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 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-

  * vim: set shiftwidth=4 tabstop=8 autoindent cindent expandtab: */

 /* 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 nsAutoRef_h_

 #define nsAutoRef_h_

 #include "mozilla/Attributes.h"

 #include "nscore.h" // for nullptr, bool

 template <class T> class nsSimpleRef;

 template <class T> class nsAutoRefBase;

 template <class T> class nsReturnRef;

 template <class T> class nsReturningRef;

 /**

  * template <class T> class nsAutoRef

  *

  * A class that holds a handle to a resource that must be released.

  * No reference is added on construction.

  *

  * No copy constructor nor copy assignment operators are available, so the

  * resource will be held until released on destruction or explicitly

  * |reset()| or transferred through provided methods.

  *

  * The publicly available methods are the public methods on this class and its

  * public base classes |nsAutoRefBase<T>| and |nsSimpleRef<T>|.

  *

  * For ref-counted resources see also |nsCountedRef<T>|.

  * For function return values see |nsReturnRef<T>|.

  *

  * For each class |T|, |nsAutoRefTraits<T>| or |nsSimpleRef<T>| must be

  * specialized to use |nsAutoRef<T>| and |nsCountedRef<T>|.

  *

  * @param T  A class identifying the type of reference held by the

  *           |nsAutoRef<T>| and the unique set methods for managing references

  *           to the resource (defined by |nsAutoRefTraits<T>| or

  *           |nsSimpleRef<T>|).

  *

  *           Often this is the class representing the resource.  Sometimes a

  *           new possibly-incomplete class may need to be declared.

  *

  *

  * Example:  An Automatically closing file descriptor

  *

  * // References that are simple integral types (as file-descriptors are)

  * // usually need a new class to represent the resource and how to handle its

  * // references.

  * class nsRawFD;

  *

  * // Specializing nsAutoRefTraits<nsRawFD> describes how to manage file

  * // descriptors, so that nsAutoRef<nsRawFD> provides automatic closing of

  * // its file descriptor on destruction.

  * template <>

  * class nsAutoRefTraits<nsRawFD> {

  * public:

  *     // The file descriptor is held in an int.

  *     typedef int RawRef;

  *     // -1 means that there is no file associated with the handle.

  *     static int Void() { return -1; }

  *     // The file associated with a file descriptor is released with close().

  *     static void Release(RawRef aFD) { close(aFD); }

  * };

  *

  * // A function returning a file descriptor that must be closed.

  * nsReturnRef<nsRawFD> get_file(const char *filename) {

  *     // Constructing from a raw file descriptor assumes ownership.

  *     nsAutoRef<nsRawFD> fd(open(filename, O_RDONLY));

  *     fcntl(fd, F_SETFD, FD_CLOEXEC);

  *     return fd.out();

  * }

  *

  * void f() {

  *     unsigned char buf[1024];

  *

  *     // Hold a file descriptor for /etc/hosts in fd1.

  *     nsAutoRef<nsRawFD> fd1(get_file("/etc/hosts"));

  *

  *     nsAutoRef<nsRawFD> fd2;

  *     fd2.steal(fd1); // fd2 takes the file descriptor from fd1

  *     ssize_t count = read(fd1, buf, 1024); // error fd1 has no file

  *     count = read(fd2, buf, 1024); // reads from /etc/hosts

  *

  *     // If the file descriptor is not stored then it is closed.

  *     get_file("/etc/login.defs"); // login.defs is closed

  *

  *     // Now use fd1 to hold a file descriptor for /etc/passwd.

  *     fd1 = get_file("/etc/passwd");

  *

  *     // The nsAutoRef<nsRawFD> can give up the file descriptor if explicitly

  *     // instructed, but the caller must then ensure that the file is closed.

  *     int rawfd = fd1.disown();

  *

  *     // Assume ownership of another file descriptor.

  *     fd1.own(open("/proc/1/maps");

  *

  *     // On destruction, fd1 closes /proc/1/maps and fd2 closes /etc/hosts,

  *     // but /etc/passwd is not closed.

