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Pods/abseil/absl/base/nullability.h

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+// Copyright 2023 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.
+//
+// -----------------------------------------------------------------------------
+// File: nullability.h
+// -----------------------------------------------------------------------------
+//
+// This header file defines a set of "templated annotations" for designating the
+// expected nullability of pointers. These annotations allow you to designate
+// pointers in one of three classification states:
+//
+//  * "Non-null" (for pointers annotated `Nonnull<T>`), indicating that it is
+//    invalid for the given pointer to ever be null.
+//  * "Nullable" (for pointers annotated `Nullable<T>`), indicating that it is
+//    valid for the given pointer to be null.
+//  * "Unknown" (for pointers annotated `NullabilityUnknown<T>`), indicating
+//    that the given pointer has not been yet classified as either nullable or
+//    non-null. This is the default state of unannotated pointers.
+//
+// NOTE: unannotated pointers implicitly bear the annotation
+// `NullabilityUnknown<T>`; you should rarely, if ever, see this annotation used
+// in the codebase explicitly.
+//
+// -----------------------------------------------------------------------------
+// Nullability and Contracts
+// -----------------------------------------------------------------------------
+//
+// These nullability annotations allow you to more clearly specify contracts on
+// software components by narrowing the *preconditions*, *postconditions*, and
+// *invariants* of pointer state(s) in any given interface. It then depends on
+// context who is responsible for fulfilling the annotation's requirements.
+//
+// For example, a function may receive a pointer argument. Designating that
+// pointer argument as "non-null" tightens the precondition of the contract of
+// that function. It is then the responsibility of anyone calling such a
+// function to ensure that the passed pointer is not null.
+//
+// Similarly, a function may have a pointer as a return value. Designating that
+// return value as "non-null" tightens the postcondition of the contract of that
+// function. In this case, however, it is the responsibility of the function
+// itself to ensure that the returned pointer is not null.
+//
+// Clearly defining these contracts allows providers (and consumers) of such
+// pointers to have more confidence in their null state. If a function declares
+// a return value as "non-null", for example, the caller should not need to
+// check whether the returned value is `nullptr`; it can simply assume the
+// pointer is valid.
+//
+// Of course most interfaces already have expectations on the nullability state
+// of pointers, and these expectations are, in effect, a contract; often,
+// however, those contracts are either poorly or partially specified, assumed,
+// or misunderstood. These nullability annotations are designed to allow you to
+// formalize those contracts within the codebase.
+//
+// -----------------------------------------------------------------------------
+// Using Nullability Annotations
+// -----------------------------------------------------------------------------
+//
+// It is important to note that these annotations are not distinct strong
+// *types*. They are alias templates defined to be equal to the underlying
+// pointer type. A pointer annotated `Nonnull<T*>`, for example, is simply a
+// pointer of type `T*`. Each annotation acts as a form of documentation about
+// the contract for the given pointer. Each annotation requires providers or
+// consumers of these pointers across API boundaries to take appropriate steps
+// when setting or using these pointers:
+//
+// * "Non-null" pointers should never be null. It is the responsibility of the
+//   provider of this pointer to ensure that the pointer may never be set to
+//   null. Consumers of such pointers can treat such pointers as non-null.
+// * "Nullable" pointers may or may not be null. Consumers of such pointers
+//   should precede any usage of that pointer (e.g. a dereference operation)
+//   with a a `nullptr` check.
+// * "Unknown" pointers may be either "non-null" or "nullable" but have not been
+//   definitively determined to be in either classification state. Providers of
+//   such pointers across API boundaries should determine --  over time -- to
+//   annotate the pointer in either of the above two states. Consumers of such
+//   pointers across an API boundary should continue to treat such pointers as
+//   they currently do.
+//
+// Example:
+//
+// // PaySalary() requires the passed pointer to an `Employee` to be non-null.
+// void PaySalary(absl::Nonnull<Employee *> e) {
+//   pay(e->salary);  // OK to dereference
+// }
+//
+// // CompleteTransaction() guarantees the returned pointer to an `Account` to
+// // be non-null.
+// absl::Nonnull<Account *> balance CompleteTransaction(double fee) {
+// ...
+// }
+//
+// // Note that specifying a nullability annotation does not prevent someone
+// // from violating the contract:
+//
+// Nullable<Employee *> find(Map& employees, std::string_view name);
+//
+// void g(Map& employees) {
+//   Employee *e = find(employees, "Pat");
+//   // `e` can now be null.
