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

[Igor Zhukov via Phabricator via libcxx-commits]

fsb4000 updated this revision to Diff 495474.
fsb4000 marked 4 inline comments as done.
fsb4000 added a comment.

applying code review's suggestions.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D142864/new/

https://reviews.llvm.org/D142864

Files:
  libcxx/include/__algorithm/ranges_minmax.h
  libcxx/test/std/algorithms/alg.sorting/alg.min.max/ranges.minmax.pass.cpp


Index: libcxx/test/std/algorithms/alg.sorting/alg.min.max/ranges.minmax.pass.cpp
===================================================================
--- libcxx/test/std/algorithms/alg.sorting/alg.min.max/ranges.minmax.pass.cpp
+++ libcxx/test/std/algorithms/alg.sorting/alg.min.max/ranges.minmax.pass.cpp
@@ -339,7 +339,7 @@
   static_assert(test());
 
   {
-    // check that the iterator isn't moved from multiple times
+    // check that the random access iterator isn't moved from multiple times
     std::shared_ptr<int> a[] = { std::make_shared<int>(42) };
     auto range = std::ranges::subrange(std::move_iterator(a), std::move_iterator(a + 1));
     auto [min, max] = std::ranges::minmax(range);
@@ -347,6 +347,16 @@
     assert(min != nullptr);
     assert(max == min);
   }
+  {
+    // check that the input iterator isn't moved from multiple times
+    using input_move_iterator = cpp20_input_iterator<std::move_iterator<std::string>>;
+    std::shared_ptr<int> a[] = {std::make_shared<int>(42)};
+    auto range               = std::ranges::subrange(input_move_iterator(a), input_move_iterator(a + 1));
+    auto [min, max]          = std::ranges::minmax(range);
+    assert(a[0] == nullptr);
+    assert(min != nullptr);
+    assert(max == min);
+  }
 
   return 0;
 }
Index: libcxx/include/__algorithm/ranges_minmax.h
===================================================================
--- libcxx/include/__algorithm/ranges_minmax.h
+++ libcxx/include/__algorithm/ranges_minmax.h
@@ -76,8 +76,20 @@
     _LIBCPP_ASSERT(__first != __last, "range has to contain at least one element");
 
     if constexpr (forward_range<_Range>) {
+      // Special-case the one element case. Avoid repeatedly initializing objects from the result of an iterator
+      // dereference when doing so might not be idempotent. The `if constexpr` avoids the extra branch in cases where
+      // it's not needed.
+      if constexpr (!same_as<remove_cvref_t<range_reference_t<_Range>>, _ValueT>
+                    || is_rvalue_reference_v<range_reference_t<_Range>>) {
+        auto __test_for_one_element = __first;
+        if (++__test_for_one_element == __last) {
+          // This initialization is correct, similar to the N4928 [dcl.init.aggr]/6 example
+          minmax_result<_ValueT> __result = {static_cast<_ValueT>(*__result.min), __result.min};
+          return __result;
+        }
+      }
       auto __result = std::__minmax_element_impl(__first, __last, __comp, __proj);
-      return {*__result.first, *__result.second};
+      return {static_cast<_ValueT>(*__result.first), static_cast<_ValueT>(*__result.second)};
     } else {
       // input_iterators can't be copied, so the implementation for input_iterators has to store
       // the values instead of a pointer to the correct values
@@ -86,7 +98,8 @@
                                    std::invoke(__proj, std::forward<decltype(__b)>(__b)));
       };
 
-      ranges::minmax_result<_ValueT> __result = {*__first, __result.min};
+      // This initialization is correct, similar to the N4928 [dcl.init.aggr]/6 example
+      ranges::minmax_result<_ValueT> __result = {static_cast<_ValueT>(*__first), __result.min};
       if (__first == __last || ++__first == __last)
         return __result;
 


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