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Edit File : rc_string_base.h \| Size : 23.18 KB
// Reference-counted versatile string base -- C++ -- // Copyright (C) 2005-2018 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 3, 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. // Under Section 7 of GPL version 3, you are granted additional // permissions described in the GCC Runtime Library Exception, version // 3.1, as published by the Free Software Foundation. // You should have received a copy of the GNU General Public License and // a copy of the GCC Runtime Library Exception along with this program; // see the files COPYING3 and COPYING.RUNTIME respectively. If not, see // <http://www.gnu.org/licenses/>. /** @file ext/rc_string_base.h * This is an internal header file, included by other library headers. * Do not attempt to use it directly. @headername{ext/vstring.h} / #ifndef _RC_STRING_BASE_H #define _RC_STRING_BASE_H 1 #include <ext/atomicity.h> #include <bits/stl_iterator_base_funcs.h> namespace __gnu_cxx _GLIBCXX_VISIBILITY(default) { _GLIBCXX_BEGIN_NAMESPACE_VERSION /* * Documentation? What's that? * Nathan Myers <ncm@cantrip.org>. * * 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 @a 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 <em>empty string</em> _Rep object already @a 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. / 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_dispatch(&_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) // + sizeof(_Rep) - 1 // (NB: last two terms for rounding reasons, see _M_create below) // Solving for m: // m = ((npos - 2 * sizeof(_Rep) + 1) / sizeof(_CharT)) - 1 // In addition, this implementation halves this amount. enum { _S_max_size = (((static_cast<size_type>(-1) - 2 * sizeof(_Rep) + 1) / sizeof(_CharT)) - 1) / 2 }; // 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() { // Be race-detector-friendly. For more info see bits/c++config. _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_rep()->_M_info. _M_refcount); if (__exchange_and_add_dispatch(&_M_rep()->_M_info._M_refcount, -1) <= 0) { _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_rep()->_M_info. _M_refcount); _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, std::__false_type) { typedef typename iterator_traits<_InIterator>::iterator_category _Tag; return _S_construct(__beg, __end, __a, _Tag()); } // _GLIBCXX_RESOLVE_LIB_DEFECTS // 438. Ambiguity in the "do the right thing" clause template<typename _Integer> static _CharT* _S_construct_aux(_Integer __beg, _Integer __end, const _Alloc& __a, std::__true_type) { return _S_construct_aux_2(static_cast<size_type>(__beg), __end, __a); } static _CharT* _S_construct_aux_2(size_type __req, _CharT __c, const _Alloc& __a) { return _S_construct(__req, __c, __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(_S_empty_rep._M_refcopy()) { } __rc_string_base(const _Alloc& __a); __rc_string_base(const __rc_string_base& __rcs); #if __cplusplus >= 201103L __rc_string_base(__rc_string_base&& __rcs) : _M_dataplus(__rcs._M_dataplus) { __rcs._M_data(_S_empty_rep._M_refcopy()); } #endif __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); void _M_clear() { _M_dispose(); _M_data(_S_empty_rep._M_refcopy()); } 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) { // _GLIBCXX_RESOLVE_LIB_DEFECTS // 83. String::npos vs. string::max_size() if (__capacity > size_type(_S_max_size)) std::__throw_length_error(__N("__rc_string_base::_Rep::_S_create")); // The standard places no restriction on allocating more memory // than is strictly needed within this layer at the moment or as // requested by an explicit application call to reserve(). // Many malloc implementations perform quite poorly when an // application attempts to allocate memory in a stepwise fashion // growing each allocation size by only 1 char. Additionally, // it makes little sense to allocate less linear memory than the // natural blocking size of the malloc implementation. // Unfortunately, we would need a somewhat low-level calculation // with tuned parameters to get this perfect for any particular // malloc implementation. Fortunately, generalizations about // common features seen among implementations seems to suffice. // __pagesize need not match the actual VM page size for good // results in practice, thus we pick a common value on the low // side. __malloc_header_size is an estimate of the amount of // overhead per memory allocation (in practice seen N * sizeof // (void) where N is 0, 2 or 4). According to folklore, // picking this value on the high side is better than // low-balling it (especially when this algorithm is used with // malloc implementations that allocate memory blocks rounded up // to a size which is a power of 2). const size_type __pagesize = 4096; const size_type __malloc_header_size = 4 sizeof(void); // The below implements an exponential growth policy, necessary to // meet amortized linear time requirements of the library: see // http://gcc.gnu.org/ml/libstdc++/2001-07/msg00085.html. if (__capacity > __old_capacity && __capacity < 2 __old_capacity) { __capacity = 2 * __old_capacity; // Never allocate a string bigger than _S_max_size. if (__capacity > size_type(_S_max_size)) __capacity = size_type(_S_max_size); } // NB: Need an array of char_type[__capacity], plus a terminating // null char_type() element, plus enough for the _Rep data structure, // plus sizeof(_Rep) - 1 to upper round to a size multiple of // sizeof(_Rep). // Whew. Seemingly so needy, yet so elemental. size_type __size = ((__capacity + 1) * sizeof(_CharT) + 2 * sizeof(_Rep) - 1); const size_type __adj_size = __size + __malloc_header_size; if (__adj_size > __pagesize && __capacity > __old_capacity) { const size_type __extra = __pagesize - __adj_size % __pagesize; __capacity += __extra / sizeof(_CharT); if (__capacity > size_type(_S_max_size)) __capacity = size_type(_S_max_size); __size = (__capacity + 1) * sizeof(_CharT) + 2 * sizeof(_Rep) - 1; } // NB: Might throw, but no worries about a leak, mate: _Rep() // does not throw. _Rep* __place = _Rep_alloc_type(__alloc).allocate(__size / sizeof(_Rep)); _Rep* __p = new (__place) _Rep; __p->_M_info._M_capacity = __capacity; return __p; } template<typename _CharT, typename _Traits, typename _Alloc> void __rc_string_base<_CharT, _Traits, _Alloc>::_Rep:: _M_destroy(const _Alloc& __a) throw () { const size_type __size = ((_M_info._M_capacity + 1) * sizeof(_CharT) + 2 * sizeof(_Rep) - 1); _Rep_alloc_type(__a).deallocate(this, __size / sizeof(_Rep)); } template<typename _CharT, typename _Traits, typename _Alloc> _CharT* __rc_string_base<_CharT, _Traits, _Alloc>::_Rep:: _M_clone(const _Alloc& __alloc, size_type __res) { // Requested capacity of the clone. const size_type __requested_cap = _M_info._M_length + __res; _Rep* __r = _Rep::_S_create(__requested_cap, _M_info._M_capacity, __alloc); if (_M_info._M_length) __rc_string_base::_S_copy(__r->_M_refdata(), _M_refdata(), _M_info._M_length); __r->_M_set_length(_M_info._M_length); return __r->_M_refdata(); } template<typename _CharT, typename _Traits, typename _Alloc> __rc_string_base<_CharT, _Traits, _Alloc>:: __rc_string_base(const _Alloc& __a) : _M_dataplus(__a, _S_construct(size_type(), _CharT(), __a)) { } template<typename _CharT, typename _Traits, typename _Alloc> __rc_string_base<_CharT, _Traits, _Alloc>:: __rc_string_base(const __rc_string_base& __rcs) : _M_dataplus(__rcs._M_get_allocator(), __rcs._M_grab(__rcs._M_get_allocator())) { } template<typename _CharT, typename _Traits, typename _Alloc> __rc_string_base<_CharT, _Traits, _Alloc>:: __rc_string_base(size_type __n, _CharT __c, const _Alloc& __a) : _M_dataplus(__a, _S_construct(__n, __c, __a)) { } template<typename _CharT, typename _Traits, typename _Alloc> template<typename _InputIterator> __rc_string_base<_CharT, _Traits, _Alloc>:: __rc_string_base(_InputIterator __beg, _InputIterator __end, const _Alloc& __a) : _M_dataplus(__a, _S_construct(__beg, __end, __a)) { } template<typename _CharT, typename _Traits, typename _Alloc> void __rc_string_base<_CharT, _Traits, _Alloc>:: _M_leak_hard() { if (_M_is_shared()) _M_erase(0, 0); _M_set_leaked(); } // NB: This is the special case for Input Iterators, used in // istreambuf_iterators, etc. // Input Iterators have a cost structure very different from // pointers, calling for a different coding style. template<typename _CharT, typename _Traits, typename _Alloc> template<typename _InIterator> _CharT* __rc_string_base<_CharT, _Traits, _Alloc>:: _S_construct(_InIterator __beg, _InIterator __end, const _Alloc& __a, std::input_iterator_tag) { if (__beg == __end && __a == _Alloc()) return _S_empty_rep._M_refcopy(); // Avoid reallocation for common case. _CharT __buf[128]; size_type __len = 0; while (__beg != __end && __len < sizeof(__buf) / sizeof(_CharT)) { __buf[__len++] = __beg; ++__beg; } _Rep __r = _Rep::_S_create(__len, size_type(0), __a); _S_copy(__r->_M_refdata(), __buf, __len); __try { while (__beg != __end) { if (__len == __r->_M_info._M_capacity) { // Allocate more space. _Rep* __another = _Rep::_S_create(__len + 1, __len, __a); _S_copy(__another->_M_refdata(), __r->_M_refdata(), __len); __r->_M_destroy(__a); __r = __another; } __r->_M_refdata()[__len++] = __beg; ++__beg; } } __catch(...) { __r->_M_destroy(__a); __throw_exception_again; } __r->_M_set_length(__len); return __r->_M_refdata(); } template<typename _CharT, typename _Traits, typename _Alloc> template<typename _InIterator> _CharT __rc_string_base<_CharT, _Traits, _Alloc>:: _S_construct(_InIterator __beg, _InIterator __end, const _Alloc& __a, std::forward_iterator_tag) { if (__beg == __end && __a == _Alloc()) return _S_empty_rep._M_refcopy(); // NB: Not required, but considered best practice. if (__is_null_pointer(__beg) && __beg != __end) std::__throw_logic_error(__N("__rc_string_base::" "_S_construct null not valid")); const size_type __dnew = static_cast<size_type>(std::distance(__beg, __end)); // Check for out_of_range and length_error exceptions. _Rep* __r = _Rep::_S_create(__dnew, size_type(0), __a); __try { __rc_string_base::_S_copy_chars(__r->_M_refdata(), __beg, __end); } __catch(...) { __r->_M_destroy(__a); __throw_exception_again; } __r->_M_set_length(__dnew); return __r->_M_refdata(); } template<typename _CharT, typename _Traits, typename _Alloc> _CharT* __rc_string_base<_CharT, _Traits, _Alloc>:: _S_construct(size_type __n, _CharT __c, const _Alloc& __a) { if (__n == 0 && __a == _Alloc()) return _S_empty_rep._M_refcopy(); // Check for out_of_range and length_error exceptions. _Rep* __r = _Rep::_S_create(__n, size_type(0), __a); if (__n) __rc_string_base::_S_assign(__r->_M_refdata(), __n, __c); __r->_M_set_length(__n); return __r->_M_refdata(); } template<typename _CharT, typename _Traits, typename _Alloc> void __rc_string_base<_CharT, _Traits, _Alloc>:: _M_swap(__rc_string_base& __rcs) { if (_M_is_leaked()) _M_set_sharable(); if (__rcs._M_is_leaked()) __rcs._M_set_sharable(); _CharT* __tmp = _M_data(); _M_data(__rcs._M_data()); __rcs._M_data(__tmp); // _GLIBCXX_RESOLVE_LIB_DEFECTS // 431. Swapping containers with unequal allocators. std::__alloc_swap<allocator_type>::_S_do_it(_M_get_allocator(), __rcs._M_get_allocator()); } template<typename _CharT, typename _Traits, typename _Alloc> void __rc_string_base<_CharT, _Traits, _Alloc>:: _M_assign(const __rc_string_base& __rcs) { if (_M_rep() != __rcs._M_rep()) { _CharT* __tmp = __rcs._M_grab(_M_get_allocator()); _M_dispose(); _M_data(__tmp); } } template<typename _CharT, typename _Traits, typename _Alloc> void __rc_string_base<_CharT, _Traits, _Alloc>:: _M_reserve(size_type __res) { // Make sure we don't shrink below the current size. if (__res < _M_length()) __res = _M_length(); if (__res != _M_capacity() \|\| _M_is_shared()) { _CharT* __tmp = _M_rep()->_M_clone(_M_get_allocator(), __res - _M_length()); _M_dispose(); _M_data(__tmp); } } template<typename _CharT, typename _Traits, typename _Alloc> void __rc_string_base<_CharT, _Traits, _Alloc>:: _M_mutate(size_type __pos, size_type __len1, const _CharT* __s, size_type __len2) { const size_type __how_much = _M_length() - __pos - __len1; _Rep* __r = _Rep::_S_create(_M_length() + __len2 - __len1, _M_capacity(), _M_get_allocator()); if (__pos) this->_S_copy(__r->_M_refdata(), _M_data(), __pos); if (__s && __len2) this->_S_copy(__r->_M_refdata() + __pos, __s, __len2); if (__how_much) this->_S_copy(__r->_M_refdata() + __pos + __len2, _M_data() + __pos + __len1, __how_much); _M_dispose(); _M_data(__r->_M_refdata()); } template<typename _CharT, typename _Traits, typename _Alloc> void __rc_string_base<_CharT, _Traits, _Alloc>:: _M_erase(size_type __pos, size_type __n) { const size_type __new_size = _M_length() - __n; const size_type __how_much = _M_length() - __pos - __n; if (_M_is_shared()) { // Must reallocate. _Rep* __r = _Rep::_S_create(__new_size, _M_capacity(), _M_get_allocator()); if (__pos) this->_S_copy(__r->_M_refdata(), _M_data(), __pos); if (__how_much) this->_S_copy(__r->_M_refdata() + __pos, _M_data() + __pos + __n, __how_much); _M_dispose(); _M_data(__r->_M_refdata()); } else if (__how_much && __n) { // Work in-place. this->_S_move(_M_data() + __pos, _M_data() + __pos + __n, __how_much); } _M_rep()->_M_set_length(__new_size); } template<> inline bool __rc_string_base<char, std::char_traits<char>, std::allocator<char> >:: _M_compare(const __rc_string_base& __rcs) const { if (_M_rep() == __rcs._M_rep()) return true; return false; } #ifdef _GLIBCXX_USE_WCHAR_T template<> inline bool __rc_string_base<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> >:: _M_compare(const __rc_string_base& __rcs) const { if (_M_rep() == __rcs._M_rep()) return true; return false; } #endif _GLIBCXX_END_NAMESPACE_VERSION } // namespace #endif /* _RC_STRING_BASE_H */ Back

