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Diffstat (limited to 'absl/memory/memory.h')
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diff --git a/absl/memory/memory.h b/absl/memory/memory.h new file mode 100644 index 00000000..c6799608 --- /dev/null +++ b/absl/memory/memory.h @@ -0,0 +1,622 @@ +// Copyright 2017 The Abseil Authors. +// +// Licensed under the Apache License, Version 2.0 (the "License"); +// you may not use this file except in compliance with the License. +// You may obtain a copy of the License at +// +// http://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, software +// distributed under the License is distributed on an "AS IS" BASIS, +// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +// See the License for the specific language governing permissions and +// limitations under the License. +// +// ----------------------------------------------------------------------------- +// File: memory.h +// ----------------------------------------------------------------------------- +// +// This header file contains utility functions for managing the creation and +// conversion of smart pointers. This file is an extension to the C++ +// standard <memory> library header file. + +#ifndef ABSL_MEMORY_MEMORY_H_ +#define ABSL_MEMORY_MEMORY_H_ + +#include <cstddef> +#include <limits> +#include <memory> +#include <new> +#include <type_traits> +#include <utility> + +#include "absl/meta/type_traits.h" + +namespace absl { + +// ----------------------------------------------------------------------------- +// Function Template: WrapUnique() +// ----------------------------------------------------------------------------- +// +// Transfers ownership of a raw pointer to a `std::unique_ptr`. The returned +// value is a `std::unique_ptr` of deduced type. +// +// Example: +// X* NewX(int, int); +// auto x = WrapUnique(NewX(1, 2)); // 'x' is std::unique_ptr<X>. +// +// `absl::WrapUnique` is useful for capturing the output of a raw pointer +// factory. However, prefer 'absl::make_unique<T>(args...) over +// 'absl::WrapUnique(new T(args...))'. +// +// auto x = WrapUnique(new X(1, 2)); // works, but nonideal. +// auto x = make_unique<X>(1, 2); // safer, standard, avoids raw 'new'. +// +// Note that `absl::WrapUnique(p)` is valid only if `delete p` is a valid +// expression. In particular, `absl::WrapUnique()` cannot wrap pointers to +// arrays, functions or void, and it must not be used to capture pointers +// obtained from array-new expressions (even though that would compile!). +template <typename T> +std::unique_ptr<T> WrapUnique(T* ptr) { + static_assert(!std::is_array<T>::value, "array types are unsupported"); + static_assert(std::is_object<T>::value, "non-object types are unsupported"); + return std::unique_ptr<T>(ptr); +} + +namespace memory_internal { + +// Traits to select proper overload and return type for `absl::make_unique<>`. +template <typename T> +struct MakeUniqueResult { + using scalar = std::unique_ptr<T>; +}; +template <typename T> +struct MakeUniqueResult<T[]> { + using array = std::unique_ptr<T[]>; +}; +template <typename T, size_t N> +struct MakeUniqueResult<T[N]> { + using invalid = void; +}; + +} // namespace memory_internal + +// ----------------------------------------------------------------------------- +// Function Template: make_unique<T>() +// ----------------------------------------------------------------------------- +// +// Creates a `std::unique_ptr<>`, while avoiding issues creating temporaries +// during the construction process. `absl::make_unique<>` also avoids redundant +// type declarations, by avoiding the need to explicitly use the `new` operator. +// +// This implementation of `absl::make_unique<>` is designed for C++11 code and +// will be replaced in C++14 by the equivalent `std::make_unique<>` abstraction. +// `absl::make_unique<>` is designed to be 100% compatible with +// `std::make_unique<>` so that the eventual migration will involve a simple +// rename operation. +// +// For more background on why `std::unique_ptr<T>(new T(a,b))` is problematic, +// see Herb Sutter's explanation on +// (Exception-Safe Function Calls)[http://herbsutter.com/gotw/_102/]. +// (In general, reviewers should treat `new T(a,b)` with scrutiny.) +// +// Example usage: +// +// auto p = make_unique<X>(args...); // 'p' is a std::unique_ptr<X> +// auto pa = make_unique<X[]>(5); // 'pa' is a std::unique_ptr<X[]> +// +// Three overloads of `absl::make_unique` are required: +// +// - For non-array T: +// +// Allocates a T with `new T(std::forward<Args> args...)