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/* -*- 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/. */
/* Smart pointer managing sole ownership of a resource. */
#ifndef mozilla_UniquePtr_h
#define mozilla_UniquePtr_h
#include <type_traits>
#include <utility>
#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
#include "mozilla/CompactPair.h"
#include "mozilla/Compiler.h"
namespace mozilla {
template <typename T>
class DefaultDelete;
template <typename T, class D = DefaultDelete<T>>
class UniquePtr;
} // namespace mozilla
namespace mozilla {
namespace detail {
struct HasPointerTypeHelper {
template <class U>
static double Test(...);
template <class U>
static char Test(typename U::pointer* = 0);
};
template <class T>
class HasPointerType
: public std::integral_constant<bool, sizeof(HasPointerTypeHelper::Test<T>(
0)) == 1> {};
template <class T, class D, bool = HasPointerType<D>::value>
struct PointerTypeImpl {
typedef typename D::pointer Type;
};
template <class T, class D>
struct PointerTypeImpl<T, D, false> {
typedef T* Type;
};
template <class T, class D>
struct PointerType {
typedef typename PointerTypeImpl<T, std::remove_reference_t<D>>::Type Type;
};
} // namespace detail
/**
* UniquePtr is a smart pointer that wholly owns a resource. Ownership may be
* transferred out of a UniquePtr through explicit action, but otherwise the
* resource is destroyed when the UniquePtr is destroyed.
*
* UniquePtr is similar to C++98's std::auto_ptr, but it improves upon auto_ptr
* in one crucial way: it's impossible to copy a UniquePtr. Copying an auto_ptr
* obviously *can't* copy ownership of its singly-owned resource. So what
* happens if you try to copy one? Bizarrely, ownership is implicitly
* *transferred*, preserving single ownership but breaking code that assumes a
* copy of an object is identical to the original. (This is why auto_ptr is
* prohibited in STL containers.)
*
* UniquePtr solves this problem by being *movable* rather than copyable.
* Instead of passing a |UniquePtr u| directly to the constructor or assignment
* operator, you pass |Move(u)|. In doing so you indicate that you're *moving*
* ownership out of |u|, into the target of the construction/assignment. After
* the transfer completes, |u| contains |nullptr| and may be safely destroyed.
* This preserves single ownership but also allows UniquePtr to be moved by
* algorithms that have been made move-safe. (Note: if |u| is instead a
* temporary expression, don't use |Move()|: just pass the expression, because
* it's already move-ready. For more information see Move.h.)
*
* UniquePtr is also better than std::auto_ptr in that the deletion operation is
* customizable. An optional second template parameter specifies a class that
* (through its operator()(T*)) implements the desired deletion policy. If no
* policy is specified, mozilla::DefaultDelete<T> is used -- which will either
* |delete| or |delete[]| the resource, depending whether the resource is an
* array. Custom deletion policies ideally should be empty classes (no member
* fields, no member fields in base classes, no virtual methods/inheritance),
* because then UniquePtr can be just as efficient as a raw pointer.
*
* Use of UniquePtr proceeds like so:
*
* UniquePtr<int> g1; // initializes to nullptr
* g1.reset(new int); // switch resources using reset()
* g1 = nullptr; // clears g1, deletes the int
*
* UniquePtr<int> g2(new int); // owns that int
* int* p = g2.release(); // g2 leaks its int -- still requires deletion
* delete p; // now freed
*
* struct S { int x; S(int x) : x(x) {} };
* UniquePtr<S> g3, g4(new S(5));
* g3 = std::move(g4); // g3 owns the S, g4 cleared
* S* p = g3.get(); // g3 still owns |p|
* assert(g3->x == 5); // operator-> works (if .get() != nullptr)
* assert((*g3).x == 5); // also operator* (again, if not cleared)
* std::swap(g3, g4); // g4 now owns the S, g3 cleared
* g3.swap(g4); // g3 now owns the S, g4 cleared
* UniquePtr<S> g5(std::move(g3)); // g5 owns the S, g3 cleared
* g5.reset(); // deletes the S, g5 cleared
*
* struct FreePolicy { void operator()(void* p) { free(p); } };
* UniquePtr<int, FreePolicy> g6(static_cast<int*>(malloc(sizeof(int))));
* int* ptr = g6.get();
* g6 = nullptr; // calls free(ptr)
*
* Now, carefully note a few things you *can't* do:
*
* UniquePtr<int> b1;
* b1 = new int; // BAD: can only assign another UniquePtr
* int* ptr = b1; // BAD: no auto-conversion to pointer, use get()
*
* UniquePtr<int> b2(b1); // BAD: can't copy a UniquePtr
* UniquePtr<int> b3 = b1; // BAD: can't copy-assign a UniquePtr
*
* (Note that changing a UniquePtr to store a direct |new| expression is
* permitted, but usually you should use MakeUnique, defined at the end of this
* header.)
