77 lines
3.4 KiB
C
77 lines
3.4 KiB
C
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// Copyright 2017 The Abseil Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// https://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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//
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// -----------------------------------------------------------------------------
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// kConstInit
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// -----------------------------------------------------------------------------
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//
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// A constructor tag used to mark an object as safe for use as a global
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// variable, avoiding the usual lifetime issues that can affect globals.
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#ifndef ABSL_BASE_CONST_INIT_H_
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#define ABSL_BASE_CONST_INIT_H_
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#include "absl/base/config.h"
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// In general, objects with static storage duration (such as global variables)
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// can trigger tricky object lifetime situations. Attempting to access them
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// from the constructors or destructors of other global objects can result in
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// undefined behavior, unless their constructors and destructors are designed
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// with this issue in mind.
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//
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// The normal way to deal with this issue in C++11 is to use constant
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// initialization and trivial destructors.
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//
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// Constant initialization is guaranteed to occur before any other code
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// executes. Constructors that are declared 'constexpr' are eligible for
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// constant initialization. You can annotate a variable declaration with the
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// ABSL_CONST_INIT macro to express this intent. For compilers that support
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// it, this annotation will cause a compilation error for declarations that
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// aren't subject to constant initialization (perhaps because a runtime value
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// was passed as a constructor argument).
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//
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// On program shutdown, lifetime issues can be avoided on global objects by
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// ensuring that they contain trivial destructors. A class has a trivial
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// destructor unless it has a user-defined destructor, a virtual method or base
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// class, or a data member or base class with a non-trivial destructor of its
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// own. Objects with static storage duration and a trivial destructor are not
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// cleaned up on program shutdown, and are thus safe to access from other code
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// running during shutdown.
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//
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// For a few core Abseil classes, we make a best effort to allow for safe global
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// instances, even though these classes have non-trivial destructors. These
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// objects can be created with the absl::kConstInit tag. For example:
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// ABSL_CONST_INIT absl::Mutex global_mutex(absl::kConstInit);
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//
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// The line above declares a global variable of type absl::Mutex which can be
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// accessed at any point during startup or shutdown. global_mutex's destructor
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// will still run, but will not invalidate the object. Note that C++ specifies
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// that accessing an object after its destructor has run results in undefined
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// behavior, but this pattern works on the toolchains we support.
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//
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// The absl::kConstInit tag should only be used to define objects with static
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// or thread_local storage duration.
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namespace absl {
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ABSL_NAMESPACE_BEGIN
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enum ConstInitType {
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kConstInit,
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};
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ABSL_NAMESPACE_END
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} // namespace absl
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#endif // ABSL_BASE_CONST_INIT_H_
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