`, +// forwarding all `args` to T's constructor. +// Returns a `std::unique_ptr<T>` owning that object. +// +// - For an array of unknown bounds T[]: +// +// `absl::make_unique<>` will allocate an array T of type U[] with +// `new U[n]()` and return a `std::unique_ptr<U[]>` owning that array. +// +// Note that 'U[n]()' is different from 'U[n]', and elements will be +// value-initialized. Note as well that `std::unique_ptr` will perform its +// own destruction of the array elements upon leaving scope, even though +// the array [] does not have a default destructor. +// +// NOTE: an array of unknown bounds T[] may still be (and often will be) +// initialized to have a size, and will still use this overload. E.g: +// +// auto my_array = absl::make_unique<int[]>(10); +// +// - For an array of known bounds T[N]: +// +// `absl::make_unique<>` is deleted (like with `std::make_unique<>`) as +// this overload is not useful. +// +// NOTE: an array of known bounds T[N] is not considered a useful +// construction, and may cause undefined behavior in templates. E.g: +// +// auto my_array = absl::make_unique<int[10]>(); +// +// In those cases, of course, you can still use the overload above and +// simply initialize it to its desired size: +// +// auto my_array = absl::make_unique<int[]>(10); + +// `absl::make_unique` overload for non-array types. +template <typename T, typename... Args> +typename memory_internal::MakeUniqueResult<T>::scalar make_unique( + Args&&... args) { + return std::unique_ptr<T>(new T(std::forward<Args>(args)...)); +} + +// `absl::make_unique` overload for an array T[] of unknown bounds. +// The array allocation needs to use the `new T[size]` form and cannot take +// element constructor arguments. The `std::unique_ptr` will manage destructing +// these array elements. +template <typename T> +typename memory_internal::MakeUniqueResult<T>::array make_unique(size_t n) { + return std::unique_ptr<T>(new typename absl::remove_extent_t<T>[n]()); +} + +// `absl::make_unique` overload for an array T[N] of known bounds. +// This construction will be rejected. +template <typename T, typename... Args> +typename memory_internal::MakeUniqueResult<T>::invalid make_unique( + Args&&... /* args */) = delete; + +// ----------------------------------------------------------------------------- +// Function Template: RawPtr() +// ----------------------------------------------------------------------------- +// +// Extracts the raw pointer from a pointer-like 'ptr'. `absl::RawPtr` is useful +// within templates that need to handle a complement of raw pointers, +// `std::nullptr_t`, and smart pointers. +template <typename T> +auto RawPtr(T&& ptr) -> decltype(&*ptr) { + // ptr is a forwarding reference to support Ts with non-const operators. + return (ptr != nullptr) ? &*ptr : nullptr; +} +inline std::nullptr_t RawPtr(std::nullptr_t) { return nullptr; } + +// ----------------------------------------------------------------------------- +// Function Template: ShareUniquePtr() +// ----------------------------------------------------------------------------- +// +// Transforms a `std::unique_ptr` rvalue into a `std::shared_ptr`. The returned +// value is a `std::shared_ptr` of deduced type and ownership is transferred to +// the shared pointer. +// +// Example: +// +// auto up = absl::make_unique<int>(10); +// auto sp = absl::ShareUniquePtr(std::move(up)); // shared_ptr<int> +// CHECK_EQ(*sp, 10); +// CHECK(up == nullptr); +// +// Note that this conversion is correct even when T is an array type, although +// the resulting shared pointer may not be very useful. +// +// Implements the resolution of [LWG 2415](http://wg21.link/lwg2415), by which a +// null shared pointer does not attempt to call the deleter. +template <typename T, typename D> +std::shared_ptr<T> ShareUniquePtr(std::unique_ptr<T, D>&& ptr) { + return ptr ? std::shared_ptr<T>(std::move(ptr)) : std::shared_ptr<T>(); +} + +// ----------------------------------------------------------------------------- +// Function Template: WeakenPtr() +// ----------------------------------------------------------------------------- +// +// Creates a weak pointer associated with a given shared pointer. The returned +// value is a `std::weak_ptr` of deduced type. +// +// Example: +// +// auto sp = std::make_shared<int>(10); +// auto wp = absl::WeakenPtr(sp); +// CHECK_EQ(sp.get(), wp.lock().get()); +// sp.reset(); +// CHECK(wp.lock() == nullptr); +// +template <typename T> +std::weak_ptr<T> WeakenPtr(const std::shared_ptr<T>& ptr) { + return std::weak_ptr<T>(ptr); +} + +namespace memory_internal { + +// ExtractOr<E, O, D>::type evaluates to E<O> if possible. Otherwise, D. +template <template <typename> class Extract, typename Obj, typename Default, + typename> +struct ExtractOr { + using type = Default; +}; + +template <template <typename> class Extract, typename Obj, typename Default> +struct ExtractOr<Extract, Obj, Default, void_t<Extract<Obj>>> { + using type = Extract<Obj>; +}; + +template <template <typename> class Extract, typename Obj, typename Default> +using ExtractOrT = typename ExtractOr<Extract, Obj, Default, void>::type; + +// Extractors for the features of allocators. +template <typename T> +using GetPointer = typename T::pointer; + +template <typename T> +using GetConstPointer = typename T::const_pointer; + +template <typename T> +using GetVoidPointer = typename T::void_pointer; + +template <typename T> +using GetConstVoidPointer = typename T::const_void_pointer; + +template <typename T> +using GetDifferenceType = typename T::difference_type; + +template <typename T> +using GetSizeType = typename T::size_type; + +template <typename T> +using GetPropagateOnContainerCopyAssignment = + typename T::propagate_on_container_copy_assignment; + +template <typename T> +using GetPropagateOnContainerMoveAssignment = + typename T::propagate_on_container_move_assignment; + +template <typename T> +using GetPropagateOnContainerSwap = typename T::propagate_on_container_swap; + +template <typename T> +using GetIsAlwaysEqual = typename T::is_always_equal; + +template <typename T> +struct GetFirstArg; + +template <template <typename...> class Class, typename T, typename... Args> +struct GetFirstArg<Class<T, Args...>> { + using type = T; +}; + +template <typename Ptr, typename = void> +struct ElementType { + using type = typename GetFirstArg<Ptr>::type; +}; + +template <typename T> +struct ElementType<T, void_t<typename T::element_type>> { + using type = typename T::element_type; +}; + +template <typename T, typename U> +struct RebindFirstArg; + +template <template <typename...> class Class, typename T, typename... Args, + typename U> +struct RebindFirstArg<Class<T, Args...>, U> { + using type = Class<U, Args...>; +}; + +template <typename T, typename U, typename = void> +struct RebindPtr { + using type = typename RebindFirstArg<T, U>::type; +}; + +template <typename T, typename U> +struct RebindPtr<T, U, void_t<typename T::template rebind<U>>> { + using type = typename T::template rebind<U>; +}; + +template <typename T, typename U, typename = void> +struct RebindAlloc { + using type = typename RebindFirstArg<T, U>::type; +}; + +template <typename T, typename U> +struct RebindAlloc<T, U, void_t<typename T::template rebind<U>::other>> { + using type = typename T::template rebind<U>::other; +}; + +} // namespace memory_internal + +// ----------------------------------------------------------------------------- +// Class Template: pointer_traits +// ----------------------------------------------------------------------------- +// +// An implementation of C++11's std::pointer_traits. +// +// Provided for portability on toolchains that have a working C++11 compiler, +// but the standard library is lacking in C++11 support. For example, some +// version of the Android NDK. +// + +template <typename Ptr> +struct pointer_traits { + using pointer = Ptr; + + // element_type: + // Ptr::element_type if present. Otherwise T if Ptr is a template + // instantiation Template<T, Args...> + using element_type = typename memory_internal::ElementType<Ptr>::type; + + // difference_type: + // Ptr::difference_type if present, otherwise std::ptrdiff_t + using difference_type = + memory_internal::ExtractOrT<memory_internal::GetDifferenceType, Ptr, + std::ptrdiff_t>; + + // rebind: + // Ptr::rebind<U> if exists, otherwise Template<U, Args...> if Ptr is a + // template instantiation Template<T, Args...