default_init_allocator.h

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/*
  Copyright (c) 2020, 2022, Oracle and/or its affiliates.
 
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  it under the terms of the GNU General Public License, version 2.0,
  as published by the Free Software Foundation.
 
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  as designated in a particular file or component or in included license
  documentation.  The authors of MySQL hereby grant you an additional
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  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.
 
  You should have received a copy of the GNU General Public License
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*/
 
#ifndef MYSQL_HARNESS_DEFAULT_INIT_ALLOCATOR_H_
#define MYSQL_HARNESS_DEFAULT_INIT_ALLOCATOR_H_
 
#include <memory>
 
/**
 * allocator which leaves newly constructed fields "default initialized".
 *
 * in case of std::vector<int> after a resize() the newly added fields would
 * be uninitialized.
 *
 * useful in case of network-buffers it will be read() into right afterwards.
 */
template <class T, class A = std::allocator<T>>
class default_init_allocator : public A {
  using a_t = std::allocator_traits<A>;
 
 public:
  template <class U>
  struct rebind {
    using other =
        default_init_allocator<U, typename a_t::template rebind_alloc<U>>;
  };
 
  // inherit the constructors of the allocator
  using A::A;
 
  template <class U>
  void construct(U *ptr) noexcept(
      std::is_nothrow_default_constructible<U>::value) {
    ::new (static_cast<void *>(ptr)) U;
  }
  template <class U, class... Args>
  void construct(U *ptr, Args &&... args) {
    a_t::construct(static_cast<A &>(*this), ptr, std::forward<Args>(args)...);
  }
};
 
#endif