chromium/third_party/abseil-cpp/absl/base/internal/atomic_hook.h

// 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.

#ifndef ABSL_BASE_INTERNAL_ATOMIC_HOOK_H_
#define ABSL_BASE_INTERNAL_ATOMIC_HOOK_H_

#include <atomic>
#include <cassert>
#include <cstdint>
#include <utility>

#include "absl/base/attributes.h"
#include "absl/base/config.h"

#if defined(_MSC_VER) && !defined(__clang__)
#define ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT …
#else
#define ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT …
#endif

#if defined(_MSC_VER)
#define ABSL_HAVE_WORKING_ATOMIC_POINTER …
#else
#define ABSL_HAVE_WORKING_ATOMIC_POINTER …
#endif

namespace absl {
ABSL_NAMESPACE_BEGIN
namespace base_internal {

template <typename T>
class AtomicHook;

// To workaround AtomicHook not being constant-initializable on some platforms,
// prefer to annotate instances with `ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES`
// instead of `ABSL_CONST_INIT`.
#if ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT
#define ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES …
#else
#define ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES
#endif

// `AtomicHook` is a helper class, templatized on a raw function pointer type,
// for implementing Abseil customization hooks.  It is a callable object that
// dispatches to the registered hook.  Objects of type `AtomicHook` must have
// static or thread storage duration.
//
// A default constructed object performs a no-op (and returns a default
// constructed object) if no hook has been registered.
//
// Hooks can be pre-registered via constant initialization, for example:
//
// ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES static AtomicHook<void(*)()>
//     my_hook(DefaultAction);
//
// and then changed at runtime via a call to `Store()`.
//
// Reads and writes guarantee memory_order_acquire/memory_order_release
// semantics.
AtomicHook<ReturnType (*)(Args...)>;

#undef ABSL_HAVE_WORKING_ATOMIC_POINTER
#undef ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT

}  // namespace base_internal
ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_BASE_INTERNAL_ATOMIC_HOOK_H_