  * }

  *

  */

 template <class T>

 class nsAutoRef : public nsAutoRefBase<T>

 {

 protected:

     typedef nsAutoRef<T> ThisClass;

     typedef nsAutoRefBase<T> BaseClass;

     typedef nsSimpleRef<T> SimpleRef;

     typedef typename BaseClass::RawRefOnly RawRefOnly;

     typedef typename BaseClass::LocalSimpleRef LocalSimpleRef;

 public:

     nsAutoRef()

     {

     }

     // Explicit construction is required so as not to risk unintentionally

     // releasing the resource associated with a raw ref.

     explicit nsAutoRef(RawRefOnly aRefToRelease)

         : BaseClass(aRefToRelease)

     {

     }

     // Construction from a nsReturnRef<T> function return value, which expects

     // to give up ownership, transfers ownership.

     // (nsReturnRef<T> is converted to const nsReturningRef<T>.)

     explicit nsAutoRef(const nsReturningRef<T>& aReturning)

         : BaseClass(aReturning)

     {

     }

     // The only assignment operator provided is for transferring from an

     // nsReturnRef smart reference, which expects to pass its ownership to

     // another object.

     //

     // With raw references and other smart references, the type of the lhs and

     // its taking and releasing nature is often not obvious from an assignment

     // statement.  Assignment from a raw ptr especially is not normally

     // expected to release the reference.

     //

     // Use |steal| for taking ownership from other smart refs.

     //

     // For raw references, use |own| to indicate intention to have the

     // resource released.

     //

     // Or, to create another owner of the same reference, use an nsCountedRef.

     ThisClass& operator=(const nsReturningRef<T>& aReturning)

     {

         BaseClass::steal(aReturning.mReturnRef);

         return *this;

     }

     // Conversion to a raw reference allow the nsAutoRef<T> to often be used

     // like a raw reference.

     operator typename SimpleRef::RawRef() const

     {

         return this->get();

     }

     // Transfer ownership from another smart reference.

     void steal(ThisClass& aOtherRef)

     {

         BaseClass::steal(aOtherRef);

     }

     // Assume ownership of a raw ref.

     //

     // |own| has similar function to |steal|, and is useful for receiving

     // ownership from a return value of a function.  It is named differently

     // because |own| requires more care to ensure that the function intends to

     // give away ownership, and so that |steal| can be safely used, knowing

     // that it won't steal ownership from any methods returning raw ptrs to

     // data owned by a foreign object.

     void own(RawRefOnly aRefToRelease)

     {

         BaseClass::own(aRefToRelease);

     }

     // Exchange ownership with |aOther|

     void swap(ThisClass& aOther)

     {

         LocalSimpleRef temp;

         temp.SimpleRef::operator=(*this);

         SimpleRef::operator=(aOther);

         aOther.SimpleRef::operator=(temp);

     }

     // Release the reference now.

     void reset()

     {

         this->SafeRelease();

         LocalSimpleRef empty;

         SimpleRef::operator=(empty);

     }

     // Pass out the reference for a function return values.

     nsReturnRef<T> out()

     {

         return nsReturnRef<T>(this->disown());

     }

     // operator->() and disown() are provided by nsAutoRefBase<T>.

     // The default nsSimpleRef<T> provides get().

 private:

     // No copy constructor

     explicit nsAutoRef(ThisClass& aRefToSteal);

 };

 /**

  * template <class T> class nsCountedRef

  *

  * A class that creates (adds) a new reference to a resource on construction

  * or assignment and releases on destruction.

  *

  * This class is similar to nsAutoRef<T> and inherits its methods, but also

  * provides copy construction and assignment operators that enable more than

  * one concurrent reference to the same resource.

  *

  * Specialize |nsAutoRefTraits<T>| or |nsSimpleRef<T>| to use this.  This

  * class assumes that the resource itself counts references and so can only be

  * used when |T| represents a reference-counting resource.

