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// 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_