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.
47 template <class _Alloc>
48 struct __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
__allocation_guard__allocation_guard53   _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 
~__allocation_guard__allocation_guard59   _LIBCPP_HIDE_FROM_ABI ~__allocation_guard() _NOEXCEPT { __destroy(); }
60 
61   _LIBCPP_HIDE_FROM_ABI __allocation_guard(const __allocation_guard&) = delete;
__allocation_guard__allocation_guard62   _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
__release_ptr__allocation_guard84   __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 
__get__allocation_guard90   _LIBCPP_HIDE_FROM_ABI _Pointer __get() const _NOEXCEPT { return __ptr_; }
91 
92 private:
__destroy__allocation_guard93   _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