  */

 template <class T>

 class nsCountedRef : public nsAutoRef<T>

 {

 protected:

     typedef nsCountedRef<T> ThisClass;

     typedef nsAutoRef<T> BaseClass;

     typedef nsSimpleRef<T> SimpleRef;

     typedef typename BaseClass::RawRef RawRef;

 public:

     nsCountedRef()

     {

     }

     // Construction and assignment from a another nsCountedRef

     // or a raw ref copies and increments the ref count.

     nsCountedRef(const ThisClass& aRefToCopy)

     {

         SimpleRef::operator=(aRefToCopy);

         SafeAddRef();

     }

     ThisClass& operator=(const ThisClass& aRefToCopy)

     {

         if (this == &aRefToCopy)

             return *this;

         this->SafeRelease();

         SimpleRef::operator=(aRefToCopy);

         SafeAddRef();

         return *this;

     }

     // Implicit conversion from another smart ref argument (to a raw ref) is

     // accepted here because construction and assignment safely creates a new

     // reference without interfering with the reference to copy.

     explicit nsCountedRef(RawRef aRefToCopy)

         : BaseClass(aRefToCopy)

     {

         SafeAddRef();

     }

     ThisClass& operator=(RawRef aRefToCopy)

     {

         this->own(aRefToCopy);

         SafeAddRef();

         return *this;

     }

     // Construction and assignment from an nsReturnRef function return value,

     // which expects to give up ownership, transfers ownership.

     explicit nsCountedRef(const nsReturningRef<T>& aReturning)

         : BaseClass(aReturning)

     {

     }

     ThisClass& operator=(const nsReturningRef<T>& aReturning)

     {

         BaseClass::operator=(aReturning);

         return *this;

     }

 protected:

     // Increase the reference count if there is a resource.

     void SafeAddRef()

     {

         if (this->HaveResource())

             this->AddRef(this->get());

     }

 };

 /**

  * template <class T> class nsReturnRef

  *

  * A type for function return values that hold a reference to a resource that

  * must be released.  See also |nsAutoRef<T>::out()|.

  */

 template <class T>

 class nsReturnRef : public nsAutoRefBase<T>

 {

 protected:

     typedef nsAutoRefBase<T> BaseClass;

     typedef typename BaseClass::RawRefOnly RawRefOnly;

 public:

     // For constructing a return value with no resource

     nsReturnRef()

     {

     }

     // For returning a smart reference from a raw reference that must be

     // released.  Explicit construction is required so as not to risk

     // unintentionally releasing the resource associated with a raw ref.

     explicit nsReturnRef(RawRefOnly aRefToRelease)

         : BaseClass(aRefToRelease)

     {

     }

     // Copy construction transfers ownership

     nsReturnRef(nsReturnRef<T>& aRefToSteal)

         : BaseClass(aRefToSteal)

     {

     }

     nsReturnRef(const nsReturningRef<T>& aReturning)

         : BaseClass(aReturning)

     {

     }

     // Conversion to a temporary (const) object referring to this object so

     // that the reference may be passed from a function return value

     // (temporary) to another smart reference.  There is no need to use this

     // explicitly.  Simply assign a nsReturnRef<T> function return value to a

     // smart reference.

     operator nsReturningRef<T>()

     {

         return nsReturningRef<T>(*this);

     }

     // No conversion to RawRef operator is provided on nsReturnRef, to ensure

     // that the return value is not carelessly assigned to a raw ptr (and the

     // resource then released).  If passing to a function that takes a raw

     // ptr, use get or disown as appropriate.

 };

 /**

  * template <class T> class nsReturningRef

  *

  * A class to allow ownership to be transferred from nsReturnRef function

  * return values.

  *

  * It should not be necessary for clients to reference this

  * class directly.  Simply pass an nsReturnRef<T> to a parameter taking an

  * |nsReturningRef<T>|.

  *

  * The conversion operator on nsReturnRef constructs a temporary wrapper of

  * class nsReturningRef<T> around a non-const reference to the nsReturnRef.

  * The wrapper can then be passed as an rvalue parameter.

  */

 template <class T>

 class nsReturningRef

 {

 private:

     friend class nsReturnRef<T>;

     explicit nsReturningRef(nsReturnRef<T>& aReturnRef)

         : mReturnRef(aReturnRef)

     {

     }

 public:

     nsReturnRef<T>& mReturnRef;

 };

 /**

  * template <class T> class nsAutoRefTraits

  *

  * A class describing traits of references managed by the default

  * |nsSimpleRef<T>| implementation and thus |nsAutoRef<T>| and |nsCountedRef|.

  * The default |nsSimpleRef<T> is suitable for resources with handles that

  * have a void value.  (If there is no such void value for a handle,

  * specialize |nsSimpleRef<T>|.)