Edit File : rc_string_base.h | Size : 23.18 KB

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

// Copyright (C) 2005-2018 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 3, 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.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/** @file ext/rc_string_base.h
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly. @headername{ext/vstring.h}
 */

#ifndef _RC_STRING_BASE_H
#define _RC_STRING_BASE_H 1

#include <ext/atomicity.h>
#include <bits/stl_iterator_base_funcs.h>

namespace __gnu_cxx _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

/**
   *  Documentation?  What's that?
   *  Nathan Myers <ncm@cantrip.org>.
   *
   *  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 @a 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 <em>empty string</em> _Rep object already @a 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.
  */
 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_dispatch(&_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)
      //        + sizeof(_Rep) - 1
      // (NB: last two terms for rounding reasons, see _M_create below)
      // Solving for m:
      // m = ((npos - 2 * sizeof(_Rep) + 1) / sizeof(_CharT)) - 1
      // In addition, this implementation halves this amount.
      enum { _S_max_size = (((static_cast<size_type>(-1) - 2 * sizeof(_Rep)
			      + 1) / sizeof(_CharT)) - 1) / 2 };

// 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()
      {
	// Be race-detector-friendly.  For more info see bits/c++config.
	_GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_rep()->_M_info.
						_M_refcount);
	if (__exchange_and_add_dispatch(&_M_rep()->_M_info._M_refcount,
					-1) <= 0)
	  {
	    _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_rep()->_M_info.
						   _M_refcount);
	    _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, std::__false_type)
	{
	  typedef typename iterator_traits<_InIterator>::iterator_category _Tag;
	  return _S_construct(__beg, __end, __a, _Tag());
	}

// _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 438. Ambiguity in the "do the right thing" clause
      template<typename _Integer>
	static _CharT*
	_S_construct_aux(_Integer __beg, _Integer __end,
			 const _Alloc& __a, std::__true_type)
	{ return _S_construct_aux_2(static_cast<size_type>(__beg),
				    __end, __a); }

static _CharT*
      _S_construct_aux_2(size_type __req, _CharT __c, const _Alloc& __a)
      { return _S_construct(__req, __c, __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(_S_empty_rep._M_refcopy()) { }

__rc_string_base(const _Alloc& __a);

__rc_string_base(const __rc_string_base& __rcs);

#if __cplusplus >= 201103L
      __rc_string_base(__rc_string_base&& __rcs)
      : _M_dataplus(__rcs._M_dataplus)
      { __rcs._M_data(_S_empty_rep._M_refcopy()); }
#endif

__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);

void
      _M_clear()
      {
	_M_dispose();
	_M_data(_S_empty_rep._M_refcopy());
      }

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)
    {
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 83.  String::npos vs. string::max_size()
      if (__capacity > size_type(_S_max_size))
	std::__throw_length_error(__N("__rc_string_base::_Rep::_S_create"));

// The standard places no restriction on allocating more memory
      // than is strictly needed within this layer at the moment or as
      // requested by an explicit application call to reserve().

// Many malloc implementations perform quite poorly when an
      // application attempts to allocate memory in a stepwise fashion
      // growing each allocation size by only 1 char.  Additionally,
      // it makes little sense to allocate less linear memory than the
      // natural blocking size of the malloc implementation.
      // Unfortunately, we would need a somewhat low-level calculation
      // with tuned parameters to get this perfect for any particular
      // malloc implementation.  Fortunately, generalizations about
      // common features seen among implementations seems to suffice.

// __pagesize need not match the actual VM page size for good
      // results in practice, thus we pick a common value on the low
      // side.  __malloc_header_size is an estimate of the amount of
      // overhead per memory allocation (in practice seen N * sizeof
      // (void*) where N is 0, 2 or 4).  According to folklore,
      // picking this value on the high side is better than
      // low-balling it (especially when this algorithm is used with
      // malloc implementations that allocate memory blocks rounded up
      // to a size which is a power of 2).
      const size_type __pagesize = 4096;
      const size_type __malloc_header_size = 4 * sizeof(void*);

// The below implements an exponential growth policy, necessary to
      // meet amortized linear time requirements of the library: see
      // http://gcc.gnu.org/ml/libstdc++/2001-07/msg00085.html.
      if (__capacity > __old_capacity && __capacity < 2 * __old_capacity)
	{
	  __capacity = 2 * __old_capacity;
	  // Never allocate a string bigger than _S_max_size.
	  if (__capacity > size_type(_S_max_size))
	    __capacity = size_type(_S_max_size);
	}