*
* A few miscellaneous notes:
*
* UniquePtr, when not instantiated for an array type, can be move-constructed
* and move-assigned, not only from itself but from "derived" UniquePtr<U, E>
* instantiations where U converts to T and E converts to D. If you want to use
* this, you're going to have to specify a deletion policy for both UniquePtr
* instantations, and T pretty much has to have a virtual destructor. In other
* words, this doesn't work:
*
* struct Base { virtual ~Base() {} };
* struct Derived : Base {};
*
* UniquePtr<Base> b1;
* // BAD: DefaultDelete<Base> and DefaultDelete<Derived> don't interconvert
* UniquePtr<Derived> d1(std::move(b));
*
* UniquePtr<Base> b2;
* UniquePtr<Derived, DefaultDelete<Base>> d2(std::move(b2)); // okay
*
* UniquePtr is specialized for array types. Specializing with an array type
* creates a smart-pointer version of that array -- not a pointer to such an
* array.
*
* UniquePtr<int[]> arr(new int[5]);
* arr[0] = 4;
*
* What else is different? Deletion of course uses |delete[]|. An operator[]
* is provided. Functionality that doesn't make sense for arrays is removed.
* The constructors and mutating methods only accept array pointers (not T*, U*
* that converts to T*, or UniquePtr<U[]> or UniquePtr<U>) or |nullptr|.
*
* It's perfectly okay for a function to return a UniquePtr. This transfers
* the UniquePtr's sole ownership of the data, to the fresh UniquePtr created
* in the calling function, that will then solely own that data. Such functions
* can return a local variable UniquePtr, |nullptr|, |UniquePtr(ptr)| where
* |ptr| is a |T*|, or a UniquePtr |Move()|'d from elsewhere.
*
* UniquePtr will commonly be a member of a class, with lifetime equivalent to
* that of that class. If you want to expose the related resource, you could
* expose a raw pointer via |get()|, but ownership of a raw pointer is
* inherently unclear. So it's better to expose a |const UniquePtr&| instead.
* This prohibits mutation but still allows use of |get()| when needed (but
* operator-> is preferred). Of course, you can only use this smart pointer as
* long as the enclosing class instance remains live -- no different than if you
* exposed the |get()| raw pointer.
*
* To pass a UniquePtr-managed resource as a pointer, use a |const UniquePtr&|
* argument. To specify an inout parameter (where the method may or may not
* take ownership of the resource, or reset it), or to specify an out parameter
* (where simply returning a |UniquePtr| isn't possible), use a |UniquePtr&|
* argument. To unconditionally transfer ownership of a UniquePtr
* into a method, use a |UniquePtr| argument. To conditionally transfer
* ownership of a resource into a method, should the method want it, use a
* |UniquePtr&&| argument.
*/
template <typename T, class D>
class UniquePtr {
public:
typedef T ElementType;
typedef D DeleterType;
typedef typename detail::PointerType<T, DeleterType>::Type Pointer;
private:
mozilla::CompactPair<Pointer, DeleterType> mTuple;
Pointer& ptr() { return mTuple.first(); }
const Pointer& ptr() const { return mTuple.first(); }
DeleterType& del() { return mTuple.second(); }
const DeleterType& del() const { return mTuple.second(); }
public:
/**
* Construct a UniquePtr containing |nullptr|.
*/
constexpr UniquePtr() : mTuple(static_cast<Pointer>(nullptr), DeleterType()) {
static_assert(!std::is_pointer_v<D>, "must provide a deleter instance");
static_assert(!std::is_reference_v<D>, "must provide a deleter instance");
}
/**
* Construct a UniquePtr containing |aPtr|.
