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 #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
  53     _LIBCPP_HIDE_FROM_ABI
  54     explicit __allocation_guard(_AllocT __alloc, _Size __n)
  55         : __alloc_(_VSTD::move(__alloc))
  56         , __n_(__n)
  57         , __ptr_(allocator_traits<_Alloc>::allocate(__alloc_, __n_)) // initialization order is important
  58     { }
  59
  60     _LIBCPP_HIDE_FROM_ABI
  61     ~__allocation_guard() _NOEXCEPT {
  62         __destroy();
  63     }
  64
  65     _LIBCPP_HIDE_FROM_ABI __allocation_guard(const __allocation_guard&) = delete;
  66     _LIBCPP_HIDE_FROM_ABI __allocation_guard(__allocation_guard&& __other) _NOEXCEPT
  67         : __alloc_(std::move(__other.__alloc_))
  68         , __n_(__other.__n_)
  69         , __ptr_(__other.__ptr_) {
  70       __other.__ptr_ = nullptr;
  71     }
  72
  73     _LIBCPP_HIDE_FROM_ABI __allocation_guard& operator=(const __allocation_guard& __other) = delete;
  74     _LIBCPP_HIDE_FROM_ABI __allocation_guard& operator=(__allocation_guard&& __other) _NOEXCEPT {
  75         if (std::addressof(__other) != this) {
  76             __destroy();
  77
  78             __alloc_ = std::move(__other.__alloc_);
  79             __n_ = __other.__n_;
  80             __ptr_ = __other.__ptr_;
  81             __other.__ptr_ = nullptr;
  82         }
  83
  84         return *this;
  85     }
  86
  87     _LIBCPP_HIDE_FROM_ABI
  88     _Pointer __release_ptr() _NOEXCEPT { // not called __release() because it's a keyword in objective-c++
  89         _Pointer __tmp = __ptr_;
  90         __ptr_ = nullptr;
  91         return __tmp;
  92     }
  93
  94     _LIBCPP_HIDE_FROM_ABI
  95     _Pointer __get() const _NOEXCEPT {
  96         return __ptr_;
  97     }
  98
  99 private:
 100     _LIBCPP_HIDE_FROM_ABI
 101     void __destroy() _NOEXCEPT {
 102         if (__ptr_ != nullptr) {
 103             allocator_traits<_Alloc>::deallocate(__alloc_, __ptr_, __n_);
 104         }
 105     }
 106
 107     _Alloc __alloc_;
 108     _Size __n_;
 109     _Pointer __ptr_;
 110 };
 111
 112 _LIBCPP_END_NAMESPACE_STD
 113
 114 _LIBCPP_POP_MACROS
 115
 116 #endif // _LIBCPP___MEMORY_ALLOCATION_GUARD_H