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Diffstat (limited to 'absl/base/nullability.h')
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diff --git a/absl/base/nullability.h b/absl/base/nullability.h new file mode 100644 index 00000000..42525dd0 --- /dev/null +++ b/absl/base/nullability.h @@ -0,0 +1,224 @@ +// 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::NotNull<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_ |