const_init.h
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00001 // Copyright 2017 The Abseil Authors.
00002 //
00003 // Licensed under the Apache License, Version 2.0 (the "License");
00004 // you may not use this file except in compliance with the License.
00005 // You may obtain a copy of the License at
00006 //
00007 //      https://www.apache.org/licenses/LICENSE-2.0
00008 //
00009 // Unless required by applicable law or agreed to in writing, software
00010 // distributed under the License is distributed on an "AS IS" BASIS,
00011 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
00012 // See the License for the specific language governing permissions and
00013 // limitations under the License.
00014 //
00015 // -----------------------------------------------------------------------------
00016 // kConstInit
00017 // -----------------------------------------------------------------------------
00018 //
00019 // A constructor tag used to mark an object as safe for use as a global
00020 // variable, avoiding the usual lifetime issues that can affect globals.
00021 
00022 #ifndef ABSL_BASE_CONST_INIT_H_
00023 #define ABSL_BASE_CONST_INIT_H_
00024 
00025 // In general, objects with static storage duration (such as global variables)
00026 // can trigger tricky object lifetime situations.  Attempting to access them
00027 // from the constructors or destructors of other global objects can result in
00028 // undefined behavior, unless their constructors and destructors are designed
00029 // with this issue in mind.
00030 //
00031 // The normal way to deal with this issue in C++11 is to use constant
00032 // initialization and trivial destructors.
00033 //
00034 // Constant initialization is guaranteed to occur before any other code
00035 // executes.  Constructors that are declared 'constexpr' are eligible for
00036 // constant initialization.  You can annotate a variable declaration with the
00037 // ABSL_CONST_INIT macro to express this intent.  For compilers that support
00038 // it, this annotation will cause a compilation error for declarations that
00039 // aren't subject to constant initialization (perhaps because a runtime value
00040 // was passed as a constructor argument).
00041 //
00042 // On program shutdown, lifetime issues can be avoided on global objects by
00043 // ensuring that they contain  trivial destructors.  A class has a trivial
00044 // destructor unless it has a user-defined destructor, a virtual method or base
00045 // class, or a data member or base class with a non-trivial destructor of its
00046 // own.  Objects with static storage duration and a trivial destructor are not
00047 // cleaned up on program shutdown, and are thus safe to access from other code
00048 // running during shutdown.
00049 //
00050 // For a few core Abseil classes, we make a best effort to allow for safe global
00051 // instances, even though these classes have non-trivial destructors.  These
00052 // objects can be created with the absl::kConstInit tag.  For example:
00053 //   ABSL_CONST_INIT absl::Mutex global_mutex(absl::kConstInit);
00054 //
00055 // The line above declares a global variable of type absl::Mutex which can be
00056 // accessed at any point during startup or shutdown.  global_mutex's destructor
00057 // will still run, but will not invalidate the object.  Note that C++ specifies
00058 // that accessing an object after its destructor has run results in undefined
00059 // behavior, but this pattern works on the toolchains we support.
00060 //
00061 // The absl::kConstInit tag should only be used to define objects with static
00062 // or thread_local storage duration.
00063 
00064 namespace absl {
00065 
00066 enum ConstInitType {
00067   kConstInit,
00068 };
00069 
00070 }  // namespace absl
00071 
00072 #endif  // ABSL_BASE_CONST_INIT_H_


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autogenerated on Wed Jun 19 2019 19:42:14