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✏️ Editing: mem_root_array.h

This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; version 2 of the License.

This program is distributed in the hope that it will be useful,
   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
   along with this program; if not, write to the Free Software
   Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1335  USA */

#ifndef MEM_ROOT_ARRAY_INCLUDED
#define MEM_ROOT_ARRAY_INCLUDED

#include <my_alloc.h>

/**
   A typesafe replacement for DYNAMIC_ARRAY.
   We use MEM_ROOT for allocating storage, rather than the C++ heap.
   The interface is chosen to be similar to std::vector.

@remark
   Unlike DYNAMIC_ARRAY, elements are properly copied
   (rather than memcpy()d) if the underlying array needs to be expanded.

@remark
   Depending on has_trivial_destructor, we destroy objects which are
   removed from the array (including when the array object itself is destroyed).

@remark
   Note that MEM_ROOT has no facility for reusing free space,
   so don't use this if multiple re-expansions are likely to happen.

@param Element_type The type of the elements of the container.
          Elements must be copyable.
   @param has_trivial_destructor If true, we don't destroy elements.
          We could have used type traits to determine this.
          __has_trivial_destructor is supported by some (but not all)
          compilers we use.
*/
template<typename Element_type, bool has_trivial_destructor>
class Mem_root_array
{
public:
  /// Convenience typedef, same typedef name as std::vector
  typedef Element_type value_type;

Mem_root_array(MEM_ROOT *root)
    : m_root(root), m_array(NULL), m_size(0), m_capacity(0)
  {
    DBUG_ASSERT(m_root != NULL);
  }

Mem_root_array(MEM_ROOT *root, size_t n, const value_type &val= value_type())
    : m_root(root), m_array(NULL), m_size(0), m_capacity(0)
  {
    resize(n, val);
  }

~Mem_root_array()
  {
    clear();
  }

Element_type &at(size_t n)
  {
    DBUG_ASSERT(n < size());
    return m_array[n];
  }

const Element_type &at(size_t n) const
  {
    DBUG_ASSERT(n < size());
    return m_array[n];
  }

Element_type &operator[](size_t n) { return at(n); }
  const Element_type &operator[](size_t n) const { return at(n); }

Element_type &back() { return at(size() - 1); }
  const Element_type &back() const { return at(size() - 1); }

// Returns a pointer to the first element in the array.
  Element_type *begin() { return &m_array[0]; }
  const Element_type *begin() const { return &m_array[0]; }

// Returns a pointer to the past-the-end element in the array.
  Element_type *end() { return &m_array[size()]; }
  const Element_type *end() const { return &m_array[size()]; }

// Erases all of the elements. 
  void clear()
  {
    if (!empty())
      chop(0);
  }

/*
    Chops the tail off the array, erasing all tail elements.
    @param pos Index of first element to erase.
  */
  void chop(const size_t pos)
  {
    DBUG_ASSERT(pos < m_size);
    if (!has_trivial_destructor)
    {
      for (size_t ix= pos; ix < m_size; ++ix)
      {
        Element_type *p= &m_array[ix];
        p->~Element_type();              // Destroy discarded element.
      }
    }
    m_size= pos;
  }

/*
    Reserves space for array elements.
    Copies over existing elements, in case we are re-expanding the array.

@param  n number of elements.
    @retval true if out-of-memory, false otherwise.
  */
  bool reserve(size_t n)
  {
    if (n <= m_capacity)
      return false;

void *mem= alloc_root(m_root, n * element_size());
    if (!mem)
      return true;
    Element_type *array= static_cast<Element_type*>(mem);

// Copy all the existing elements into the new array.
    for (size_t ix= 0; ix < m_size; ++ix)
    {
      Element_type *new_p= &array[ix];
      Element_type *old_p= &m_array[ix];
      new (new_p) Element_type(*old_p);         // Copy into new location.
      if (!has_trivial_destructor)
        old_p->~Element_type();                 // Destroy the old element.
    }

// Forget the old array.
    m_array= array;
    m_capacity= n;
    return false;
  }

/*
    Adds a new element at the end of the array, after its current last
    element. The content of this new element is initialized to a copy of
    the input argument.

@param  element Object to copy.
    @retval true if out-of-memory, false otherwise.
  */
  bool push_back(const Element_type &element)
  {
    const size_t min_capacity= 20;
    const size_t expansion_factor= 2;
    if (0 == m_capacity && reserve(min_capacity))
      return true;
    if (m_size == m_capacity && reserve(m_capacity * expansion_factor))
      return true;
    Element_type *p= &m_array[m_size++];
    new (p) Element_type(element);
    return false;
  }

/**
    Removes the last element in the array, effectively reducing the
    container size by one. This destroys the removed element.
   */
  void pop_back()
  {
    DBUG_ASSERT(!empty());
    if (!has_trivial_destructor)
      back().~Element_type();
    m_size-= 1;
  }

/**
    Resizes the container so that it contains n elements.

If n is smaller than the current container size, the content is
    reduced to its first n elements, removing those beyond (and
    destroying them).

If n is greater than the current container size, the content is
    expanded by inserting at the end as many elements as needed to
    reach a size of n. If val is specified, the new elements are
    initialized as copies of val, otherwise, they are
    value-initialized.

If n is also greater than the current container capacity, an automatic
    reallocation of the allocated storage space takes place.

Notice that this function changes the actual content of the
    container by inserting or erasing elements from it.
   */
  void resize(size_t n, const value_type &val= value_type())
  {
    if (n == m_size)
      return;
    if (n > m_size)
    {
      if (!reserve(n))
      {
        while (n != m_size)
          push_back(val);
      }
      return;
    }
    if (!has_trivial_destructor)
    {
      while (n != m_size)
        pop_back();
    }
    m_size= n;
  }

size_t capacity()     const { return m_capacity; }
  size_t element_size() const { return sizeof(Element_type); }
  bool   empty()        const { return size() == 0; }
  size_t size()         const { return m_size; }
  const MEM_ROOT *mem_root() const { return m_root; }

private:
  MEM_ROOT *const m_root;
  Element_type   *m_array;
  size_t          m_size;
  size_t          m_capacity;

// Not (yet) implemented.
  Mem_root_array(const Mem_root_array&);
  Mem_root_array &operator=(const Mem_root_array&);
};

#endif  // MEM_ROOT_ARRAY_INCLUDED

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