1 //===----------------------------------------------------------------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 
9 #ifndef _LIBCPP___UTILITY_NO_DESTROY_H
10 #define _LIBCPP___UTILITY_NO_DESTROY_H
11 
12 #include <__config>
13 #include <__type_traits/is_constant_evaluated.h>
14 #include <__utility/forward.h>
15 
16 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
17 #  pragma GCC system_header
18 #endif
19 
20 _LIBCPP_BEGIN_NAMESPACE_STD
21 
22 struct __uninitialized_tag {};
23 
24 // This class stores an object of type _Tp but never destroys it.
25 //
26 // This is akin to using __attribute__((no_destroy)), except that it is possible
27 // to control the lifetime of the object with more flexibility by deciding e.g.
28 // whether to initialize the object at construction or to defer to a later
29 // initialization using __emplace.
30 template <class _Tp>
31 struct __no_destroy {
__no_destroy__no_destroy32   _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI explicit __no_destroy(__uninitialized_tag) : __dummy_() {
33     if (__libcpp_is_constant_evaluated()) {
34       __dummy_ = char();
35     }
36   }
~__no_destroy__no_destroy37   _LIBCPP_HIDE_FROM_ABI ~__no_destroy() {
38     // nothing
39   }
40 
41   template <class... _Args>
__no_destroy__no_destroy42   _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI explicit __no_destroy(_Args&&... __args)
43       : __obj_(std::forward<_Args>(__args)...) {}
44 
45   template <class... _Args>
__emplace__no_destroy46   _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI _Tp& __emplace(_Args&&... __args) {
47     new (&__obj_) _Tp(std::forward<_Args>(__args)...);
48     return __obj_;
49   }
50 
__get__no_destroy51   _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI _Tp& __get() { return __obj_; }
__get__no_destroy52   _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI _Tp const& __get() const { return __obj_; }
53 
54 private:
55   union {
56     _Tp __obj_;
57     char __dummy_; // so we can initialize a member even with __uninitialized_tag for constexpr-friendliness
58   };
59 };
60 
61 _LIBCPP_END_NAMESPACE_STD
62 
63 #endif // _LIBCPP___UTILITY_NO_DESTROY_H
64