  *

  * Specializations must be provided for each class |T| according to the

  * following pattern:

  *

  * // The template parameter |T| should be a class such that the set of fields

  * // in class nsAutoRefTraits<T> is unique for class |T|.  Usually the

  * // resource object class is sufficient.  For handles that are simple

  * // integral typedefs, a new unique possibly-incomplete class may need to be

  * // declared.

  *

  * template <>

  * class nsAutoRefTraits<T>

  * {

  *     // Specializations must provide a typedef for RawRef, describing the

  *     // type of the handle to the resource.

  *     typedef <handle-type> RawRef;

  *

  *     // Specializations should define Void(), a function returning a value

  *     // suitable for a handle that does not have an associated resource.

  *     //

  *     // The return type must be a suitable as the parameter to a RawRef

  *     // constructor and operator==.

  *     //

  *     // If this method is not accessible then some limited nsAutoRef

  *     // functionality will still be available, but the default constructor,

  *     // |reset|, and most transfer of ownership methods will not be available.

  *     static <return-type> Void();

  *

  *     // Specializations must define Release() to properly finalize the

  *     // handle to a non-void custom-deleted or reference-counted resource.

  *     static void Release(RawRef aRawRef);

  *

  *     // For reference-counted resources, if |nsCountedRef<T>| is to be used,

  *     // specializations must define AddRef to increment the reference count

  *     // held by a non-void handle.

  *     // (AddRef() is not necessary for |nsAutoRef<T>|.)

  *     static void AddRef(RawRef aRawRef);

  * };

  *

  * See nsPointerRefTraits for example specializations for simple pointer

  * references.  See nsAutoRef for an example specialization for a non-pointer

  * reference.

  */

 template <class T> class nsAutoRefTraits;

 /**

  * template <class T> class nsPointerRefTraits

  *

  * A convenience class useful as a base class for specializations of

  * |nsAutoRefTraits<T>| where the handle to the resource is a pointer to |T|.

  * By inheriting from this class, definitions of only Release(RawRef) and

  * possibly AddRef(RawRef) need to be added.

  *

  * Examples of use:

  *

  * template <>

  * class nsAutoRefTraits<PRFileDesc> : public nsPointerRefTraits<PRFileDesc>

  * {

  * public:

  *     static void Release(PRFileDesc *ptr) { PR_Close(ptr); }

  * };

  *

  * template <>

  * class nsAutoRefTraits<FcPattern> : public nsPointerRefTraits<FcPattern>

  * {

  * public:

  *     static void Release(FcPattern *ptr) { FcPatternDestroy(ptr); }

  *     static void AddRef(FcPattern *ptr) { FcPatternReference(ptr); }

  * };

  */

 template <class T>

 class nsPointerRefTraits

 {

 public:

     // The handle is a pointer to T.

     typedef T* RawRef;

     // A nullptr does not have a resource.

     static RawRef Void() { return nullptr; }

 };

 /**

  * template <class T> class nsSimpleRef

  *

  * Constructs a non-smart reference, and provides methods to test whether

  * there is an associated resource and (if so) get its raw handle.

  *

  * A default implementation is suitable for resources with handles that have a

  * void value.  This is not intended for direct use but used by |nsAutoRef<T>|

  * and thus |nsCountedRef<T>|.

  *

  * Specialize this class if there is no particular void value for the resource

  * handle.  A specialized implementation must also provide Release(RawRef),

  * and, if |nsCountedRef<T>| is required, AddRef(RawRef), as described in

  * nsAutoRefTraits<T>.

  */

 template <class T>

 class nsSimpleRef : protected nsAutoRefTraits<T>

 {

 protected:

     // The default implementation uses nsAutoRefTrait<T>.

     // Specializations need not define this typedef.

     typedef nsAutoRefTraits<T> Traits;

     // The type of the handle to the resource.

     // A specialization must provide a typedef for RawRef.

     typedef typename Traits::RawRef RawRef;

     // Construct with no resource.

     //

     // If this constructor is not accessible then some limited nsAutoRef

     // functionality will still be available, but the default constructor,

     // |reset|, and most transfer of ownership methods will not be available.

     nsSimpleRef()

         : mRawRef(Traits::Void())

     {

     }

     // Construct with a handle to a resource.

