const_init.h
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1 // Copyright 2017 The Abseil Authors.
2 //
3 // Licensed under the Apache License, Version 2.0 (the "License");
4 // you may not use this file except in compliance with the License.
5 // You may obtain a copy of the License at
6 //
7 // https://www.apache.org/licenses/LICENSE-2.0
8 //
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS,
11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 // See the License for the specific language governing permissions and
13 // limitations under the License.
14 //
15 // -----------------------------------------------------------------------------
16 // kConstInit
17 // -----------------------------------------------------------------------------
18 //
19 // A constructor tag used to mark an object as safe for use as a global
20 // variable, avoiding the usual lifetime issues that can affect globals.
21 
22 #ifndef ABSL_BASE_CONST_INIT_H_
23 #define ABSL_BASE_CONST_INIT_H_
24 
25 // In general, objects with static storage duration (such as global variables)
26 // can trigger tricky object lifetime situations. Attempting to access them
27 // from the constructors or destructors of other global objects can result in
28 // undefined behavior, unless their constructors and destructors are designed
29 // with this issue in mind.
30 //
31 // The normal way to deal with this issue in C++11 is to use constant
32 // initialization and trivial destructors.
33 //
34 // Constant initialization is guaranteed to occur before any other code
35 // executes. Constructors that are declared 'constexpr' are eligible for
36 // constant initialization. You can annotate a variable declaration with the
37 // ABSL_CONST_INIT macro to express this intent. For compilers that support
38 // it, this annotation will cause a compilation error for declarations that
39 // aren't subject to constant initialization (perhaps because a runtime value
40 // was passed as a constructor argument).
41 //
42 // On program shutdown, lifetime issues can be avoided on global objects by
43 // ensuring that they contain trivial destructors. A class has a trivial
44 // destructor unless it has a user-defined destructor, a virtual method or base
45 // class, or a data member or base class with a non-trivial destructor of its
46 // own. Objects with static storage duration and a trivial destructor are not
47 // cleaned up on program shutdown, and are thus safe to access from other code
48 // running during shutdown.
49 //
50 // For a few core Abseil classes, we make a best effort to allow for safe global
51 // instances, even though these classes have non-trivial destructors. These
52 // objects can be created with the absl::kConstInit tag. For example:
53 // ABSL_CONST_INIT absl::Mutex global_mutex(absl::kConstInit);
54 //
55 // The line above declares a global variable of type absl::Mutex which can be
56 // accessed at any point during startup or shutdown. global_mutex's destructor
57 // will still run, but will not invalidate the object. Note that C++ specifies
58 // that accessing an object after its destructor has run results in undefined
59 // behavior, but this pattern works on the toolchains we support.
60 //
61 // The absl::kConstInit tag should only be used to define objects with static
62 // or thread_local storage duration.
63 
64 namespace absl {
65 
68 };
69 
70 } // namespace absl
71 
72 #endif // ABSL_BASE_CONST_INIT_H_
Definition: algorithm.h:29
ConstInitType
Definition: const_init.h:66


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autogenerated on Tue Jun 18 2019 19:44:35