gdbsupport/default-init-alloc.h

   1 /* Copyright (C) 2017-2020 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 namespace gdb {
  22
  23 /* An allocator that default constructs using default-initialization
  24    rather than value-initialization.  The idea is to use this when you
  25    don't want to default construct elements of containers of trivial
  26    types using zero-initialization.  */
  27
  28 /* Mostly as implementation convenience, this is implemented as an
  29    adapter that given an allocator A, overrides 'A::construct()'.  'A'
  30    defaults to std::allocator<T>.  */
  31
  32 template<typename T, typename A = std::allocator<T>>
  33 class default_init_allocator : public A
  34 {
  35 public:
  36   /* Pull in A's ctors.  */
  37   using A::A;
  38
  39   /* Override rebind.  */
  40   template<typename U>
  41   struct rebind
  42   {
  43     /* A couple helpers just to make it a bit more readable.  */
  44     typedef std::allocator_traits<A> traits_;
  45     typedef typename traits_::template rebind_alloc<U> alloc_;
  46
  47     /* This is what we're after.  */
  48     typedef default_init_allocator<U, alloc_> other;
  49   };
  50
  51   /* Make the base allocator's construct method(s) visible.  */
  52   using A::construct;
  53
  54   /* .. and provide an override/overload for the case of default
  55      construction (i.e., no arguments).  This is where we construct
  56      with default-init.  */
  57   template <typename U>
  58   void construct (U *ptr)
  59     noexcept (std::is_nothrow_default_constructible<U>::value)
  60   {
  61     ::new ((void *) ptr) U; /* default-init */
  62   }
  63 };
  64
  65 } /* namespace gdb */
  66
  67 #endif /* COMMON_DEFAULT_INIT_ALLOC_H */