> + template <typename U> + using rebind = typename memory_internal::RebindPtr<Ptr, U>::type; + + // pointer_to: + // Calls Ptr::pointer_to(r) + static pointer pointer_to(element_type& r) { // NOLINT(runtime/references) + return Ptr::pointer_to(r); + } +}; + +// Specialization for T*. +template <typename T> +struct pointer_traits<T*> { + using pointer = T*; + using element_type = T; + using difference_type = std::ptrdiff_t; + + template <typename U> + using rebind = U*; + + // pointer_to: + // Calls std::addressof(r) + static pointer pointer_to( + element_type& r) noexcept { // NOLINT(runtime/references) + return std::addressof(r); + } +}; + +// ----------------------------------------------------------------------------- +// Class Template: allocator_traits +// ----------------------------------------------------------------------------- +// +// A C++11 compatible implementation of C++17's std::allocator_traits. +// +template <typename Alloc> +struct allocator_traits { + using allocator_type = Alloc; + + // value_type: + // Alloc::value_type + using value_type = typename Alloc::value_type; + + // pointer: + // Alloc::pointer if present, otherwise value_type* + using pointer = memory_internal::ExtractOrT<memory_internal::GetPointer, + Alloc, value_type*>; + + // const_pointer: + // Alloc::const_pointer if present, otherwise + // absl::pointer_traits<pointer>::rebind<const value_type> + using const_pointer = + memory_internal::ExtractOrT<memory_internal::GetConstPointer, Alloc, + typename absl::pointer_traits<pointer>:: + template rebind<const value_type>>; + + // void_pointer: + // Alloc::void_pointer if present, otherwise + // absl::pointer_traits<pointer>::rebind<void> + using void_pointer = memory_internal::ExtractOrT< + memory_internal::GetVoidPointer, Alloc, + typename absl::pointer_traits<pointer>::template rebind<void>>; + + // const_void_pointer: + // Alloc::const_void_pointer if present, otherwise + // absl::pointer_traits<pointer>::rebind<const void> + using const_void_pointer = memory_internal::ExtractOrT< + memory_internal::GetConstVoidPointer, Alloc, + typename absl::pointer_traits<pointer>::template rebind<const void>>; + + // difference_type: + // Alloc::difference_type if present, otherwise + // absl::pointer_traits<pointer>::difference_type + using difference_type = memory_internal::ExtractOrT< + memory_internal::GetDifferenceType, Alloc, + typename absl::pointer_traits<pointer>::difference_type>; + + // size_type: + // Alloc::size_type if present, otherwise + // std::make_unsigned<difference_type>::type + using size_type = memory_internal::ExtractOrT< + memory_internal::GetSizeType, Alloc, + typename std::make_unsigned<difference_type>::type>; + + // propagate_on_container_copy_assignment: + // Alloc::propagate_on_container_copy_assignment if present, otherwise + // std::false_type + using propagate_on_container_copy_assignment = memory_internal::ExtractOrT< + memory_internal::GetPropagateOnContainerCopyAssignment, Alloc, + std::false_type>; + + // propagate_on_container_move_assignment: + // Alloc::propagate_on_container_move_assignment if present, otherwise + // std::false_type + using propagate_on_container_move_assignment = memory_internal::ExtractOrT< + memory_internal::GetPropagateOnContainerMoveAssignment, Alloc, + std::false_type>; + + // propagate_on_container_swap: + // Alloc::propagate_on_container_swap if present, otherwise std::false_type + using propagate_on_container_swap = + memory_internal::ExtractOrT<memory_internal::GetPropagateOnContainerSwap, + Alloc, std::false_type>; + + // is_always_equal: + // Alloc::is_always_equal if present, otherwise std::is_empty<Alloc>::type + using is_always_equal = + memory_internal::ExtractOrT<memory_internal::GetIsAlwaysEqual, Alloc, + typename std::is_empty<Alloc>::type>; + + // rebind_alloc: + // Alloc::rebind<T>::other if present, otherwise Alloc<T, Args> if this Alloc + // is Alloc<U, Args> + template <typename T> + using rebind_alloc = typename memory_internal::RebindAlloc<Alloc, T>::type; + + // rebind_traits: + // absl::allocator_traits<rebind_alloc<T>> + template <typename T> + using rebind_traits = absl::allocator_traits<rebind_alloc<T>>; + + // allocate(Alloc& a, size_type n): + // Calls a.allocate(n) + static pointer allocate(Alloc& a, // NOLINT(runtime/references) + size_type n) { + return a.allocate(n); + } + + // allocate(Alloc& a, size_type n, const_void_pointer hint): + // Calls a.allocate(n, hint) if possible. + // If not possible, calls a.allocate(n) + static pointer allocate(Alloc& a, size_type n, // NOLINT(runtime/references) + const_void_pointer hint) { + return allocate_impl(0, a, n, hint); + } + + // deallocate(Alloc& a, pointer p, size_type n): + // Calls a.deallocate(p, n) + static void deallocate(Alloc& a, pointer p, // NOLINT(runtime/references) + size_type n) { + a.deallocate(p, n); + } + + // construct(Alloc& a, T* p, Args&&... args): + // Calls a.construct(p, std::forward<Args>(args)...) if possible. + // If