// NB: Need an array of char_type[__capacity], plus a terminating
      // null char_type() element, plus enough for the _Rep data structure,
      // plus sizeof(_Rep) - 1 to upper round to a size multiple of
      // sizeof(_Rep).
      // Whew. Seemingly so needy, yet so elemental.
      size_type __size = ((__capacity + 1) * sizeof(_CharT)
			  + 2 * sizeof(_Rep) - 1);

const size_type __adj_size = __size + __malloc_header_size;
      if (__adj_size > __pagesize && __capacity > __old_capacity)
	{
	  const size_type __extra = __pagesize - __adj_size % __pagesize;
	  __capacity += __extra / sizeof(_CharT);
	  if (__capacity > size_type(_S_max_size))
	    __capacity = size_type(_S_max_size);
	  __size = (__capacity + 1) * sizeof(_CharT) + 2 * sizeof(_Rep) - 1;
	}

// NB: Might throw, but no worries about a leak, mate: _Rep()
      // does not throw.
      _Rep* __place = _Rep_alloc_type(__alloc).allocate(__size / sizeof(_Rep));
      _Rep* __p = new (__place) _Rep;
      __p->_M_info._M_capacity = __capacity;
      return __p;
    }

template<typename _CharT, typename _Traits, typename _Alloc>
    void
    __rc_string_base<_CharT, _Traits, _Alloc>::_Rep::
    _M_destroy(const _Alloc& __a) throw ()
    {
      const size_type __size = ((_M_info._M_capacity + 1) * sizeof(_CharT)
				+ 2 * sizeof(_Rep) - 1);
      _Rep_alloc_type(__a).deallocate(this, __size / sizeof(_Rep));
    }

template<typename _CharT, typename _Traits, typename _Alloc>
    _CharT*
    __rc_string_base<_CharT, _Traits, _Alloc>::_Rep::
    _M_clone(const _Alloc& __alloc, size_type __res)
    {
      // Requested capacity of the clone.
      const size_type __requested_cap = _M_info._M_length + __res;
      _Rep* __r = _Rep::_S_create(__requested_cap, _M_info._M_capacity,
				  __alloc);

if (_M_info._M_length)
	__rc_string_base::_S_copy(__r->_M_refdata(), _M_refdata(), _M_info._M_length);

__r->_M_set_length(_M_info._M_length);
      return __r->_M_refdata();
    }

template<typename _CharT, typename _Traits, typename _Alloc>
    __rc_string_base<_CharT, _Traits, _Alloc>::
    __rc_string_base(const _Alloc& __a)
    : _M_dataplus(__a, _S_construct(size_type(), _CharT(), __a)) { }

template<typename _CharT, typename _Traits, typename _Alloc>
    __rc_string_base<_CharT, _Traits, _Alloc>::
    __rc_string_base(const __rc_string_base& __rcs)
    : _M_dataplus(__rcs._M_get_allocator(),
		  __rcs._M_grab(__rcs._M_get_allocator())) { }

template<typename _CharT, typename _Traits, typename _Alloc>
    __rc_string_base<_CharT, _Traits, _Alloc>::
    __rc_string_base(size_type __n, _CharT __c, const _Alloc& __a)
    : _M_dataplus(__a, _S_construct(__n, __c, __a)) { }

template<typename _CharT, typename _Traits, typename _Alloc>
    template<typename _InputIterator>
    __rc_string_base<_CharT, _Traits, _Alloc>::
    __rc_string_base(_InputIterator __beg, _InputIterator __end,
		     const _Alloc& __a)
    : _M_dataplus(__a, _S_construct(__beg, __end, __a)) { }

template<typename _CharT, typename _Traits, typename _Alloc>
    void
    __rc_string_base<_CharT, _Traits, _Alloc>::
    _M_leak_hard()
    {
      if (_M_is_shared())
	_M_erase(0, 0);
      _M_set_leaked();
    }

