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Diffstat (limited to 'absl/base/const_init.h')
| -rw-r--r-- | absl/base/const_init.h | 74 |
1 files changed, 74 insertions, 0 deletions
diff --git a/absl/base/const_init.h b/absl/base/const_init.h new file mode 100644 index 00000000..30d6a3ca --- /dev/null +++ b/absl/base/const_init.h @@ -0,0 +1,74 @@ +// 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 +// +// https://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 { +inline namespace lts_2019_08_08 { + +enum ConstInitType { + kConstInit, +}; + +} // inline namespace lts_2019_08_08 +} // namespace absl + +#endif // ABSL_BASE_CONST_INIT_H_ |