Improve map insertions

util/stdlib/map_insert.h

+// Copyright 2015 The Crashpad Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef CRASHPAD_UTIL_STDLIB_MAP_INSERT_H_
+#define CRASHPAD_UTIL_STDLIB_MAP_INSERT_H_
+
+#include <map>
+#include <utility>
+
+namespace crashpad {
+
+//! \brief Inserts a mapping from \a key to \a value into \a map, or replaces
+//!     an existing mapping so that \a key maps to \value.
+//!
+//! This behaves similarly to `std::map<>::insert_or_assign()` proposed for
+//! C++17, except that the \a old_value parameter is added.
+//!
+//! \param[inout] map The map to operate on.
+//! \param[in] key The key that should be mapped to \a value.
+//! \param[in] value The value that \a key should map to.
+//! \param[out] old_value If \a key was previously present in \a map, this will
+//!     be set to its previous value. This parameter is optional and may be
+//!     `nullptr` if this information is not required.
+//!
+//! \return `false` if \a key was previously present in \a map. If \a old_value
+//!     is not `nullptr`, it will be set to the previous value. `true` if \a
+//!     key was not present in the map and was inserted.
+template <typename T>
+bool MapInsertOrReplace(T* map,
+                        const typename T::key_type& key,
+                        const typename T::mapped_type& value,
+                        typename T::mapped_type* old_value) {
+  const auto result = map->insert(std::make_pair(key, value));
+  if (!result.second) {
+    if (old_value) {
+      *old_value = result.first->second;
+    }
+    result.first->second = value;
+  }
+  return result.second;
+}
+
+}  // namespace crashpad
+
+#endif  // CRASHPAD_UTIL_STDLIB_MAP_INSERT_H_