*/
explicit UniquePtr(Pointer aPtr) : mTuple(aPtr, DeleterType()) {
static_assert(!std::is_pointer_v<D>, "must provide a deleter instance");
static_assert(!std::is_reference_v<D>, "must provide a deleter instance");
}
UniquePtr(Pointer aPtr,
std::conditional_t<std::is_reference_v<D>, D, const D&> aD1)
: mTuple(aPtr, aD1) {}
UniquePtr(Pointer aPtr, std::remove_reference_t<D>&& aD2)
: mTuple(aPtr, std::move(aD2)) {
static_assert(!std::is_reference_v<D>,
"rvalue deleter can't be stored by reference");
}
UniquePtr(UniquePtr&& aOther)
: mTuple(aOther.release(),
std::forward<DeleterType>(aOther.get_deleter())) {}
MOZ_IMPLICIT
UniquePtr(decltype(nullptr)) : mTuple(nullptr, DeleterType()) {
static_assert(!std::is_pointer_v<D>, "must provide a deleter instance");
static_assert(!std::is_reference_v<D>, "must provide a deleter instance");
}
template <typename U, class E>
MOZ_IMPLICIT UniquePtr(
UniquePtr<U, E>&& aOther,
std::enable_if_t<
std::is_convertible_v<typename UniquePtr<U, E>::Pointer, Pointer> &&
!std::is_array_v<U> &&
(std::is_reference_v<D> ? std::is_same_v<D, E>
: std::is_convertible_v<E, D>),
int>
aDummy = 0)
: mTuple(aOther.release(), std::forward<E>(aOther.get_deleter())) {}
~UniquePtr() { reset(nullptr); }
UniquePtr& operator=(UniquePtr&& aOther) {
reset(aOther.release());
get_deleter() = std::forward<DeleterType>(aOther.get_deleter());
return *this;
}
template <typename U, typename E>
UniquePtr& operator=(UniquePtr<U, E>&& aOther) {
static_assert(
std::is_convertible_v<typename UniquePtr<U, E>::Pointer, Pointer>,
"incompatible UniquePtr pointees");
static_assert(!std::is_array_v<U>,
"can't assign from UniquePtr holding an array");
reset(aOther.release());
get_deleter() = std::forward<E>(aOther.get_deleter());
return *this;
}
UniquePtr& operator=(decltype(nullptr)) {
reset(nullptr);
return *this;
}
std::add_lvalue_reference_t<T> operator*() const { return *get(); }
Pointer operator->() const {
MOZ_ASSERT(get(), "dereferencing a UniquePtr containing nullptr");
return get();
}
explicit operator bool() const { return get() != nullptr; }
Pointer get() const { return ptr(); }
DeleterType& get_deleter() { return del(); }
const DeleterType& get_deleter() const { return del(); }
MOZ_MUST_USE Pointer release() {
Pointer p = ptr();
ptr() = nullptr;
return p;
}
void reset(Pointer aPtr = Pointer()) {
Pointer old = ptr();
ptr() = aPtr;
if (old != nullptr) {
get_deleter()(old);
}
}
void swap(UniquePtr& aOther) { mTuple.swap(aOther.mTuple); }
UniquePtr(const UniquePtr& aOther) = delete; // construct using std::move()!
void operator=(const UniquePtr& aOther) =
delete; // assign using std::move()!
};
// In case you didn't read the comment by the main definition (you should!): the
// UniquePtr<T[]> specialization exists to manage array pointers. It deletes
// such pointers using delete[], it will reject construction and modification
// attempts using U* or U[]. Otherwise it works like the normal UniquePtr.
template <typename T, class D>
class UniquePtr<T[], D> {
public:
typedef T* Pointer;
typedef T ElementType;
typedef D DeleterType;
private:
mozilla::CompactPair<Pointer, DeleterType> mTuple;
public:
/**
* Construct a UniquePtr containing nullptr.
*/
constexpr UniquePtr() : mTuple(static_cast<Pointer>(nullptr), DeleterType()) {
static_assert(!std::is_pointer_v<D>, "must provide a deleter instance");
static_assert(!std::is_reference_v<D>, "must provide a deleter instance");
}
/**
* Construct a UniquePtr containing |aPtr|.
*/
explicit UniquePtr(Pointer aPtr) : mTuple(aPtr, DeleterType()) {
static_assert(!std::is_pointer_v<D>, "must provide a deleter instance");
static_assert(!std::is_reference_v<D>, "must provide a deleter instance");
}
// delete[] knows how to handle *only* an array of a single class type. For
// delete[] to work correctly, it must know the size of each element, the
// fields and base classes of each element requiring destruction, and so on.
