 Answers1