folly/folly/SingletonThreadLocal.h

/*
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * 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.
 */

#pragma once

#include <thread>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>

#include <folly/ScopeGuard.h>
#include <folly/ThreadLocal.h>
#include <folly/detail/Iterators.h>
#include <folly/detail/Singleton.h>
#include <folly/detail/UniqueInstance.h>
#include <folly/functional/Invoke.h>
#include <folly/lang/Hint.h>

folly // namespace folly

/// FOLLY_DECLARE_REUSED
///
/// Useful for local variables of container types, where it is desired to avoid
/// the overhead associated with the local variable entering and leaving scope.
/// Rather, where it is desired that the memory be reused between invocations
/// of the same scope in the same thread rather than deallocated and reallocated
/// between invocations of the same scope in the same thread. Note that the
/// container will always be cleared between invocations; it is only the backing
/// memory allocation which is reused.
///
/// Example:
///
///   void traverse_perform(int root);
///   template <typename F>
///   void traverse_each_child_r(int root, F const&);
///   void traverse_depthwise(int root) {
///     // preserves some of the memory backing these per-thread data structures
///     FOLLY_DECLARE_REUSED(seen, std::unordered_set<int>);
///     FOLLY_DECLARE_REUSED(work, std::vector<int>);
///     // example algorithm that uses these per-thread data structures
///     work.push_back(root);
///     while (!work.empty()) {
///       root = work.back();
///       work.pop_back();
///       seen.insert(root);
///       traverse_perform(root);
///       traverse_each_child_r(root, [&](int item) {
///         if (!seen.count(item)) {
///           work.push_back(item);
///         }
///       });
///     }
///   }
#define FOLLY_DECLARE_REUSED(name, ...) …