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      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 */
     79