libcxx/include/__memory/allocation_guard.h

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