Quelle OwningNonNull.h Sprache: C

/* -*- 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/. */

/* A class for non-null strong pointers to reference-counted objects. */

#ifndef mozilla_OwningNonNull_h
#define mozilla_OwningNonNull_h

#include "nsCOMPtr.h"
#include "nsCycleCollectionNoteChild.h"

namespace mozilla {

// OwningNonNull<T> is similar to a RefPtr<T>, which is not null after initial
// initialization. It has a restricted interface compared to RefPtr, with some
// additional operations defined. The main use is in DOM bindings. Use it
// outside DOM bindings only if you can ensure it never escapes without being
// properly initialized, and you don't need to move it. Otherwise, use a
// RefPtr<T> instead.
//
// Compared to a plain RefPtr<T>, in particular
// - it is copyable but not movable
// - it can be constructed and assigned from T& and is convertible to T&
// implicitly
// - it cannot be cleared by the user once initialized, though it can be
// re-assigned a new (non-null) value
// - it is not convertible to bool, but there is an explicit isInitialized
// member function
//
// Beware that there are two cases where an OwningNonNull<T> actually is nullptr
// - it was default-constructed and not yet initialized
// - it was cleared during CC unlinking.
// All attempts to use it in an invalid state will trigger an assertion in debug
// builds.
//
// The original intent of OwningNonNull<T> was to implement a class with the
// same auto-conversion and annotation semantics as mozilla::dom::NonNull<T>
// (i.e. never null once you have properly initialized it, auto-converts to T&),
// but that holds a strong reference to the object involved. This was designed
// for use in DOM bindings and in particular for storing what WebIDL represents
// as InterfaceName (as opposed to `InterfaceName?`) in various containers
// (dictionaries, sequences). DOM bindings never allow a default-constructed
// uninitialized OwningNonNull to escape. RefPtr could have been used for this
// use case, just like we could have used T* instead of NonNull<T>, but it
// seemed desirable to explicitly annotate the non-null nature of the things
// involved to eliminate pointless null-checks, which otherwise tend to
// proliferate.
template <class T>
class MOZ_IS_SMARTPTR_TO_REFCOUNTED OwningNonNull {
public:
 using element_type = T;

 OwningNonNull() = default;

 MOZ_IMPLICIT OwningNonNull(T& aValue) { init(&aValue); }

 template <class U>
 MOZ_IMPLICIT OwningNonNull(already_AddRefed&& aValue) {
 init(aValue);
 }

 template <class U>
 MOZ_IMPLICIT OwningNonNull(RefPtr&& aValue) {
 init(std::move(aValue));
 }

 template <class U>
 MOZ_IMPLICIT OwningNonNull(const OwningNonNull& aValue) {
 init(aValue);
 }

 // This is no worse than get() in terms of const handling.
 operator T&() const { return ref(); }

 operator T*() const { return get(); }

 // Conversion to bool is always true, so delete to catch errors
 explicit operator bool() const = delete;

 T* operator->() const { return get(); }

 T& operator*() const { return ref(); }

 OwningNonNull<T>& operator=(T* aValue) {
 init(aValue);
 return *this;
 }

 OwningNonNull<T>& operator=(T& aValue) {
 init(&aValue);
 return *this;
 }

 template <class U>
 OwningNonNull<T>& operator=(already_AddRefed&& aValue) {
 init(aValue);
 return *this;
 }

 template <class U>
 OwningNonNull<T>& operator=(RefPtr&& aValue) {
 init(std::move(aValue));
 return *this;
 }

 template <class U>
 OwningNonNull<T>& operator=(const OwningNonNull& aValue) {
 init(aValue);
 return *this;
 }

 // Don't allow assigning nullptr, it makes no sense
 void operator=(decltype(nullptr)) = delete;

 T& ref() const {
 MOZ_ASSERT(mInited);
 MOZ_ASSERT(mPtr, "OwningNonNull was set to null");
 return *mPtr;
 }

 // Make us work with smart pointer helpers that expect a get().
 T* get() const {
 MOZ_ASSERT(mInited);
 MOZ_ASSERT(mPtr, "OwningNonNull was set to null");
 return mPtr;
 }

 template <typename U>
 void swap(U& aOther) {
 mPtr.swap(aOther);
#ifdef DEBUG
 mInited = mPtr;
#endif
 }

 // We have some consumers who want to check whether we're inited in non-debug
 // builds as well. Luckily, we have the invariant that we're inited precisely
 // when mPtr is non-null.
 bool isInitialized() const {
 MOZ_ASSERT(!!mPtr == mInited, "mInited out of sync with mPtr?");
 return mPtr;
 }

private:
 void unlinkForCC() {
#ifdef DEBUG
 mInited = false;
#endif
 mPtr = nullptr;
 }

 // Allow ImplCycleCollectionUnlink to call unlinkForCC().
 template <typename U>
 friend void ImplCycleCollectionUnlink(OwningNonNull& aField);

protected:
 template <typename U>
 void init(U&& aValue) {
 mPtr = std::move(aValue);
 MOZ_ASSERT(mPtr);
#ifdef DEBUG
 mInited = true;
#endif
 }

 RefPtr<T> mPtr;
#ifdef DEBUG
 bool mInited = false;
#endif
};

template <typename T>
inline void ImplCycleCollectionUnlink(OwningNonNull<T>& aField) {
 aField.unlinkForCC();
}

template <typename T>
inline void ImplCycleCollectionTraverse(
 nsCycleCollectionTraversalCallback& aCallback, OwningNonNull<T>& aField,
 const char* aName, uint32_t aFlags = 0) {
 CycleCollectionNoteChild(aCallback, aField.get(), aName, aFlags);
}

} // namespace mozilla

// Declared in nsCOMPtr.h
template <class T>
template <class U>
nsCOMPtr<T>::nsCOMPtr(const mozilla::OwningNonNull& aOther)
 : nsCOMPtr(aOther.get()) {}

template <class T>
template <class U>
nsCOMPtr<T>& nsCOMPtr<T>::operator=(const mozilla::OwningNonNull& aOther) {
 return operator=(aOther.get());
}

// Declared in mozilla/RefPtr.h
template <class T>
template <class U>
RefPtr<T>::RefPtr(const mozilla::OwningNonNull& aOther)
 : RefPtr(aOther.get()) {}

template <class T>
template <class U>
RefPtr<T>& RefPtr<T>::operator=(const mozilla::OwningNonNull& aOther) {
 return operator=(aOther.get());
}

#endif // mozilla_OwningNonNull_h

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