/* -*- 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. */
/** * 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; typedeftypename detail::PointerType<T, DeleterType>::Type Pointer;
UniquePtr(const UniquePtr& aOther) = delete; // construct using std::move()! voidoperator=(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;
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;
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(const UniquePtr& aOther) = delete; // construct using std::move()! voidoperator=(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;
/** * 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.)
*/
/** * 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.
*/
Make a temporary class to support assigning to UniquePtr/unique_ptr via passing a pointer to the callee.
Often, APIs will be shaped like this trivial example: ``` nsresult Foo::NewChildBar(Bar** out) { if (!IsOk()) return NS_ERROR_FAILURE; *out = new Bar(this); return NS_OK; } ```
In order to make this work with unique ptrs, it's often either risky or overwrought: ``` Bar* bar = nullptr; const auto cleanup = MakeScopeExit([&]() { if (bar) { delete bar; } }); if (FAILED(foo->NewChildBar(&bar)) { // handle it } ```
``` UniquePtr<Bar> bar; { Bar* raw = nullptr; const auto res = foo->NewChildBar(&bar); bar.reset(raw); if (FAILED(res) { // handle it } } ``` TempPtrToSettable is a shorthand for the latter approach, allowing something cleaner but also safe:
``` UniquePtr<Bar> bar; if (FAILED(foo->NewChildBar(TempPtrToSetter(&bar))) { // handle it } ```
*/
namespace mozilla { namespace detail {
template <class T, class UniquePtrT> class MOZ_TEMPORARY_CLASS TempPtrToSetterT final { private:
UniquePtrT* const mDest;
T* mNewVal;
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