1 // Copyright 2013 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_STRINGS_STRING16_H_ 6 #define BASE_STRINGS_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 <stddef.h> 30 #include <stdint.h> 31 #include <stdio.h> 32 33 #include <functional> 34 #include <string> 35 36 #include "base/base_export.h" 37 #include "build/build_config.h" 38 39 #if defined(WCHAR_T_IS_UTF16) 40 41 namespace base { 42 43 typedef wchar_t char16; 44 typedef std::wstring string16; 45 46 } // namespace base 47 48 #elif defined(WCHAR_T_IS_UTF32) 49 50 #include <wchar.h> // for mbstate_t 51 52 namespace base { 53 54 typedef uint16_t char16; 55 56 // char16 versions of the functions required by string16_char_traits; these 57 // are based on the wide character functions of similar names ("w" or "wcs" 58 // instead of "c16"). 59 BASE_EXPORT int c16memcmp(const char16* s1, const char16* s2, size_t n); 60 BASE_EXPORT size_t c16len(const char16* s); 61 BASE_EXPORT const char16* c16memchr(const char16* s, char16 c, size_t n); 62 BASE_EXPORT char16* c16memmove(char16* s1, const char16* s2, size_t n); 63 BASE_EXPORT char16* c16memcpy(char16* s1, const char16* s2, size_t n); 64 BASE_EXPORT char16* c16memset(char16* s, char16 c, size_t n); 65 66 // This namespace contains the implementation of base::string16 along with 67 // things that need to be found via argument-dependent lookup from a 68 // base::string16. 69 namespace string16_internals { 70 71 struct string16_char_traits { 72 typedef char16 char_type; 73 typedef int int_type; 74 75 // int_type needs to be able to hold each possible value of char_type, and in 76 // addition, the distinct value of eof(). 77 static_assert(sizeof(int_type) > sizeof(char_type), 78 "int must be larger than 16 bits wide"); 79 80 typedef std::streamoff off_type; 81 typedef mbstate_t state_type; 82 typedef std::fpos<state_type> pos_type; 83 assignstring16_char_traits84 static void assign(char_type& c1, const char_type& c2) { 85 c1 = c2; 86 } 87 eqstring16_char_traits88 static bool eq(const char_type& c1, const char_type& c2) { 89 return c1 == c2; 90 } ltstring16_char_traits91 static bool lt(const char_type& c1, const char_type& c2) { 92 return c1 < c2; 93 } 94 comparestring16_char_traits95 static int compare(const char_type* s1, const char_type* s2, size_t n) { 96 return c16memcmp(s1, s2, n); 97 } 98 lengthstring16_char_traits99 static size_t length(const char_type* s) { 100 return c16len(s); 101 } 102 findstring16_char_traits103 static const char_type* find(const char_type* s, size_t n, 104 const char_type& a) { 105 return c16memchr(s, a, n); 106 } 107 movestring16_char_traits108 static char_type* move(char_type* s1, const char_type* s2, size_t n) { 109 return c16memmove(s1, s2, n); 110 } 111 copystring16_char_traits112 static char_type* copy(char_type* s1, const char_type* s2, size_t n) { 113 return c16memcpy(s1, s2, n); 114 } 115 assignstring16_char_traits116 static char_type* assign(char_type* s, size_t n, char_type a) { 117 return c16memset(s, a, n); 118 } 119 not_eofstring16_char_traits120 static int_type not_eof(const int_type& c) { 121 return eq_int_type(c, eof()) ? 0 : c; 122 } 123 to_char_typestring16_char_traits124 static char_type to_char_type(const int_type& c) { 125 return char_type(c); 126 } 127 to_int_typestring16_char_traits128 static int_type to_int_type(const char_type& c) { 129 return int_type(c); 130 } 131 eq_int_typestring16_char_traits132 static bool eq_int_type(const int_type& c1, const int_type& c2) { 133 return c1 == c2; 134 } 135 eofstring16_char_traits136 static int_type eof() { 137 return static_cast<int_type>(EOF); 138 } 139 }; 140 141 } // namespace string16_internals 142 143 typedef std::basic_string<char16, 144 base::string16_internals::string16_char_traits> 145 string16; 146 147 namespace string16_internals { 148 149 BASE_EXPORT extern std::ostream& operator<<(std::ostream& out, 150 const string16& str); 151 152 // This is required by googletest to print a readable output on test failures. 153 BASE_EXPORT extern void PrintTo(const string16& str, std::ostream* out); 154 155 } // namespace string16_internals 156 157 } // namespace base 158 159 // The string class will be explicitly instantiated only once, in string16.cc. 160 // 161 // std::basic_string<> in GNU libstdc++ contains a static data member, 162 // _S_empty_rep_storage, to represent empty strings. When an operation such 163 // as assignment or destruction is performed on a string, causing its existing 164 // data member to be invalidated, it must not be freed if this static data 165 // member is being used. Otherwise, it counts as an attempt to free static 166 // (and not allocated) data, which is a memory error. 167 // 168 // Generally, due to C++ template magic, _S_empty_rep_storage will be marked 169 // as a coalesced symbol, meaning that the linker will combine multiple 170 // instances into a single one when generating output. 171 // 172 // If a string class is used by multiple shared libraries, a problem occurs. 173 // Each library will get its own copy of _S_empty_rep_storage. When strings 174 // are passed across a library boundary for alteration or destruction, memory 175 // errors will result. GNU libstdc++ contains a configuration option, 176 // --enable-fully-dynamic-string (_GLIBCXX_FULLY_DYNAMIC_STRING), which 177 // disables the static data member optimization, but it's a good optimization 178 // and non-STL code is generally at the mercy of the system's STL 179 // configuration. Fully-dynamic strings are not the default for GNU libstdc++ 180 // libstdc++ itself or for the libstdc++ installations on the systems we care 181 // about, such as Mac OS X and relevant flavors of Linux. 182 // 183 // See also http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24196 . 184 // 185 // To avoid problems, string classes need to be explicitly instantiated only 186 // once, in exactly one library. All other string users see it via an "extern" 187 // declaration. This is precisely how GNU libstdc++ handles 188 // std::basic_string<char> (string) and std::basic_string<wchar_t> (wstring). 189 // 190 // This also works around a Mac OS X linker bug in ld64-85.2.1 (Xcode 3.1.2), 191 // in which the linker does not fully coalesce symbols when dead code 192 // stripping is enabled. This bug causes the memory errors described above 193 // to occur even when a std::basic_string<> does not cross shared library 194 // boundaries, such as in statically-linked executables. 195 // 196 // TODO(mark): File this bug with Apple and update this note with a bug number. 197 198 extern template class BASE_EXPORT 199 std::basic_string<base::char16, 200 base::string16_internals::string16_char_traits>; 201 202 // Specialize std::hash for base::string16. Although the style guide forbids 203 // this in general, it is necessary for consistency with WCHAR_T_IS_UTF16 204 // platforms, where base::string16 is a type alias for std::wstring. 205 namespace std { 206 template <> 207 struct hash<base::string16> { 208 std::size_t operator()(const base::string16& s) const { 209 std::size_t result = 0; 210 for (base::char16 c : s) 211 result = (result * 131) + c; 212 return result; 213 } 214 }; 215 } // namespace std 216 217 #endif // WCHAR_T_IS_UTF32 218 219 #endif // BASE_STRINGS_STRING16_H_ 220