libcxx/include/__memory/allocation_guard.h

// -*- 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_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

#endif // _LIBCPP___MEMORY_ALLOCATION_GUARD_H