libcxx/include/__utility/no_destroy.h

   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 #include <new>
  16
  17 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
  18 #  pragma GCC system_header
  19 #endif
  20
  21 _LIBCPP_BEGIN_NAMESPACE_STD
  22
  23 struct __uninitialized_tag {};
  24
  25 // This class stores an object of type _Tp but never destroys it.
  26 //
  27 // This is akin to using __attribute__((no_destroy)), except that it is possible
  28 // to control the lifetime of the object with more flexibility by deciding e.g.
  29 // whether to initialize the object at construction or to defer to a later
  30 // initialization using __emplace.
  31 template <class _Tp>
  32 struct __no_destroy {
  33   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR explicit __no_destroy(__uninitialized_tag) : __obj_() {}
  34
  35   template <class... _Args>
  36   _LIBCPP_HIDE_FROM_ABI explicit __no_destroy(_Args&&... __args) {
  37     ::new ((void*)__obj_) _Tp(std::forward<_Args>(__args)...);
  38   }
  39
  40   template <class... _Args>
  41   _LIBCPP_HIDE_FROM_ABI _Tp& __emplace(_Args&&... __args) {
  42     return *(::new ((void*)__obj_) _Tp(std::forward<_Args>(__args)...));
  43   }
  44
  45   _LIBCPP_HIDE_FROM_ABI _Tp& __get() { return *reinterpret_cast<_Tp*>(__obj_); }
  46   _LIBCPP_HIDE_FROM_ABI _Tp const& __get() const { return *reinterpret_cast<const _Tp*>(__obj_); }
  47
  48 private:
  49   _ALIGNAS_TYPE(_Tp) char __obj_[sizeof(_Tp)];
  50 };
  51
  52 _LIBCPP_END_NAMESPACE_STD
  53
  54 #endif // _LIBCPP___UTILITY_NO_DESTROY_H