gdbsupport/default-init-alloc.h

   1 /* Copyright (C) 2017-2024 Free Software Foundation, Inc.
   2
   3    This file is part of GDB.
   4
   5    This program is free software; you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation; either version 3 of the License, or
   8    (at your option) any later version.
   9
  10    This program is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13    GNU General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License
  16    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  17
  18 #ifndef COMMON_DEFAULT_INIT_ALLOC_H
  19 #define COMMON_DEFAULT_INIT_ALLOC_H
  20
  21 #if __cplusplus >= 202002L
  22 #include <memory_resource>
  23 #endif
  24
  25 namespace gdb {
  26
  27 /* An allocator that default constructs using default-initialization
  28    rather than value-initialization.  The idea is to use this when you
  29    don't want to default construct elements of containers of trivial
  30    types using zero-initialization.  */
  31
  32 /* Mostly as implementation convenience, this is implemented as an
  33    adapter that given an allocator A, overrides 'A::construct()'.  'A'
  34    defaults to std::allocator<T>.  */
  35
  36 template<typename T,
  37          typename A
  38 #if __cplusplus >= 202002L
  39          = std::pmr::polymorphic_allocator<T>
  40 #else
  41          = std::allocator<T>
  42 #endif
  43          >
  44 class default_init_allocator : public A
  45 {
  46 public:
  47   /* Pull in A's ctors.  */
  48   using A::A;
  49
  50   /* Override rebind.  */
  51   template<typename U>
  52   struct rebind
  53   {
  54     /* A couple helpers just to make it a bit more readable.  */
  55     typedef std::allocator_traits<A> traits_;
  56     typedef typename traits_::template rebind_alloc<U> alloc_;
  57
  58     /* This is what we're after.  */
  59     typedef default_init_allocator<U, alloc_> other;
  60   };
  61
  62   /* Make the base allocator's construct method(s) visible.  */
  63   using A::construct;
  64
  65   /* .. and provide an override/overload for the case of default
  66      construction (i.e., no arguments).  This is where we construct
  67      with default-init.  */
  68   template <typename U>
  69   void construct (U *ptr)
  70     noexcept (std::is_nothrow_default_constructible<U>::value)
  71   {
  72     ::new ((void *) ptr) U; /* default-init */
  73   }
  74 };
  75
  76 } /* namespace gdb */
  77
  78 #endif /* COMMON_DEFAULT_INIT_ALLOC_H */