1// -*- C++ -*-
2//===----------------------------------------------------------------------===//
3//
4// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
5// See https://llvm.org/LICENSE.txt for license information.
6// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
7//
8//===----------------------------------------------------------------------===//
9
10#ifndef _LIBCPP___MEMORY_ALLOCATION_GUARD_H
11#define _LIBCPP___MEMORY_ALLOCATION_GUARD_H
12
13#include <__config>
14#include <__memory/addressof.h>
15#include <__memory/allocator_traits.h>
16#include <__utility/move.h>
17#include <cstddef>
18
19#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
20#  pragma GCC system_header
21#endif
22
23_LIBCPP_PUSH_MACROS
24#include <__undef_macros>
25
26_LIBCPP_BEGIN_NAMESPACE_STD
27
28// Helper class to allocate memory using an Allocator in an exception safe
29// manner.
30//
31// The intended usage of this class is as follows:
32//
33// 0
34// 1     __allocation_guard<SomeAllocator> guard(alloc, 10);
35// 2     do_some_initialization_that_may_throw(guard.__get());
36// 3     save_allocated_pointer_in_a_noexcept_operation(guard.__release_ptr());
37// 4
38//
39// If line (2) throws an exception during initialization of the memory, the
40// guard's destructor will be called, and the memory will be released using
41// Allocator deallocation. Otherwise, we release the memory from the guard on
42// line (3) in an operation that can't throw -- after that, the guard is not
43// responsible for the memory anymore.
44//
45// This is similar to a unique_ptr, except it's easier to use with a
46// custom allocator.
47template <class _Alloc>
48struct __allocation_guard {
49  using _Pointer = typename allocator_traits<_Alloc>::pointer;
50  using _Size    = typename allocator_traits<_Alloc>::size_type;
51
52  template <class _AllocT> // we perform the allocator conversion inside the constructor
53  _LIBCPP_HIDE_FROM_ABI explicit __allocation_guard(_AllocT __alloc, _Size __n)
54      : __alloc_(std::move(__alloc)),
55        __n_(__n),
56        __ptr_(allocator_traits<_Alloc>::allocate(__alloc_, __n_)) // initialization order is important
57  {}
58
59  _LIBCPP_HIDE_FROM_ABI ~__allocation_guard() _NOEXCEPT { __destroy(); }
60
61  _LIBCPP_HIDE_FROM_ABI __allocation_guard(const __allocation_guard&) = delete;
62  _LIBCPP_HIDE_FROM_ABI __allocation_guard(__allocation_guard&& __other) _NOEXCEPT
63      : __alloc_(std::move(__other.__alloc_)),
64        __n_(__other.__n_),
65        __ptr_(__other.__ptr_) {
66    __other.__ptr_ = nullptr;
67  }
68
69  _LIBCPP_HIDE_FROM_ABI __allocation_guard& operator=(const __allocation_guard& __other) = delete;
70  _LIBCPP_HIDE_FROM_ABI __allocation_guard& operator=(__allocation_guard&& __other) _NOEXCEPT {
71    if (std::addressof(__other) != this) {
72      __destroy();
73
74      __alloc_       = std::move(__other.__alloc_);
75      __n_           = __other.__n_;
76      __ptr_         = __other.__ptr_;
77      __other.__ptr_ = nullptr;
78    }
79
80    return *this;
81  }
82
83  _LIBCPP_HIDE_FROM_ABI _Pointer
84  __release_ptr() _NOEXCEPT { // not called __release() because it's a keyword in objective-c++
85    _Pointer __tmp = __ptr_;
86    __ptr_         = nullptr;
87    return __tmp;
88  }
89
90  _LIBCPP_HIDE_FROM_ABI _Pointer __get() const _NOEXCEPT { return __ptr_; }
91
92private:
93  _LIBCPP_HIDE_FROM_ABI void __destroy() _NOEXCEPT {
94    if (__ptr_ != nullptr) {
95      allocator_traits<_Alloc>::deallocate(__alloc_, __ptr_, __n_);
96    }
97  }
98
99  _Alloc __alloc_;
100  _Size __n_;
101  _Pointer __ptr_;
102};
103
104_LIBCPP_END_NAMESPACE_STD
105
106_LIBCPP_POP_MACROS
107
108#endif // _LIBCPP___MEMORY_ALLOCATION_GUARD_H
109