     // A specialization must provide this. 

     nsSimpleRef(RawRef aRawRef)

         : mRawRef(aRawRef)

     {

     }

     // Test whether there is an associated resource.  A specialization must

     // provide this.  The function is permitted to always return true if the

     // default constructor is not accessible, or if Release (and AddRef) can

     // deal with void handles.

     bool HaveResource() const

     {

         return mRawRef != Traits::Void();

     }

 public:

     // A specialization must provide get() or loose some functionality.  This

     // is inherited by derived classes and the specialization may choose

     // whether it is public or protected.

     RawRef get() const

     {

         return mRawRef;

     }

 private:

     RawRef mRawRef;

 };

 /**

  * template <class T> class nsAutoRefBase

  *

  * Internal base class for |nsAutoRef<T>| and |nsReturnRef<T>|.

  * Adds release on destruction to a |nsSimpleRef<T>|.

  */

 template <class T>

 class nsAutoRefBase : public nsSimpleRef<T>

 {

 protected:

     typedef nsAutoRefBase<T> ThisClass;

     typedef nsSimpleRef<T> SimpleRef;

     typedef typename SimpleRef::RawRef RawRef;

     nsAutoRefBase()

     {

     }

     // A type for parameters that should be passed a raw ref but should not

     // accept implicit conversions (from another smart ref).  (The only

     // conversion to this type is from a raw ref so only raw refs will be

     // accepted.)

     class RawRefOnly

     {

     public:

         RawRefOnly(RawRef aRawRef)

             : mRawRef(aRawRef)

         {

         }

         operator RawRef() const

         {

             return mRawRef;

         }

     private:

         RawRef mRawRef;

     };

     // Construction from a raw ref assumes ownership

     explicit nsAutoRefBase(RawRefOnly aRefToRelease)

         : SimpleRef(aRefToRelease)

     {

     }

     // Constructors that steal ownership

     explicit nsAutoRefBase(ThisClass& aRefToSteal)

         : SimpleRef(aRefToSteal.disown())

     {

     }

     explicit nsAutoRefBase(const nsReturningRef<T>& aReturning)

         : SimpleRef(aReturning.mReturnRef.disown())

     {

     }

     ~nsAutoRefBase()

     {

         SafeRelease();

     }

     // An internal class providing access to protected nsSimpleRef<T>

     // constructors for construction of temporary simple references (that are

     // not ThisClass).

     class LocalSimpleRef : public SimpleRef

     {

     public:

         LocalSimpleRef()

         {

         }

         explicit LocalSimpleRef(RawRef aRawRef)

             : SimpleRef(aRawRef)

         {

         }

     };

 private:

     ThisClass& operator=(const ThisClass& aSmartRef) MOZ_DELETE;

 public:

     RawRef operator->() const

     {

         return this->get();

     }

     // Transfer ownership to a raw reference.

     //

     // THE CALLER MUST ENSURE THAT THE REFERENCE IS EXPLICITLY RELEASED.

     //

     // Is this really what you want to use?  Using this removes any guarantee

     // of release.  Use nsAutoRef<T>::out() for return values, or an

     // nsAutoRef<T> modifiable lvalue for an out parameter.  Use disown() when

     // the reference must be stored in a POD type object, such as may be

     // preferred for a namespace-scope object with static storage duration,

     // for example.

     RawRef disown()

     {

         RawRef temp = this->get();

         LocalSimpleRef empty;

         SimpleRef::operator=(empty);

         return temp;

     }

 protected:

     // steal and own are protected because they make no sense on nsReturnRef,

     // but steal is implemented on this class for access to aOtherRef.disown()

     // when aOtherRef is an nsReturnRef;

     // Transfer ownership from another smart reference.

     void steal(ThisClass& aOtherRef)

     {

         own(aOtherRef.disown());

     }

     // Assume ownership of a raw ref.

     void own(RawRefOnly aRefToRelease)

     {

         SafeRelease();

         LocalSimpleRef ref(aRefToRelease);

         SimpleRef::operator=(ref);

     }

     // Release a resource if there is one.

     void SafeRelease()

     {

         if (this->HaveResource())

             this->Release(this->get());

     }

 };

 #endif // !defined(nsAutoRef_h_)