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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
/* Smart pointer managing sole ownership of a resource. */
#ifndef mozilla_UniquePtr_h
#define mozilla_UniquePtr_h
#include <memory>
#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>>
using UniquePtr = std::unique_ptr<T, D>;
} // 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
/**
* 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;
};
// 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
/**
TempPtrToSetter(UniquePtr<T>*) -> T**-ish
TempPtrToSetter(std::unique_ptr<T>*) -> T**-ish
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;
public:
explicit TempPtrToSetterT(UniquePtrT* dest)
: mDest(dest), mNewVal(mDest->get()) {}
operator T**() { return &mNewVal; }
~TempPtrToSetterT() {
if (mDest->get() != mNewVal) {
mDest->reset(mNewVal);
}
}
};
} // namespace detail
template <class T, class Deleter>
auto TempPtrToSetter(UniquePtr<T, Deleter>* const p) {
return detail::TempPtrToSetterT<T, UniquePtr<T, Deleter>>{p};
}
} // namespace mozilla
namespace std {
template <typename T, class D>
bool operator==(const mozilla::UniquePtr<T, D>& aX, const T* aY) {
return aX.get() == aY;
}
template <typename T, class D>
bool operator==(const T* aY, const mozilla::UniquePtr<T, D>& aX) {
return aY == aX.get();
}
template <typename T, class D>
bool operator!=(const mozilla::UniquePtr<T, D>& aX, const T* aY) {
return aX.get() != aY;
}
template <typename T, class D>
bool operator!=(const T* aY, const mozilla::UniquePtr<T, D>& aX) {
return aY != aX.get();
}
} // namespace std
#endif /* mozilla_UniquePtr_h */