// 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.
// 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_
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