// So forbid all overloads which would end up invoking delete[] on a pointer
// of the wrong type.
template <typename U>
UniquePtr(U&& aU,
std::enable_if_t<
std::is_pointer_v<U> && std::is_convertible_v<U, Pointer>, int>
aDummy = 0) = delete;
UniquePtr(Pointer aPtr,
std::conditional_t<std::is_reference_v<D>, D, const D&> aD1)
: mTuple(aPtr, aD1) {}
UniquePtr(Pointer aPtr, std::remove_reference_t<D>&& aD2)
: mTuple(aPtr, std::move(aD2)) {
static_assert(!std::is_reference_v<D>,
"rvalue deleter can't be stored by reference");
}
// Forbidden for the same reasons as stated above.
template <typename U, typename V>
UniquePtr(U&& aU, V&& aV,
std::enable_if_t<
std::is_pointer_v<U> && std::is_convertible_v<U, Pointer>, int>
aDummy = 0) = delete;
UniquePtr(UniquePtr&& aOther)
: mTuple(aOther.release(),
std::forward<DeleterType>(aOther.get_deleter())) {}
MOZ_IMPLICIT
UniquePtr(decltype(nullptr)) : mTuple(nullptr, DeleterType()) {
static_assert(!std::is_pointer_v<D>, "must provide a deleter instance");
static_assert(!std::is_reference_v<D>, "must provide a deleter instance");
}
~UniquePtr() { reset(nullptr); }
UniquePtr& operator=(UniquePtr&& aOther) {
reset(aOther.release());
get_deleter() = std::forward<DeleterType>(aOther.get_deleter());
return *this;
}
UniquePtr& operator=(decltype(nullptr)) {
reset();
return *this;
}
explicit operator bool() const { return get() != nullptr; }
T& operator[](decltype(sizeof(int)) aIndex) const { return get()[aIndex]; }
Pointer get() const { return mTuple.first(); }
DeleterType& get_deleter() { return mTuple.second(); }
const DeleterType& get_deleter() const { return mTuple.second(); }
MOZ_MUST_USE Pointer release() {
Pointer p = mTuple.first();
mTuple.first() = nullptr;
return p;
}
void reset(Pointer aPtr = Pointer()) {
Pointer old = mTuple.first();
mTuple.first() = aPtr;
if (old != nullptr) {
mTuple.second()(old);
}
}
void reset(decltype(nullptr)) {
Pointer old = mTuple.first();
mTuple.first() = nullptr;
if (old != nullptr) {
mTuple.second()(old);
}
}
template <typename U>
void reset(U) = delete;
void swap(UniquePtr& aOther) { mTuple.swap(aOther.mTuple); }
UniquePtr(const UniquePtr& aOther) = delete; // construct using std::move()!
void operator=(const UniquePtr& aOther) =
delete; // assign using std::move()!
};
/**
* A default deletion policy using plain old operator delete.
*
* Note that this type can be specialized, but authors should beware of the risk
* that the specialization may at some point cease to match (either because it
* gets moved to a different compilation unit or the signature changes). If the
* non-specialized (|delete|-based) version compiles for that type but does the
* wrong thing, bad things could happen.
*
* This is a non-issue for types which are always incomplete (i.e. opaque handle
* types), since |delete|-ing such a type will always trigger a compilation
* error.
*/
template <typename T>
class DefaultDelete {
public:
constexpr DefaultDelete() = default;
template <typename U>
MOZ_IMPLICIT DefaultDelete(
const DefaultDelete<U>& aOther,
std::enable_if_t<std::is_convertible_v<U*, T*>, int> aDummy = 0) {}
void operator()(T* aPtr) const {
static_assert(sizeof(T) > 0, "T must be complete");
delete aPtr;
}
};
/** A default deletion policy using operator delete[]. */
template <typename T>
class DefaultDelete<T[]> {
public:
constexpr DefaultDelete() = default;
void operator()(T* aPtr) const {
static_assert(sizeof(T) > 0, "T must be complete");
delete[] aPtr;
}
template <typename U>
void operator()(U* aPtr) const = delete;
};
template <typename T, class D, typename U, class E>
bool operator==(const UniquePtr<T, D>& aX, const UniquePtr<U, E>& aY) {
return aX.get() == aY.get();
}
template <typename T, class D, typename U, class E>
bool operator!=(const UniquePtr<T, D>& aX, const UniquePtr<U, E>& aY) {
return aX.get() != aY.get();
}
template <typename T, class D>
bool operator==(const UniquePtr<T, D>& aX, const T* aY) {
return aX.get() == aY;
}
template <typename T, class D>
bool operator==(const T* aY, const UniquePtr<T, D>& aX) {
return aY == aX.get();
}
template <typename T, class D>
bool operator!=(const UniquePtr<T, D>& aX, const T* aY) {
return aX.get() != aY;
}
template <typename T, class D>
bool operator!=(const T* aY, const UniquePtr<T, D>& aX) {
return aY != aX.get();
}
template <typename T, class D>
bool operator==(const UniquePtr<T, D>& aX, decltype(nullptr)) {
return !aX;
}
template <typename T, class D>
bool operator==(decltype(nullptr), const UniquePtr<T, D>& aX) {
return !aX;
}
template <typename T, class D>
bool operator!=(const UniquePtr<T, D>& aX, decltype(nullptr)) {
return bool(aX);
}
template <typename T, class D>
bool operator!=(decltype(nullptr), const UniquePtr<T, D>& aX) {
return bool(aX);
}
// No operator<, operator>, operator<=, operator>= for now because simplicity.
