fc8dc48020
git-subtree-dir: third_party/abseil_cpp git-subtree-mainline:ffb2ae54begit-subtree-split:768eb2ca28
76 lines
3.4 KiB
C++
76 lines
3.4 KiB
C++
// 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_
|
|
|
|
#include "absl/base/config.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 {
|
|
ABSL_NAMESPACE_BEGIN
|
|
|
|
enum ConstInitType {
|
|
kConstInit,
|
|
};
|
|
|
|
ABSL_NAMESPACE_END
|
|
} // namespace absl
|
|
|
|
#endif // ABSL_BASE_CONST_INIT_H_
|