// NB: This is the special case for Input Iterators, used in
  // istreambuf_iterators, etc.
  // Input Iterators have a cost structure very different from
  // pointers, calling for a different coding style.
  template<typename _CharT, typename _Traits, typename _Alloc>
    template<typename _InIterator>
      _CharT*
      __rc_string_base<_CharT, _Traits, _Alloc>::
      _S_construct(_InIterator __beg, _InIterator __end, const _Alloc& __a,
		   std::input_iterator_tag)
      {
	if (__beg == __end && __a == _Alloc())
	  return _S_empty_rep._M_refcopy();

// Avoid reallocation for common case.
	_CharT __buf[128];
	size_type __len = 0;
	while (__beg != __end && __len < sizeof(__buf) / sizeof(_CharT))
	  {
	    __buf[__len++] = *__beg;
	    ++__beg;
	  }
	_Rep* __r = _Rep::_S_create(__len, size_type(0), __a);
	_S_copy(__r->_M_refdata(), __buf, __len);
	__try
	  {
	    while (__beg != __end)
	      {
		if (__len == __r->_M_info._M_capacity)
		  {
		    // Allocate more space.
		    _Rep* __another = _Rep::_S_create(__len + 1, __len, __a);
		    _S_copy(__another->_M_refdata(), __r->_M_refdata(), __len);
		    __r->_M_destroy(__a);
		    __r = __another;
		  }
		__r->_M_refdata()[__len++] = *__beg;
		++__beg;
	      }
	  }
	__catch(...)
	  {
	    __r->_M_destroy(__a);
	    __throw_exception_again;
	  }
	__r->_M_set_length(__len);
	return __r->_M_refdata();
      }

template<typename _CharT, typename _Traits, typename _Alloc>
    template<typename _InIterator>
      _CharT*
      __rc_string_base<_CharT, _Traits, _Alloc>::
      _S_construct(_InIterator __beg, _InIterator __end, const _Alloc& __a,
		   std::forward_iterator_tag)
      {
	if (__beg == __end && __a == _Alloc())
	  return _S_empty_rep._M_refcopy();

// NB: Not required, but considered best practice.
	if (__is_null_pointer(__beg) && __beg != __end)
	  std::__throw_logic_error(__N("__rc_string_base::"
				       "_S_construct null not valid"));

const size_type __dnew = static_cast<size_type>(std::distance(__beg,
								      __end));
	// Check for out_of_range and length_error exceptions.
	_Rep* __r = _Rep::_S_create(__dnew, size_type(0), __a);
	__try
	  { __rc_string_base::_S_copy_chars(__r->_M_refdata(), __beg, __end); }
	__catch(...)
	  {
	    __r->_M_destroy(__a);
	    __throw_exception_again;
	  }
	__r->_M_set_length(__dnew);
	return __r->_M_refdata();
      }

template<typename _CharT, typename _Traits, typename _Alloc>
    _CharT*
    __rc_string_base<_CharT, _Traits, _Alloc>::
    _S_construct(size_type __n, _CharT __c, const _Alloc& __a)
    {
      if (__n == 0 && __a == _Alloc())
	return _S_empty_rep._M_refcopy();

// Check for out_of_range and length_error exceptions.
      _Rep* __r = _Rep::_S_create(__n, size_type(0), __a);
      if (__n)
	__rc_string_base::_S_assign(__r->_M_refdata(), __n, __c);

__r->_M_set_length(__n);
      return __r->_M_refdata();
    }

template<typename _CharT, typename _Traits, typename _Alloc>
    void
    __rc_string_base<_CharT, _Traits, _Alloc>::
    _M_swap(__rc_string_base& __rcs)
    {
      if (_M_is_leaked())
	_M_set_sharable();
      if (__rcs._M_is_leaked())
	__rcs._M_set_sharable();

