1 files changed, 89 insertions, 0 deletions
diff --git a/libcxx/include/__memory/allocation_guard.h b/libcxx/include/__memory/allocation_guard.h
new file mode 100644
index 000000000000..4987af293d14
--- /dev/null
+++ b/libcxx/include/__memory/allocation_guard.h
@@ -0,0 +1,89 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___MEMORY_ALLOCATION_GUARD_H
+#define _LIBCPP___MEMORY_ALLOCATION_GUARD_H
+
+#include <__config>
+#include <__memory/allocator_traits.h>
+#include <cstddef>
+#include <utility>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+// Helper class to allocate memory using an Allocator in an exception safe
+// manner.
+//
+// The intended usage of this class is as follows:
+//
+// 0
+// 1     __allocation_guard<SomeAllocator> guard(alloc, 10);
+// 2     do_some_initialization_that_may_throw(guard.__get());
+// 3     save_allocated_pointer_in_a_noexcept_operation(guard.__release_ptr());
+// 4
+//
+// If line (2) throws an exception during initialization of the memory, the
+// guard's destructor will be called, and the memory will be released using
+// Allocator deallocation. Otherwise, we release the memory from the guard on
+// line (3) in an operation that can't throw -- after that, the guard is not
+// responsible for the memory anymore.
+//
+// This is similar to a unique_ptr, except it's easier to use with a
+// custom allocator.
+template<class _Alloc>
+struct __allocation_guard {
+    using _Pointer = typename allocator_traits<_Alloc>::pointer;
+    using _Size = typename allocator_traits<_Alloc>::size_type;
+
+    template<class _AllocT> // we perform the allocator conversion inside the constructor
+    _LIBCPP_HIDE_FROM_ABI
+    explicit __allocation_guard(_AllocT __alloc, _Size __n)
+        : __alloc_(_VSTD::move(__alloc))
+        , __n_(__n)
+        , __ptr_(allocator_traits<_Alloc>::allocate(__alloc_, __n_)) // initialization order is important
+    { }
+
+    _LIBCPP_HIDE_FROM_ABI
+    ~__allocation_guard() _NOEXCEPT {
+        if (__ptr_ != nullptr) {
+            allocator_traits<_Alloc>::deallocate(__alloc_, __ptr_, __n_);
+        }
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    _Pointer __release_ptr() _NOEXCEPT { // not called __release() because it's a keyword in objective-c++
+        _Pointer __tmp = __ptr_;
+        __ptr_ = nullptr;
+        return __tmp;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    _Pointer __get() const _NOEXCEPT {
+        return __ptr_;
+    }
+
+private:
+    _Alloc __alloc_;
+    _Size __n_;
+    _Pointer __ptr_;
+};
+
+_LIBCPP_END_NAMESPACE_STD
+
+_LIBCPP_POP_MACROS
+
+#endif // _LIBCPP___MEMORY_ALLOCATION_GUARD_H