namespace detail {
template <typename T>
struct UniqueSelector {
typedef UniquePtr<T> SingleObject;
};
template <typename T>
struct UniqueSelector<T[]> {
typedef UniquePtr<T[]> UnknownBound;
};
template <typename T, decltype(sizeof(int)) N>
struct UniqueSelector<T[N]> {
typedef UniquePtr<T[N]> KnownBound;
};
} // namespace detail
/**
* MakeUnique is a helper function for allocating new'd objects and arrays,
* returning a UniquePtr containing the resulting pointer. The semantics of
* MakeUnique<Type>(...) are as follows.
*
* If Type is an array T[n]:
* Disallowed, deleted, no overload for you!
* If Type is an array T[]:
* MakeUnique<T[]>(size_t) is the only valid overload. The pointer returned
* is as if by |new T[n]()|, which value-initializes each element. (If T
* isn't a class type, this will zero each element. If T is a class type,
* then roughly speaking, each element will be constructed using its default
* constructor. See C++11 [dcl.init]p7 for the full gory details.)
* If Type is non-array T:
* The arguments passed to MakeUnique<T>(...) are forwarded into a
* |new T(...)| call, initializing the T as would happen if executing
* |T(...)|.
*
* There are various benefits to using MakeUnique instead of |new| expressions.
*
* First, MakeUnique eliminates use of |new| from code entirely. If objects are
* only created through UniquePtr, then (assuming all explicit release() calls
* are safe, including transitively, and no type-safety casting funniness)
* correctly maintained ownership of the UniquePtr guarantees no leaks are
* possible. (This pays off best if a class is only ever created through a
* factory method on the class, using a private constructor.)
*
* Second, initializing a UniquePtr using a |new| expression requires repeating
* the name of the new'd type, whereas MakeUnique in concert with the |auto|
* keyword names it only once:
*
* UniquePtr<char> ptr1(new char()); // repetitive
* auto ptr2 = MakeUnique<char>(); // shorter
*
* Of course this assumes the reader understands the operation MakeUnique
* performs. In the long run this is probably a reasonable assumption. In the
* short run you'll have to use your judgment about what readers can be expected
* to know, or to quickly look up.
*
* Third, a call to MakeUnique can be assigned directly to a UniquePtr. In
* contrast you can't assign a pointer into a UniquePtr without using the
* cumbersome reset().
*
* UniquePtr<char> p;
* p = new char; // ERROR
* p.reset(new char); // works, but fugly
* p = MakeUnique<char>(); // preferred
*
* (And third, although not relevant to Mozilla: MakeUnique is exception-safe.
* An exception thrown after |new T| succeeds will leak that memory, unless the
* pointer is assigned to an object that will manage its ownership. UniquePtr
* ably serves this function.)
*/
template <typename T, typename... Args>
typename detail::UniqueSelector<T>::SingleObject MakeUnique(Args&&... aArgs) {
return UniquePtr<T>(new T(std::forward<Args>(aArgs)...));
}
template <typename T>
typename detail::UniqueSelector<T>::UnknownBound MakeUnique(
decltype(sizeof(int)) aN) {
using ArrayType = std::remove_extent_t<T>;
return UniquePtr<T>(new ArrayType[aN]());
}
template <typename T, typename... Args>
typename detail::UniqueSelector<T>::KnownBound MakeUnique(Args&&... aArgs) =
delete;
/**
* WrapUnique is a helper function to transfer ownership from a raw pointer
* into a UniquePtr<T>. It can only be used with a single non-array type.
*
* It is generally used this way:
*
* auto p = WrapUnique(new char);
*
* It can be used when MakeUnique is not usable, for example, when the
* constructor you are using is private, or you want to use aggregate
* initialization.
*/
template <typename T>
typename detail::UniqueSelector<T>::SingleObject WrapUnique(T* aPtr) {
return UniquePtr<T>(aPtr);
}
} // namespace mozilla
namespace std {
template <typename T, class D>
void swap(mozilla::UniquePtr<T, D>& aX, mozilla::UniquePtr<T, D>& aY) {
aX.swap(aY);
}
} // namespace std
#endif /* mozilla_UniquePtr_h */