[libcxx-commits] [libcxx] [libc++] implement `std::flat_multimap` (PR #113835)

[Louis Dionne via libcxx-commits]

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@@ -0,0 +1,97 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___FLAT_MAP_UTILS_H
+#define _LIBCPP___FLAT_MAP_UTILS_H
+
+#include <__config>
+#include <__type_traits/container_traits.h>
+#include <__utility/exception_guard.h>
+#include <__utility/forward.h>
+#include <__utility/move.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+#if _LIBCPP_STD_VER >= 23
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+// It is easier to be-friend to have a struct with static functions
+struct __flat_map_utils {
+  template <class _Map, class _IterK, class _IterM, class _KeyArg, class... _MArgs>
+  _LIBCPP_HIDE_FROM_ABI static typename _Map::iterator __emplace_exact_pos(
+      _Map& __map, _IterK&& __it_key, _IterM&& __it_mapped, _KeyArg&& __key, _MArgs&&... __mapped_args) {
+    auto __on_key_failed = std::__make_exception_guard([&]() noexcept {
+      using _KeyContainer = typename _Map::key_container_type;
+      if constexpr (__container_traits<_KeyContainer>::__emplacement_has_strong_exception_safety_guarantee) {
+        // Nothing to roll back!
+      } else {
+        // we need to clear both because we don't know the state of our keys anymore
+        __map.clear() /* noexcept */;
+      }
+    });
+    auto __key_it        = __map.__containers_.keys.emplace(__it_key, std::forward<_KeyArg>(__key));
+    __on_key_failed.__complete();
+
+    auto __on_value_failed = std::__make_exception_guard([&]() noexcept {
+      using _MappedContainer = typename _Map::mapped_container_type;
+      if constexpr (!__container_traits<_MappedContainer>::__emplacement_has_strong_exception_safety_guarantee) {
+        // we need to clear both because we don't know the state of our values anymore
+        __map.clear() /* noexcept */;
+      } else {
+        // In this case, we know the values are just like before we attempted emplacement,
+        // and we also know that the keys have been emplaced successfully. Just roll back the keys.
+#  if _LIBCPP_HAS_EXCEPTIONS
+        try {
+#  endif // _LIBCPP_HAS_EXCEPTIONS
+          __map.__containers_.keys.erase(__key_it);
+#  if _LIBCPP_HAS_EXCEPTIONS
+        } catch (...) {
+          // Now things are funky for real. We're failing to rollback the keys.
+          // Just give up and clear the whole thing.
+          //
+          // Also, swallow the exception that happened during the rollback and let the
+          // original value-emplacement exception propagate normally.
+          __map.clear() /* noexcept */;
+        }
+#  endif // _LIBCPP_HAS_EXCEPTIONS
+      }
+    });
+    auto __mapped_it = __map.__containers_.values.emplace(__it_mapped, std::forward<_MArgs>(__mapped_args)...);
+    __on_value_failed.__complete();
+
+    return typename _Map::iterator(std::move(__key_it), std::move(__mapped_it));
+  }
+
+  // https://github.com/llvm/llvm-project/issues/108624
+  template <class _Map, class _InputIterator, class _Sentinel>
----------------
ldionne wrote:

Let's document what this function does since this is now a reusable utility. Let's also move the link to the Github issue under a `TODO: we could actually optimize this, see <ISSUE LINK>`.

https://github.com/llvm/llvm-project/pull/113835