BufferAllocator.h

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/*

             *
            ***
           *****
   *********************       Mercenaries Engineering SARL
     *****************              Copyright (C) 2018
       *************
         *********        http://www.mercenaries-engineering.com
        ***********
       ****     ****
      **           **

*/
#pragma once

#include <assert.h>
#include <memory>

namespace maquina
{

template<class T>
class BufferAllocator
{
public:
  using size_type = typename std::allocator<T>::size_type;
  using value_type = typename std::allocator<T>::value_type;
  using difference_type = typename std::allocator<T>::difference_type;
  using pointer            = typename std::allocator<T>::pointer;
  using const_pointer      = typename std::allocator<T>::const_pointer;
  using reference            = typename std::allocator<T>::reference;
  using const_reference      = typename std::allocator<T>::const_reference;
  using propagate_on_container_move_assignment = std::true_type;

  BufferAllocator() = delete;

  BufferAllocator(T* ptr, size_type size) : _pointer(ptr), _size(size) {}

  T* allocate( size_type n, const void * hint = 0 )
  { 
    if(_free && n <= _size)
    {
      _free = false;
      return _pointer;
    }
    _use_heap = true;
    return _fall_back.allocate(n, hint);
  }

  void deallocate( T* p, std::size_t n )
  {
    if(_pointer == p)
    {
      assert(!_free);
      _free = true;
    }
    else
      _fall_back.deallocate(p, n);
  }

  bool use_heap() const
  {
    return _use_heap;
  }
private:
  T* _pointer;
  size_type _size;
  bool _free = true;
  bool _use_heap = false;
  std::allocator<T> _fall_back;
};

}