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Diffstat (limited to 'mfbt/UniquePtr.h')
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diff --git a/mfbt/UniquePtr.h b/mfbt/UniquePtr.h new file mode 100644 index 000000000..7e1035bc6 --- /dev/null +++ b/mfbt/UniquePtr.h @@ -0,0 +1,697 @@ +/* -*- 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 "mozilla/Assertions.h" +#include "mozilla/Attributes.h" +#include "mozilla/Compiler.h" +#include "mozilla/Move.h" +#include "mozilla/Pair.h" +#include "mozilla/TypeTraits.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 IntegralConstant<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, typename RemoveReference<D>::Type>::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 = 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) + * Swap(g3, g4); // g4 now owns the S, g3 cleared + * g3.swap(g4); // g3 now owns the S, g4 cleared + * UniquePtr<S> g5(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(Move(b)); + * + * UniquePtr<Base> b2; + * UniquePtr<Derived, DefaultDelete<Base>> d2(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: + Pair<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(!IsPointer<D>::value, "must provide a deleter instance"); + static_assert(!IsReference<D>::value, "must provide a deleter instance"); + } + + /** + * Construct a UniquePtr containing |aPtr|. + */ + explicit UniquePtr(Pointer aPtr) + : mTuple(aPtr, DeleterType()) + { + static_assert(!IsPointer<D>::value, "must provide a deleter instance"); + static_assert(!IsReference<D>::value, "must provide a deleter instance"); + } + + UniquePtr(Pointer aPtr, + typename Conditional<IsReference<D>::value, + D, + const D&>::Type aD1) + : mTuple(aPtr, aD1) + {} + + // If you encounter an error with MSVC10 about RemoveReference below, along + // the lines that "more than one partial specialization matches the template + // argument list": don't use UniquePtr<T, reference to function>! Ideally + // you should make deletion use the same function every time, using a + // deleter policy: + // + // // BAD, won't compile with MSVC10, deleter doesn't need to be a + // // variable at all + // typedef void (&FreeSignature)(void*); + // UniquePtr<int, FreeSignature> ptr((int*) malloc(sizeof(int)), free); + // + // // GOOD, compiles with MSVC10, deletion behavior statically known and + // // optimizable + // struct DeleteByFreeing + // { + // void operator()(void* aPtr) { free(aPtr); } + // }; + // + // If deletion really, truly, must be a variable: you might be able to work + // around this with a deleter class that contains the function reference. + // But this workaround is untried and untested, because variable deletion + // behavior really isn't something you should use. + UniquePtr(Pointer aPtr, + typename RemoveReference<D>::Type&& aD2) + : mTuple(aPtr, Move(aD2)) + { + static_assert(!IsReference<D>::value, + "rvalue deleter can't be stored by reference"); + } + + UniquePtr(UniquePtr&& aOther) + : mTuple(aOther.release(), Forward<DeleterType>(aOther.get_deleter())) + {} + + MOZ_IMPLICIT + UniquePtr(decltype(nullptr)) + : mTuple(nullptr, DeleterType()) + { + static_assert(!IsPointer<D>::value, "must provide a deleter instance"); + static_assert(!IsReference<D>::value, "must provide a deleter instance"); + } + + template<typename U, class E> + MOZ_IMPLICIT + UniquePtr(UniquePtr<U, E>&& aOther, + typename EnableIf<IsConvertible<typename UniquePtr<U, E>::Pointer, + Pointer>::value && + !IsArray<U>::value && + (IsReference<D>::value + ? IsSame<D, E>::value + : IsConvertible<E, D>::value), + int>::Type aDummy = 0) + : mTuple(aOther.release(), Forward<E>(aOther.get_deleter())) + { + } + + ~UniquePtr() { reset(nullptr); } + + UniquePtr& operator=(UniquePtr&& aOther) + { + reset(aOther.release()); + get_deleter() = Forward<DeleterType>(aOther.get_deleter()); + return *this; + } + + template<typename U, typename E> + UniquePtr& operator=(UniquePtr<U, E>&& aOther) + { + static_assert(IsConvertible<typename UniquePtr<U, E>::Pointer, + Pointer>::value, + "incompatible UniquePtr pointees"); + static_assert(!IsArray<U>::value, + "can't assign from UniquePtr holding an array"); + + reset(aOther.release()); + get_deleter() = Forward<E>(aOther.get_deleter()); + return *this; + } + + UniquePtr& operator=(decltype(nullptr)) + { + reset(nullptr); + return *this; + } + + 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 Move()! + void operator=(const UniquePtr& aOther) = delete; // assign using 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: + Pair<Pointer, DeleterType> mTuple; + +public: + /** + * Construct a UniquePtr containing nullptr. + */ + constexpr UniquePtr() + : mTuple(static_cast<Pointer>(nullptr), DeleterType()) + { + static_assert(!IsPointer<D>::value, "must provide a deleter instance"); + static_assert(!IsReference<D>::value, "must provide a deleter instance"); + } + + /** + * Construct a UniquePtr containing |aPtr|. + */ + explicit UniquePtr(Pointer aPtr) + : mTuple(aPtr, DeleterType()) + { + static_assert(!IsPointer<D>::value, "must provide a deleter instance"); + static_assert(!IsReference<D>::value, "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, + typename EnableIf<IsPointer<U>::value && + IsConvertible<U, Pointer>::value, + int>::Type aDummy = 0) + = delete; + + UniquePtr(Pointer aPtr, + typename Conditional<IsReference<D>::value, + D, + const D&>::Type aD1) + : mTuple(aPtr, aD1) + {} + + // If you encounter an error with MSVC10 about RemoveReference below, along + // the lines that "more than one partial specialization matches the template + // argument list": don't use UniquePtr<T[], reference to function>! See the + // comment by this constructor in the non-T[] specialization above. + UniquePtr(Pointer aPtr, + typename RemoveReference<D>::Type&& aD2) + : mTuple(aPtr, Move(aD2)) + { + static_assert(!IsReference<D>::value, + "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, + typename EnableIf<IsPointer<U>::value && + IsConvertible<U, Pointer>::value, + int>::Type aDummy = 0) + = delete; + + UniquePtr(UniquePtr&& aOther) + : mTuple(aOther.release(), Forward<DeleterType>(aOther.get_deleter())) + {} + + MOZ_IMPLICIT + UniquePtr(decltype(nullptr)) + : mTuple(nullptr, DeleterType()) + { + static_assert(!IsPointer<D>::value, "must provide a deleter instance"); + static_assert(!IsReference<D>::value, "must provide a deleter instance"); + } + + ~UniquePtr() { reset(nullptr); } + + UniquePtr& operator=(UniquePtr&& aOther) + { + reset(aOther.release()); + get_deleter() = 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 Move()! + void operator=(const UniquePtr& aOther) = delete; // assign using 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() {} + + template<typename U> + MOZ_IMPLICIT DefaultDelete(const DefaultDelete<U>& aOther, + typename EnableIf<mozilla::IsConvertible<U*, T*>::value, + int>::Type 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() {} + + 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> +void +Swap(UniquePtr<T, D>& aX, UniquePtr<T, D>& aY) +{ + aX.swap(aY); +} + +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, 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(Forward<Args>(aArgs)...)); +} + +template<typename T> +typename detail::UniqueSelector<T>::UnknownBound +MakeUnique(decltype(sizeof(int)) aN) +{ + typedef typename RemoveExtent<T>::Type ArrayType; + return UniquePtr<T>(new ArrayType[aN]()); +} + +template<typename T, typename... Args> +typename detail::UniqueSelector<T>::KnownBound +MakeUnique(Args&&... aArgs) = delete; + +} // namespace mozilla + +#endif /* mozilla_UniquePtr_h */ |