rc_string_base.h

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// Reference-counted versatile string base -*- C++ -*-

// Copyright (C) 2005, 2006 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library 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; either version 2, or (at your option)
// any later version.

// This library 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 library; see the file COPYING.  If not, write to the Free
// Software Foundation, 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301,
// USA.

// As a special exception, you may use this file as part of a free software
// library without restriction.  Specifically, if other files instantiate
// templates or use macros or inline functions from this file, or you compile
// this file and link it with other files to produce an executable, this
// file does not by itself cause the resulting executable to be covered by
// the GNU General Public License.  This exception does not however
// invalidate any other reasons why the executable file might be covered by
// the GNU General Public License.

/** @file ext/rc_string_base.h
 *  This file is a GNU extension to the Standard C++ Library.
 *  This is an internal header file, included by other library headers.
 *  You should not attempt to use it directly.
 */

#ifndef _RC_STRING_BASE_H
#define _RC_STRING_BASE_H 1

#include <bits/atomicity.h>

namespace __gnu_cxx
{
  /**
   *  @if maint
   *  Documentation?  What's that?
   *  Nathan Myers <[e-mail]>.
   *
   *  A string looks like this:
   *
   *  @code
   *                                        [_Rep]
   *                                        _M_length
   *   [__rc_string_base<char_type>]        _M_capacity
   *   _M_dataplus                          _M_refcount
   *   _M_p ---------------->               unnamed array of char_type
   *  @endcode
   *
   *  Where the _M_p points to the first character in the string, and
   *  you cast it to a pointer-to-_Rep and subtract 1 to get a
   *  pointer to the header.
   *
   *  This approach has the enormous advantage that a string object
   *  requires only one allocation.  All the ugliness is confined
   *  within a single pair of inline functions, which each compile to
   *  a single "add" instruction: _Rep::_M_refdata(), and
   *  __rc_string_base::_M_rep(); and the allocation function which gets a
   *  block of raw bytes and with room enough and constructs a _Rep
   *  object at the front.
   *
   *  The reason you want _M_data pointing to the character array and
   *  not the _Rep is so that the debugger can see the string
   *  contents. (Probably we should add a non-inline member to get
   *  the _Rep for the debugger to use, so users can check the actual
   *  string length.)
   *
   *  Note that the _Rep object is a POD so that you can have a
   *  static "empty string" _Rep object already "constructed" before
   *  static constructors have run.  The reference-count encoding is
   *  chosen so that a 0 indicates one reference, so you never try to
   *  destroy the empty-string _Rep object.
   *
   *  All but the last paragraph is considered pretty conventional
   *  for a C++ string implementation.
   *  @endif
  */
 template<typename _CharT, typename _Traits, typename _Alloc>
    class __rc_string_base
    : protected __vstring_utility<_CharT, _Traits, _Alloc>
    {
    public:
      typedef _Traits					    traits_type;
      typedef typename _Traits::char_type		    value_type;
      typedef _Alloc					    allocator_type;

      typedef __vstring_utility<_CharT, _Traits, _Alloc>    _Util_Base;
      typedef typename _Util_Base::_CharT_alloc_type        _CharT_alloc_type;
      typedef typename _CharT_alloc_type::size_type	    size_type;

    private:
      // _Rep: string representation
      //   Invariants:
      //   1. String really contains _M_length + 1 characters: due to 21.3.4
      //      must be kept null-terminated.
      //   2. _M_capacity >= _M_length
      //      Allocated memory is always (_M_capacity + 1) * sizeof(_CharT).
      //   3. _M_refcount has three states:
      //      -1: leaked, one reference, no ref-copies allowed, non-const.
      //       0: one reference, non-const.
      //     n>0: n + 1 references, operations require a lock, const.
      //   4. All fields == 0 is an empty string, given the extra storage
      //      beyond-the-end for a null terminator; thus, the shared
      //      empty string representation needs no constructor.
      struct _Rep
      {
	union
	{
	  struct
	  {
	    size_type	    _M_length;
	    size_type	    _M_capacity;
	    _Atomic_word    _M_refcount;
	  }                 _M_info;
	  
	  // Only for alignment purposes.
	  _CharT            _M_align;
	};

	typedef typename _Alloc::template rebind<_Rep>::other _Rep_alloc_type;

 	_CharT*
	_M_refdata() throw()
	{ return reinterpret_cast<_CharT*>(this + 1); }

	_CharT*
	_M_refcopy() throw()
	{
	  __atomic_add(&_M_info._M_refcount, 1);
	  return _M_refdata();
	}  // XXX MT
	
	void
	_M_set_length(size_type __n)
	{ 
	  _M_info._M_refcount = 0;  // One reference.
	  _M_info._M_length = __n;
	  // grrr. (per 21.3.4)
	  // You cannot leave those LWG people alone for a second.
	  traits_type::assign(_M_refdata()[__n], _CharT());
	}

	// Create & Destroy
	static _Rep*
	_S_create(size_type, size_type, const _Alloc&);

	void
	_M_destroy(const _Alloc&) throw();

	_CharT*
	_M_clone(const _Alloc&, size_type __res = 0);
      };

      struct _Rep_empty
      : public _Rep
      {
	_CharT              _M_terminal;
      };

      static _Rep_empty     _S_empty_rep;

