1 // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style license that can be 3 // found in the LICENSE file. 4 5 #ifndef BASE_STRING16_H_ 6 #define BASE_STRING16_H_ 7 8 // WHAT: 9 // A version of std::basic_string that provides 2-byte characters even when 10 // wchar_t is not implemented as a 2-byte type. You can access this class as 11 // string16. We also define char16, which string16 is based upon. 12 // 13 // WHY: 14 // On Windows, wchar_t is 2 bytes, and it can conveniently handle UTF-16/UCS-2 15 // data. Plenty of existing code operates on strings encoded as UTF-16. 16 // 17 // On many other platforms, sizeof(wchar_t) is 4 bytes by default. We can make 18 // it 2 bytes by using the GCC flag -fshort-wchar. But then std::wstring fails 19 // at run time, because it calls some functions (like wcslen) that come from 20 // the system's native C library -- which was built with a 4-byte wchar_t! 21 // It's wasteful to use 4-byte wchar_t strings to carry UTF-16 data, and it's 22 // entirely improper on those systems where the encoding of wchar_t is defined 23 // as UTF-32. 24 // 25 // Here, we define string16, which is similar to std::wstring but replaces all 26 // libc functions with custom, 2-byte-char compatible routines. It is capable 27 // of carrying UTF-16-encoded data. 28 29 #include <stdio.h> 30 #include <string> 31 32 #include "base/basictypes.h" 33 34 #if defined(WCHAR_T_IS_UTF16) 35 36 typedef wchar_t char16; 37 typedef std::wstring string16; 38 39 #elif defined(WCHAR_T_IS_UTF32) 40 41 typedef uint16 char16; 42 43 namespace base { 44 45 // char16 versions of the functions required by string16_char_traits; these 46 // are based on the wide character functions of similar names ("w" or "wcs" 47 // instead of "c16"). 48 int c16memcmp(const char16* s1, const char16* s2, size_t n); 49 size_t c16len(const char16* s); 50 const char16* c16memchr(const char16* s, char16 c, size_t n); 51 char16* c16memmove(char16* s1, const char16* s2, size_t n); 52 char16* c16memcpy(char16* s1, const char16* s2, size_t n); 53 char16* c16memset(char16* s, char16 c, size_t n); 54 55 struct string16_char_traits { 56 typedef char16 char_type; 57 typedef int int_type; 58 59 // int_type needs to be able to hold each possible value of char_type, and in 60 // addition, the distinct value of eof(). 61 COMPILE_ASSERT(sizeof(int_type) > sizeof(char_type), unexpected_type_width); 62 63 typedef std::streamoff off_type; 64 typedef mbstate_t state_type; 65 typedef std::fpos<state_type> pos_type; 66 67 static void assign(char_type& c1, const char_type& c2) { 68 c1 = c2; 69 } 70 71 static bool eq(const char_type& c1, const char_type& c2) { 72 return c1 == c2; 73 } 74 static bool lt(const char_type& c1, const char_type& c2) { 75 return c1 < c2; 76 } 77 78 static int compare(const char_type* s1, const char_type* s2, size_t n) { 79 return c16memcmp(s1, s2, n); 80 } 81 82 static size_t length(const char_type* s) { 83 return c16len(s); 84 } 85 86 static const char_type* find(const char_type* s, size_t n, 87 const char_type& a) { 88 return c16memchr(s, a, n); 89 } 90 91 static char_type* move(char_type* s1, const char_type* s2, int_type n) { 92 return c16memmove(s1, s2, n); 93 } 94 95 static char_type* copy(char_type* s1, const char_type* s2, size_t n) { 96 return c16memcpy(s1, s2, n); 97 } 98 99 static char_type* assign(char_type* s, size_t n, char_type a) { 100 return c16memset(s, a, n); 101 } 102 103 static int_type not_eof(const int_type& c) { 104 return eq_int_type(c, eof()) ? 0 : c; 105 } 106 107 static char_type to_char_type(const int_type& c) { 108 return char_type(c); 109 } 110 111 static int_type to_int_type(const char_type& c) { 112 return int_type(c); 113 } 114 115 static bool eq_int_type(const int_type& c1, const int_type& c2) { 116 return c1 == c2; 117 } 118 119 static int_type eof() { 120 return static_cast<int_type>(EOF); 121 } 122 }; 123 124 } // namespace base 125 126 // The string class will be explicitly instantiated only once, in string16.cc. 127 // 128 // std::basic_string<> in GNU libstdc++ contains a static data member, 129 // _S_empty_rep_storage, to represent empty strings. When an operation such 130 // as assignment or destruction is performed on a string, causing its existing 131 // data member to be invalidated, it must not be freed if this static data 132 // member is being used. Otherwise, it counts as an attempt to free static 133 // (and not allocated) data, which is a memory error. 134 // 135 // Generally, due to C++ template magic, _S_empty_rep_storage will be marked 136 // as a coalesced symbol, meaning that the linker will combine multiple 137 // instances into a single one when generating output. 138 // 139 // If a string class is used by multiple shared libraries, a problem occurs. 140 // Each library will get its own copy of _S_empty_rep_storage. When strings 141 // are passed across a library boundary for alteration or destruction, memory 142 // errors will result. GNU libstdc++ contains a configuration option, 143 // --enable-fully-dynamic-string (_GLIBCXX_FULLY_DYNAMIC_STRING), which 144 // disables the static data member optimization, but it's a good optimization 145 // and non-STL code is generally at the mercy of the system's STL 146 // configuration. Fully-dynamic strings are not the default for GNU libstdc++ 147 // libstdc++ itself or for the libstdc++ installations on the systems we care 148 // about, such as Mac OS X and relevant flavors of Linux. 149 // 150 // See also http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24196 . 151 // 152 // To avoid problems, string classes need to be explicitly instantiated only 153 // once, in exactly one library. All other string users see it via an "extern" 154 // declaration. This is precisely how GNU libstdc++ handles 155 // std::basic_string<char> (string) and std::basic_string<wchar_t> (wstring). 156 // 157 // This also works around a Mac OS X linker bug in ld64-85.2.1 (Xcode 3.1.2), 158 // in which the linker does not fully coalesce symbols when dead code 159 // stripping is enabled. This bug causes the memory errors described above 160 // to occur even when a std::basic_string<> does not cross shared library 161 // boundaries, such as in statically-linked executables. 162 // 163 // TODO(mark): File this bug with Apple and update this note with a bug number. 164 165 extern template class std::basic_string<char16, base::string16_char_traits>; 166 167 typedef std::basic_string<char16, base::string16_char_traits> string16; 168 169 extern std::ostream& operator<<(std::ostream& out, const string16& str); 170 171 #endif // WCHAR_T_IS_UTF32 172 173 #endif // BASE_STRING16_H_ 174
