[libcxx-commits] [PATCH] D142864: [libc++] `<algorithm>`: `ranges::minmax` should dereference iterators only once

[Igor Zhukov via Phabricator via libcxx-commits]

fsb4000 added inline comments.


================
Comment at: libcxx/include/__algorithm/ranges_minmax.h:140
+      // This initialization is correct, similar to the N4928 [dcl.init.aggr]/6 example
+      minmax_result<_ValueT> __result = {static_cast<_ValueT>(*__found_min), __result.min};
+      return __result;
----------------
fsb4000 wrote:
> philnik wrote:
> > What are these `static_cast`s for?
> MS STL uses the `static_cast`s. I used for consistency with MS STL. I asked them why the static_cast are needed and I added more `static_cast`s here. After their answer and if the `static_cast`s are optionally needed I could remove them.
Casey answered me:
`indirectly_copyable_storable<_It, _ValueT*>` (https://eel.is/c++draft/alg.req.ind.copy#2) requires that `_ValueT` is `copyable`, but the reference type of the input range (and `_It`) isn't necessarily `const _ValueT&` so we don't have the same guarantee of implicit convertibility. In fact there's no requirement on that algorithm that lets us implicitly convert the result of an iterator dereference to `_ValueT`. There is a requirement that `constructible_from<_ValueT, iter_reference_t<_It>>` holds, however, so we can perform the conversion explicitly with `static_cast`.

https://github.com/microsoft/STL/pull/3384#discussion_r1099671173


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