+//   PaySalary(e); // Violates contract, but compiles!
+// }
+//
+// Nullability annotations, in other words, are useful for defining and
+// narrowing contracts; *enforcement* of those contracts depends on use and any
+// additional (static or dynamic analysis) tooling.
+//
+// NOTE: The "unknown" annotation state indicates that a pointer's contract has
+// not yet been positively identified. The unknown state therefore acts as a
+// form of documentation of your technical debt, and a codebase that adopts
+// nullability annotations should aspire to annotate every pointer as either
+// "non-null" or "nullable".
+//
+// -----------------------------------------------------------------------------
+// Applicability of Nullability Annotations
+// -----------------------------------------------------------------------------
+//
+// By default, nullability annotations are applicable to raw and smart
+// pointers. User-defined types can indicate compatibility with nullability
+// annotations by providing an `absl_nullability_compatible` nested type. The
+// actual definition of this inner type is not relevant as it is used merely as
+// a marker. It is common to use a using declaration of
+// `absl_nullability_compatible` set to void.
+//
+// // Example:
+// struct MyPtr {
+//   using absl_nullability_compatible = void;
+//   ...
+// };
+//
+// DISCLAIMER:
+// ===========================================================================
+// These nullability annotations are primarily a human readable signal about the
+// intended contract of the pointer. They are not *types* and do not currently
+// provide any correctness guarantees. For example, a pointer annotated as
+// `Nonnull<T*>` is *not guaranteed* to be non-null, and the compiler won't
+// alert or prevent assignment of a `Nullable<T*>` to a `Nonnull<T*>`.
+// ===========================================================================
+#ifndef ABSL_BASE_NULLABILITY_H_
+#define ABSL_BASE_NULLABILITY_H_
+
+#include "absl/base/internal/nullability_impl.h"
+
+namespace absl {
+
+// absl::Nonnull
+//
+// The indicated pointer is never null. It is the responsibility of the provider
+// of this pointer across an API boundary to ensure that the pointer is never be
+// set to null. Consumers of this pointer across an API boundary may safely
+// dereference the pointer.
+//
+// Example:
+//
+// // `employee` is designated as not null.
+// void PaySalary(absl::Nonnull<Employee *> employee) {
+//   pay(*employee);  // OK to dereference
+// }
+template <typename T>
+using Nonnull = nullability_internal::NonnullImpl<T>;
+
+// absl::Nullable
+//
+// The indicated pointer may, by design, be either null or non-null. Consumers
+// of this pointer across an API boundary should perform a `nullptr` check
+// before performing any operation using the pointer.
+//
+// Example:
+//
+// // `employee` may  be null.
+// void PaySalary(absl::Nullable<Employee *> employee) {
+//   if (employee != nullptr) {
+//     Pay(*employee);  // OK to dereference
+//   }
+// }
+template <typename T>
+using Nullable = nullability_internal::NullableImpl<T>;
+
+// absl::NullabilityUnknown (default)
+//
+// The indicated pointer has not yet been determined to be definitively
+// "non-null" or "nullable." Providers of such pointers across API boundaries
+// should, over time, annotate such pointers as either "non-null" or "nullable."
+// Consumers of these pointers across an API boundary should treat such pointers
+// with the same caution they treat currently unannotated pointers. Most
+// existing code will have "unknown"  pointers, which should eventually be
+// migrated into one of the above two nullability states: `Nonnull<T>` or
+//  `Nullable<T>`.
+//
+// NOTE: Because this annotation is the global default state, pointers without
+// any annotation are assumed to have "unknown" semantics. This assumption is
+// designed to minimize churn and reduce clutter within the codebase.
+//
+// Example:
+//
+// // `employee`s nullability state is unknown.
+// void PaySalary(absl::NullabilityUnknown<Employee *> employee) {
+//   Pay(*employee); // Potentially dangerous. API provider should investigate.
+// }
+//
+// Note that a pointer without an annotation, by default, is assumed to have the
+// annotation `NullabilityUnknown`.
+//
+// // `employee`s nullability state is unknown.
+// void PaySalary(Employee* employee) {
+//   Pay(*employee); // Potentially dangerous. API provider should investigate.
+// }
+template <typename T>
+using NullabilityUnknown = nullability_internal::NullabilityUnknownImpl<T>;
+
+}  // namespace absl
+
+#endif  // ABSL_BASE_NULLABILITY_H_