_CharT* __tmp = _M_data();
      _M_data(__rcs._M_data());
      __rcs._M_data(__tmp);

// _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 431. Swapping containers with unequal allocators.
      std::__alloc_swap<allocator_type>::_S_do_it(_M_get_allocator(),
						  __rcs._M_get_allocator());
    }

template<typename _CharT, typename _Traits, typename _Alloc>
    void
    __rc_string_base<_CharT, _Traits, _Alloc>::
    _M_assign(const __rc_string_base& __rcs)
    {
      if (_M_rep() != __rcs._M_rep())
	{
	  _CharT* __tmp = __rcs._M_grab(_M_get_allocator());
	  _M_dispose();
	  _M_data(__tmp);
	}
    }

template<typename _CharT, typename _Traits, typename _Alloc>
    void
    __rc_string_base<_CharT, _Traits, _Alloc>::
    _M_reserve(size_type __res)
    {
      // Make sure we don't shrink below the current size.
      if (__res < _M_length())
	__res = _M_length();

if (__res != _M_capacity() || _M_is_shared())
	{
	  _CharT* __tmp = _M_rep()->_M_clone(_M_get_allocator(),
					     __res - _M_length());
	  _M_dispose();
	  _M_data(__tmp);
	}
    }

template<typename _CharT, typename _Traits, typename _Alloc>
    void
    __rc_string_base<_CharT, _Traits, _Alloc>::
    _M_mutate(size_type __pos, size_type __len1, const _CharT* __s,
	      size_type __len2)
    {
      const size_type __how_much = _M_length() - __pos - __len1;

_Rep* __r = _Rep::_S_create(_M_length() + __len2 - __len1,
				  _M_capacity(), _M_get_allocator());

if (__pos)
	this->_S_copy(__r->_M_refdata(), _M_data(), __pos);
      if (__s && __len2)
	this->_S_copy(__r->_M_refdata() + __pos, __s, __len2);
      if (__how_much)
	this->_S_copy(__r->_M_refdata() + __pos + __len2,
		_M_data() + __pos + __len1, __how_much);

_M_dispose();
      _M_data(__r->_M_refdata());
    }

template<typename _CharT, typename _Traits, typename _Alloc>
    void
    __rc_string_base<_CharT, _Traits, _Alloc>::
    _M_erase(size_type __pos, size_type __n)
    {
      const size_type __new_size = _M_length() - __n;
      const size_type __how_much = _M_length() - __pos - __n;

if (_M_is_shared())
	{
	  // Must reallocate.
	  _Rep* __r = _Rep::_S_create(__new_size, _M_capacity(),
				      _M_get_allocator());

if (__pos)
	    this->_S_copy(__r->_M_refdata(), _M_data(), __pos);
	  if (__how_much)
	    this->_S_copy(__r->_M_refdata() + __pos,
		    _M_data() + __pos + __n, __how_much);

_M_dispose();
	  _M_data(__r->_M_refdata());
	}
      else if (__how_much && __n)
	{
	  // Work in-place.
	  this->_S_move(_M_data() + __pos,
		  _M_data() + __pos + __n, __how_much);
	}

_M_rep()->_M_set_length(__new_size);
    }

template<>
    inline bool
    __rc_string_base<char, std::char_traits<char>,
		     std::allocator<char> >::
    _M_compare(const __rc_string_base& __rcs) const
    {
      if (_M_rep() == __rcs._M_rep())
	return true;
      return false;
    }

#ifdef _GLIBCXX_USE_WCHAR_T
  template<>
    inline bool
    __rc_string_base<wchar_t, std::char_traits<wchar_t>,
		     std::allocator<wchar_t> >::
    _M_compare(const __rc_string_base& __rcs) const
    {
      if (_M_rep() == __rcs._M_rep())
	return true;
      return false;
    }
#endif

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace

#endif /* _RC_STRING_BASE_H */

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