xref: /freebsd/contrib/llvm-project/libcxx/include/__memory/allocation_guard.h (revision e6bfd18d21b225af6a0ed67ceeaf1293b7b9eba5)
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/allocator_traits.h>
15 #include <__utility/move.h>
16 #include <cstddef>
17 
18 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
19 #  pragma GCC system_header
20 #endif
21 
22 _LIBCPP_BEGIN_NAMESPACE_STD
23 
24 // Helper class to allocate memory using an Allocator in an exception safe
25 // manner.
26 //
27 // The intended usage of this class is as follows:
28 //
29 // 0
30 // 1     __allocation_guard<SomeAllocator> guard(alloc, 10);
31 // 2     do_some_initialization_that_may_throw(guard.__get());
32 // 3     save_allocated_pointer_in_a_noexcept_operation(guard.__release_ptr());
33 // 4
34 //
35 // If line (2) throws an exception during initialization of the memory, the
36 // guard's destructor will be called, and the memory will be released using
37 // Allocator deallocation. Otherwise, we release the memory from the guard on
38 // line (3) in an operation that can't throw -- after that, the guard is not
39 // responsible for the memory anymore.
40 //
41 // This is similar to a unique_ptr, except it's easier to use with a
42 // custom allocator.
43 template<class _Alloc>
44 struct __allocation_guard {
45     using _Pointer = typename allocator_traits<_Alloc>::pointer;
46     using _Size = typename allocator_traits<_Alloc>::size_type;
47 
48     template<class _AllocT> // we perform the allocator conversion inside the constructor
49     _LIBCPP_HIDE_FROM_ABI
50     explicit __allocation_guard(_AllocT __alloc, _Size __n)
51         : __alloc_(_VSTD::move(__alloc))
52         , __n_(__n)
53         , __ptr_(allocator_traits<_Alloc>::allocate(__alloc_, __n_)) // initialization order is important
54     { }
55 
56     _LIBCPP_HIDE_FROM_ABI
57     ~__allocation_guard() _NOEXCEPT {
58         if (__ptr_ != nullptr) {
59             allocator_traits<_Alloc>::deallocate(__alloc_, __ptr_, __n_);
60         }
61     }
62 
63     _LIBCPP_HIDE_FROM_ABI
64     _Pointer __release_ptr() _NOEXCEPT { // not called __release() because it's a keyword in objective-c++
65         _Pointer __tmp = __ptr_;
66         __ptr_ = nullptr;
67         return __tmp;
68     }
69 
70     _LIBCPP_HIDE_FROM_ABI
71     _Pointer __get() const _NOEXCEPT {
72         return __ptr_;
73     }
74 
75 private:
76     _Alloc __alloc_;
77     _Size __n_;
78     _Pointer __ptr_;
79 };
80 
81 _LIBCPP_END_NAMESPACE_STD
82 
83 #endif // _LIBCPP___MEMORY_ALLOCATION_GUARD_H
84