/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */ /* vim: set ts=8 sts=2 et sw=2 tw=80: */ /* This Source Code Form is subject to the terms of the Mozilla Public * License, v. 2.0. If a copy of the MPL was not distributed with this * file, You can obtain one at http://mozilla.org/MPL/2.0/. */ #ifndef mozilla_NotNull_h #define mozilla_NotNull_h // It's often unclear if a particular pointer, be it raw (T*) or smart // (RefPtr<T>, nsCOMPtr<T>, etc.) can be null. This leads to missing null // checks (which can cause crashes) and unnecessary null checks (which clutter // the code). // // C++ has a built-in alternative that avoids these problems: references. This // module defines another alternative, NotNull, which can be used in cases // where references are not suitable. // // In the comments below we use the word "handle" to cover all varieties of // pointers and references. // // References // ---------- // References are always non-null. (You can do |T& r = *p;| where |p| is null, // but that's undefined behaviour. C++ doesn't provide any built-in, ironclad // guarantee of non-nullness.) // // A reference works well when you need a temporary handle to an existing // single object, e.g. for passing a handle to a function, or as a local handle // within another object. (In Rust parlance, this is a "borrow".) // // A reference is less appropriate in the following cases. // // - As a primary handle to an object. E.g. code such as this is possible but // strange: |T& t = *new T(); ...; delete &t;| // // - As a handle to an array. It's common for |T*| to refer to either a single // |T| or an array of |T|, but |T&| cannot refer to an array of |T| because // you can't index off a reference (at least, not without first converting it // to a pointer). // // - When the handle identity is meaningful, e.g. if you have a hashtable of // handles, because you have to use |&| on the reference to convert it to a // pointer. // // - Some people don't like using non-const references as function parameters, // because it is not clear at the call site that the argument might be // modified. // // - When you need "smart" behaviour. E.g. we lack reference equivalents to // RefPtr and nsCOMPtr. // // - When interfacing with code that uses pointers a lot, sometimes using a // reference just feels like an odd fit. // // Furthermore, a reference is impossible in the following cases. // // - When the handle is rebound to another object. References don't allow this. // // - When the handle has type |void|. |void&| is not allowed. // // NotNull is an alternative that can be used in any of the above cases except // for the last one, where the handle type is |void|. See below. #include "mozilla/Assertions.h" namespace mozilla { // NotNull can be used to wrap a "base" pointer (raw or smart) to indicate it // is not null. Some examples: // // - NotNull<char*> // - NotNull<RefPtr<Event>> // - NotNull<nsCOMPtr<Event>> // // NotNull has the following notable properties. // // - It has zero space overhead. // // - It must be initialized explicitly. There is no default initialization. // // - It auto-converts to the base pointer type. // // - It does not auto-convert from a base pointer. Implicit conversion from a // less-constrained type (e.g. T*) to a more-constrained type (e.g. // NotNull<T*>) is dangerous. Creation and assignment from a base pointer can // only be done with WrapNotNull(), which makes them impossible to overlook, // both when writing and reading code. // // - When initialized (or assigned) it is checked, and if it is null we abort. // This guarantees that it cannot be null. // // - |operator bool()| is deleted. This means you cannot check a NotNull in a // boolean context, which eliminates the possibility of unnecessary null // checks. // // NotNull currently doesn't work with UniquePtr. See // https://github.com/Microsoft/GSL/issues/89 for some discussion. // template <typename T> class NotNull { template <typename U> friend NotNull<U> WrapNotNull(U aBasePtr); T mBasePtr; // This constructor is only used by WrapNotNull(). template <typename U> explicit NotNull(U aBasePtr) : mBasePtr(aBasePtr) {} public: // Disallow default construction. NotNull() = delete; // Construct/assign from another NotNull with a compatible base pointer type. template <typename U> MOZ_IMPLICIT NotNull(const NotNull<U>& aOther) : mBasePtr(aOther.get()) {} // Default copy/move construction and assignment. NotNull(const NotNull<T>&) = default; NotNull<T>& operator=(const NotNull<T>&) = default; NotNull(NotNull<T>&&) = default; NotNull<T>& operator=(NotNull<T>&&) = default; // Disallow null checks, which are unnecessary for this type. explicit operator bool() const = delete; // Explicit conversion to a base pointer. Use only to resolve ambiguity or to // get a castable pointer. const T& get() const { return mBasePtr; } // Implicit conversion to a base pointer. Preferable to get(). operator const T&() const { return get(); } // Dereference operators. const T& operator->() const { return get(); } decltype(*mBasePtr) operator*() const { return *mBasePtr; } }; template <typename T> NotNull<T> WrapNotNull(const T aBasePtr) { NotNull<T> notNull(aBasePtr); MOZ_RELEASE_ASSERT(aBasePtr); return notNull; } // Compare two NotNulls. template <typename T, typename U> inline bool operator==(const NotNull<T>& aLhs, const NotNull<U>& aRhs) { return aLhs.get() == aRhs.get(); } template <typename T, typename U> inline bool operator!=(const NotNull<T>& aLhs, const NotNull<U>& aRhs) { return aLhs.get() != aRhs.get(); } // Compare a NotNull to a base pointer. template <typename T, typename U> inline bool operator==(const NotNull<T>& aLhs, const U& aRhs) { return aLhs.get() == aRhs; } template <typename T, typename U> inline bool operator!=(const NotNull<T>& aLhs, const U& aRhs) { return aLhs.get() != aRhs; } // Compare a base pointer to a NotNull. template <typename T, typename U> inline bool operator==(const T& aLhs, const NotNull<U>& aRhs) { return aLhs == aRhs.get(); } template <typename T, typename U> inline bool operator!=(const T& aLhs, const NotNull<U>& aRhs) { return aLhs != aRhs.get(); } // Disallow comparing a NotNull to a nullptr. template <typename T> bool operator==(const NotNull<T>&, decltype(nullptr)) = delete; template <typename T> bool operator!=(const NotNull<T>&, decltype(nullptr)) = delete; // Disallow comparing a nullptr to a NotNull. template <typename T> bool operator==(decltype(nullptr), const NotNull<T>&) = delete; template <typename T> bool operator!=(decltype(nullptr), const NotNull<T>&) = delete; } // namespace mozilla #endif /* mozilla_NotNull_h */