      // The maximum number of individual char_type elements of an
      // individual string is determined by _S_max_size. This is the
      // value that will be returned by max_size().  (Whereas npos
      // is the maximum number of bytes the allocator can allocate.)
      // If one was to divvy up the theoretical largest size string,
      // with a terminating character and m _CharT elements, it'd
      // look like this:
      // npos = sizeof(_Rep) + (m * sizeof(_CharT)) + sizeof(_CharT)
      // Solving for m:
      // m = ((npos - sizeof(_Rep)) / sizeof(_CharT)) - 1
      // In addition, this implementation quarters this amount.
      enum { _S_max_size = (((static_cast<size_type>(-1) - sizeof(_Rep))
			     / sizeof(_CharT)) - 1) / 4 };

      // Data Member (private):
      mutable typename _Util_Base::template _Alloc_hider<_Alloc>  _M_dataplus;

      void
      _M_data(_CharT* __p)
      { _M_dataplus._M_p = __p; }

      _Rep*
      _M_rep() const
      { return &((reinterpret_cast<_Rep*>(_M_data()))[-1]); }

      _CharT*
      _M_grab(const _Alloc& __alloc) const
      {
	return (!_M_is_leaked() && _M_get_allocator() == __alloc)
	        ? _M_rep()->_M_refcopy() : _M_rep()->_M_clone(__alloc);
      }

      void
      _M_dispose()
      {
	if (__exchange_and_add(&_M_rep()->_M_info._M_refcount, -1) <= 0)
	  _M_rep()->_M_destroy(_M_get_allocator());
      }  // XXX MT

      bool
      _M_is_leaked() const
      { return _M_rep()->_M_info._M_refcount < 0; }

      void
      _M_set_sharable()
      { _M_rep()->_M_info._M_refcount = 0; }

      void
      _M_leak_hard();

      // _S_construct_aux is used to implement the 21.3.1 para 15 which
      // requires special behaviour if _InIterator is an integral type
      template<typename _InIterator>
        static _CharT*
        _S_construct_aux(_InIterator __beg, _InIterator __end,
			 const _Alloc& __a, __false_type)
	{
          typedef typename iterator_traits<_InIterator>::iterator_category _Tag;
          return _S_construct(__beg, __end, __a, _Tag());
	}

      template<typename _InIterator>
        static _CharT*
        _S_construct_aux(_InIterator __beg, _InIterator __end,
			 const _Alloc& __a, __true_type)
	{ return _S_construct(static_cast<size_type>(__beg),
			      static_cast<value_type>(__end), __a); }

      template<typename _InIterator>
        static _CharT*
        _S_construct(_InIterator __beg, _InIterator __end, const _Alloc& __a)
	{
	  typedef typename std::__is_integer<_InIterator>::__type _Integral;
	  return _S_construct_aux(__beg, __end, __a, _Integral());
        }

      // For Input Iterators, used in istreambuf_iterators, etc.
      template<typename _InIterator>
        static _CharT*
         _S_construct(_InIterator __beg, _InIterator __end, const _Alloc& __a,
		      std::input_iterator_tag);
      
      // For forward_iterators up to random_access_iterators, used for
      // string::iterator, _CharT*, etc.
      template<typename _FwdIterator>
        static _CharT*
        _S_construct(_FwdIterator __beg, _FwdIterator __end, const _Alloc& __a,
		     std::forward_iterator_tag);

      static _CharT*
      _S_construct(size_type __req, _CharT __c, const _Alloc& __a);

    public:
      size_type
      _M_max_size() const
      { return size_type(_S_max_size); }

      _CharT*
      _M_data() const
      { return _M_dataplus._M_p; }

      size_type
      _M_length() const
      { return _M_rep()->_M_info._M_length; }

      size_type
      _M_capacity() const
      { return _M_rep()->_M_info._M_capacity; }

      bool
      _M_is_shared() const
      { return _M_rep()->_M_info._M_refcount > 0; }

      void
      _M_set_leaked()
      { _M_rep()->_M_info._M_refcount = -1; }

      void
      _M_leak()    // for use in begin() & non-const op[]
      {
	if (!_M_is_leaked())
	  _M_leak_hard();
      }

      void
      _M_set_length(size_type __n)
      { _M_rep()->_M_set_length(__n); }

      __rc_string_base()
      : _M_dataplus(_Alloc(), _S_empty_rep._M_refcopy()) { }

      __rc_string_base(const _Alloc& __a);

      __rc_string_base(const __rc_string_base& __rcs);

      __rc_string_base(size_type __n, _CharT __c, const _Alloc& __a);

      template<typename _InputIterator>
        __rc_string_base(_InputIterator __beg, _InputIterator __end,
			 const _Alloc& __a);

      ~__rc_string_base()
      { _M_dispose(); }      

      allocator_type&
      _M_get_allocator()
      { return _M_dataplus; }

      const allocator_type&
      _M_get_allocator() const
      { return _M_dataplus; }

      void
      _M_swap(__rc_string_base& __rcs);

      void
      _M_assign(const __rc_string_base& __rcs);

      void
      _M_reserve(size_type __res);

      void
      _M_mutate(size_type __pos, size_type __len1, const _CharT* __s,
		size_type __len2);
      
      void
      _M_erase(size_type __pos, size_type __n);

      bool
      _M_compare(const __rc_string_base&) const
      { return false; }
    };

  template<typename _CharT, typename _Traits, typename _Alloc>
    typename __rc_string_base<_CharT, _Traits, _Alloc>::_Rep_empty
    __rc_string_base<_CharT, _Traits, _Alloc>::_S_empty_rep;

  template<typename _CharT, typename _Traits, typename _Alloc>
    typename __rc_string_base<_CharT, _Traits, _Alloc>::_Rep*
    __rc_string_base<_CharT, _Traits, _Alloc>::_Rep::
    _S_create(size_type __capacity, size_type __old_capacity,
	      const _Alloc& __alloc)
    {