not possible, calls + // ::new (static_cast<void*>(p)) T(std::forward<Args>(args)...) + template <typename T, typename... Args> + static void construct(Alloc& a, T* p, // NOLINT(runtime/references) + Args&&... args) { + construct_impl(0, a, p, std::forward<Args>(args)...); + } + + // destroy(Alloc& a, T* p): + // Calls a.destroy(p) if possible. If not possible, calls p->~T(). + template <typename T> + static void destroy(Alloc& a, T* p) { // NOLINT(runtime/references) + destroy_impl(0, a, p); + } + + // max_size(const Alloc& a): + // Returns a.max_size() if possible. If not possible, returns + // std::numeric_limits<size_type>::max() / sizeof(value_type) + static size_type max_size(const Alloc& a) { return max_size_impl(0, a); } + + // select_on_container_copy_construction(const Alloc& a): + // Returns a.select_on_container_copy_construction() if possible. + // If not possible, returns a. + static Alloc select_on_container_copy_construction(const Alloc& a) { + return select_on_container_copy_construction_impl(0, a); + } + + private: + template <typename A> + static auto allocate_impl(int, A& a, // NOLINT(runtime/references) + size_type n, const_void_pointer hint) + -> decltype(a.allocate(n, hint)) { + return a.allocate(n, hint); + } + static pointer allocate_impl(char, Alloc& a, // NOLINT(runtime/references) + size_type n, const_void_pointer) { + return a.allocate(n); + } + + template <typename A, typename... Args> + static auto construct_impl(int, A& a, // NOLINT(runtime/references) + Args&&... args) + -> decltype(a.construct(std::forward<Args>(args)...)) { + a.construct(std::forward<Args>(args)...); + } + + template <typename T, typename... Args> + static void construct_impl(char, Alloc&, T* p, Args&&... args) { + ::new (static_cast<void*>(p)) T(std::forward<Args>(args)...); + } + + template <typename A, typename T> + static auto destroy_impl(int, A& a, // NOLINT(runtime/references) + T* p) -> decltype(a.destroy(p)) { + a.destroy(p); + } + template <typename T> + static void destroy_impl(char, Alloc&, T* p) { + p->~T(); + } + + template <typename A> + static auto max_size_impl(int, const A& a) -> decltype(a.max_size()) { + return a.max_size(); + } + static size_type max_size_impl(char, const Alloc&) { + return std::numeric_limits<size_type>::max() / sizeof(value_type); + } + + template <typename A> + static auto select_on_container_copy_construction_impl(int, const A& a) + -> decltype(a.select_on_container_copy_construction()) { + return a.select_on_container_copy_construction(); + } + static Alloc select_on_container_copy_construction_impl(char, + const Alloc& a) { + return a; + } +}; + +namespace memory_internal { + +// This template alias transforms Alloc::is_nothrow into a metafunction with +// Alloc as a parameter so it can be used with ExtractOrT<>. +template <typename Alloc> +using GetIsNothrow = typename Alloc::is_nothrow; + +} // namespace memory_internal + +// ABSL_ALLOCATOR_NOTHROW is a build time configuration macro for user to +// specify whether the default allocation function can throw or never throws. +// If the allocation function never throws, user should define it to a non-zero +// value (e.g. via `-DABSL_ALLOCATOR_NOTHROW`). +// If the allocation function can throw, user should leave it undefined or +// define it to zero. +// +// allocator_is_nothrow<Alloc> is a traits class that derives from +// Alloc::is_nothrow if present, otherwise std::false_type. It's specialized +// for Alloc = std::allocator<T> for any type T according to the state of +// ABSL_ALLOCATOR_NOTHROW. +// +// default_allocator_is_nothrow is a class that derives from std::true_type +// when the default allocator (global operator new) never throws, and +// std::false_type when it can throw. It is a convenience shorthand for writing +// allocator_is_nothrow<std::allocator<T>> (T can be any type). +// NOTE: allocator_is_nothrow<std::allocator<T>> is guaranteed to derive from +// the same type for all T, because users should specialize neither +// allocator_is_nothrow nor std::allocator. +template <typename Alloc> +struct allocator_is_nothrow + : memory_internal::ExtractOrT<memory_internal::GetIsNothrow, Alloc, + std::false_type> {}; + +#if ABSL_ALLOCATOR_NOTHROW +template <typename T> +struct allocator_is_nothrow<std::allocator<T>> : std::true_type {}; +struct default_allocator_is_nothrow : std::true_type {}; +#else +struct default_allocator_is_nothrow : std::false_type {}; +#endif + +} // namespace absl + +#endif // ABSL_MEMORY_MEMORY_H_ |