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From: "ted at lyncon dot se" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/112349] New: ranges::max makes unecessary copies
Date: Thu, 02 Nov 2023 13:31:26 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112349

            Bug ID: 112349
           Summary: ranges::max makes unecessary copies
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ted at lyncon dot se
  Target Milestone: ---

From:
https://stackoverflow.com/questions/77409266/why-does-gcc-copy-object-for-e=
ach-comparison-in-stdrangesmax

OP's sample program shows a copy being made for each comparison:
```
#include <algorithm>
#include <iostream>
#include <ranges>
#include <vector>

struct A {
    A() =3D default;
    A(const A&) { std::cout << "Copy\n"; }
    A(A&&) noexcept { std::cout << "Move\n"; }

    A& operator=3D(const A&) {
        std::cout << "Copy assigned\n";
        return *this;
    }
    A& operator=3D(A&&) noexcept {
        std::cout << "Move assigned\n";
        return *this;
    }

    int x =3D 10;
};

int main() {
    std::vector<A> vec(10);
    std::cout << "Init\n";
    std::cout << std::ranges::max(vec, [](const auto& a, const auto& b) {
                     std::cout << "cmp" << std::endl;
                     return a.x < b.x;
                 }).x;
}
```
I _think_ the proper solution is to not copy `__first` into `__tmp` in
`operator()`:
```
    template<input_range _Range, typename _Proj =3D identity,
             indirect_strict_weak_order<projected<iterator_t<_Range>, _Proj=
>>
               _Comp =3D ranges::less>
      requires indirectly_copyable_storable<iterator_t<_Range>,
                                            range_value_t<_Range>*>
      constexpr range_value_t<_Range>
      operator()(_Range&& __r, _Comp __comp =3D {}, _Proj __proj =3D {}) co=
nst
      {
        auto __first =3D ranges::begin(__r);
        auto __last =3D ranges::end(__r);
        __glibcxx_assert(__first !=3D __last);
        auto __result =3D *__first;
        while (++__first !=3D __last)
          {
            auto __tmp =3D *__first;                               // <- CO=
PY
            if (std::__invoke(__comp,
                              std::__invoke(__proj, __result),
                              std::__invoke(__proj, __tmp)))
              __result =3D std::move(__tmp);
          }
        return __result;
      }
```
By changing it to
```c++
auto& __tmp =3D *__first;
```
...or just supplying `*__first` to `std::__invoke` it doesn't do copies in =
OP's
example but it may have other consequences. Worth looking in to anyway.=