| // Copyright 2017 The Abseil Authors. |
| // |
| // 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. |
| // |
| // ----------------------------------------------------------------------------- |
| // kConstInit |
| // ----------------------------------------------------------------------------- |
| // |
| // A constructor tag used to mark an object as safe for use as a global |
| // variable, avoiding the usual lifetime issues that can affect globals. |
| |
| #ifndef ABSL_BASE_CONST_INIT_H_ |
| #define ABSL_BASE_CONST_INIT_H_ |
| |
| // In general, objects with static storage duration (such as global variables) |
| // can trigger tricky object lifetime situations. Attempting to access them |
| // from the constructors or destructors of other global objects can result in |
| // undefined behavior, unless their constructors and destructors are designed |
| // with this issue in mind. |
| // |
| // The normal way to deal with this issue in C++11 is to use constant |
| // initialization and trivial destructors. |
| // |
| // Constant initialization is guaranteed to occur before any other code |
| // executes. Constructors that are declared 'constexpr' are eligible for |
| // constant initialization. You can annotate a variable declaration with the |
| // ABSL_CONST_INIT macro to express this intent. For compilers that support |
| // it, this annotation will cause a compilation error for declarations that |
| // aren't subject to constant initialization (perhaps because a runtime value |
| // was passed as a constructor argument). |
| // |
| // On program shutdown, lifetime issues can be avoided on global objects by |
| // ensuring that they contain trivial destructors. A class has a trivial |
| // destructor unless it has a user-defined destructor, a virtual method or base |
| // class, or a data member or base class with a non-trivial destructor of its |
| // own. Objects with static storage duration and a trivial destructor are not |
| // cleaned up on program shutdown, and are thus safe to access from other code |
| // running during shutdown. |
| // |
| // For a few core Abseil classes, we make a best effort to allow for safe global |
| // instances, even though these classes have non-trivial destructors. These |
| // objects can be created with the absl::kConstInit tag. For example: |
| // ABSL_CONST_INIT absl::Mutex global_mutex(absl::kConstInit); |
| // |
| // The line above declares a global variable of type absl::Mutex which can be |
| // accessed at any point during startup or shutdown. global_mutex's destructor |
| // will still run, but will not invalidate the object. Note that C++ specifies |
| // that accessing an object after its destructor has run results in undefined |
| // behavior, but this pattern works on the toolchains we support. |
| // |
| // The absl::kConstInit tag should only be used to define objects with static |
| // or thread_local storage duration. |
| // |
| |
| namespace absl { |
| |
| enum ConstInitType { |
| kConstInit, |
| }; |
| |
| } // namespace absl |
| |
| #endif // ABSL_BASE_CONST_INIT_H_ |
