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 41namespace base { 42 43typedef wchar_t char16; 44typedef std::wstring string16; 45typedef std::char_traits<wchar_t> string16_char_traits; 46 47} // namespace base 48 49#elif defined(WCHAR_T_IS_UTF32) 50 51#include <wchar.h> // for mbstate_t 52 53namespace base { 54 55typedef uint16_t char16; 56 57// char16 versions of the functions required by string16_char_traits; these 58// are based on the wide character functions of similar names ("w" or "wcs" 59// instead of "c16"). 60BASE_EXPORT int c16memcmp(const char16* s1, const char16* s2, size_t n); 61BASE_EXPORT size_t c16len(const char16* s); 62BASE_EXPORT const char16* c16memchr(const char16* s, char16 c, size_t n); 63BASE_EXPORT char16* c16memmove(char16* s1, const char16* s2, size_t n); 64BASE_EXPORT char16* c16memcpy(char16* s1, const char16* s2, size_t n); 65BASE_EXPORT char16* c16memset(char16* s, char16 c, size_t n); 66 67struct string16_char_traits { 68 typedef char16 char_type; 69 typedef int int_type; 70 71 // int_type needs to be able to hold each possible value of char_type, and in 72 // addition, the distinct value of eof(). 73 static_assert(sizeof(int_type) > sizeof(char_type), 74 "int must be larger than 16 bits wide"); 75 76 typedef std::streamoff off_type; 77 typedef mbstate_t state_type; 78 typedef std::fpos<state_type> pos_type; 79 80 static void assign(char_type& c1, const char_type& c2) { 81 c1 = c2; 82 } 83 84 static bool eq(const char_type& c1, const char_type& c2) { 85 return c1 == c2; 86 } 87 static bool lt(const char_type& c1, const char_type& c2) { 88 return c1 < c2; 89 } 90 91 static int compare(const char_type* s1, const char_type* s2, size_t n) { 92 return c16memcmp(s1, s2, n); 93 } 94 95 static size_t length(const char_type* s) { 96 return c16len(s); 97 } 98 99 static const char_type* find(const char_type* s, size_t n, 100 const char_type& a) { 101 return c16memchr(s, a, n); 102 } 103 104 static char_type* move(char_type* s1, const char_type* s2, size_t n) { 105 return c16memmove(s1, s2, n); 106 } 107 108 static char_type* copy(char_type* s1, const char_type* s2, size_t n) { 109 return c16memcpy(s1, s2, n); 110 } 111 112 static char_type* assign(char_type* s, size_t n, char_type a) { 113 return c16memset(s, a, n); 114 } 115 116 static int_type not_eof(const int_type& c) { 117 return eq_int_type(c, eof()) ? 0 : c; 118 } 119 120 static char_type to_char_type(const int_type& c) { 121 return char_type(c); 122 } 123 124 static int_type to_int_type(const char_type& c) { 125 return int_type(c); 126 } 127 128 static bool eq_int_type(const int_type& c1, const int_type& c2) { 129 return c1 == c2; 130 } 131 132 static int_type eof() { 133 return static_cast<int_type>(EOF); 134 } 135}; 136 137typedef std::basic_string<char16, base::string16_char_traits> string16; 138 139BASE_EXPORT extern std::ostream& operator<<(std::ostream& out, 140 const string16& str); 141 142// This is required by googletest to print a readable output on test failures. 143BASE_EXPORT extern void PrintTo(const string16& str, std::ostream* out); 144 145} // namespace base 146 147// The string class will be explicitly instantiated only once, in string16.cc. 148// 149// std::basic_string<> in GNU libstdc++ contains a static data member, 150// _S_empty_rep_storage, to represent empty strings. When an operation such 151// as assignment or destruction is performed on a string, causing its existing 152// data member to be invalidated, it must not be freed if this static data 153// member is being used. Otherwise, it counts as an attempt to free static 154// (and not allocated) data, which is a memory error. 155// 156// Generally, due to C++ template magic, _S_empty_rep_storage will be marked 157// as a coalesced symbol, meaning that the linker will combine multiple 158// instances into a single one when generating output. 159// 160// If a string class is used by multiple shared libraries, a problem occurs. 161// Each library will get its own copy of _S_empty_rep_storage. When strings 162// are passed across a library boundary for alteration or destruction, memory 163// errors will result. GNU libstdc++ contains a configuration option, 164// --enable-fully-dynamic-string (_GLIBCXX_FULLY_DYNAMIC_STRING), which 165// disables the static data member optimization, but it's a good optimization 166// and non-STL code is generally at the mercy of the system's STL 167// configuration. Fully-dynamic strings are not the default for GNU libstdc++ 168// libstdc++ itself or for the libstdc++ installations on the systems we care 169// about, such as Mac OS X and relevant flavors of Linux. 170// 171// See also http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24196 . 172// 173// To avoid problems, string classes need to be explicitly instantiated only 174// once, in exactly one library. All other string users see it via an "extern" 175// declaration. This is precisely how GNU libstdc++ handles 176// std::basic_string<char> (string) and std::basic_string<wchar_t> (wstring). 177// 178// This also works around a Mac OS X linker bug in ld64-85.2.1 (Xcode 3.1.2), 179// in which the linker does not fully coalesce symbols when dead code 180// stripping is enabled. This bug causes the memory errors described above 181// to occur even when a std::basic_string<> does not cross shared library 182// boundaries, such as in statically-linked executables. 183// 184// TODO(mark): File this bug with Apple and update this note with a bug number. 185 186extern template 187class BASE_EXPORT std::basic_string<base::char16, base::string16_char_traits>; 188 189// Specialize std::hash for base::string16. Although the style guide forbids 190// this in general, it is necessary for consistency with WCHAR_T_IS_UTF16 191// platforms, where base::string16 is a type alias for std::wstring. 192namespace std { 193template <> 194struct hash<base::string16> { 195 std::size_t operator()(const base::string16& s) const { 196 std::size_t result = 0; 197 for (base::char16 c : s) 198 result = (result * 131) + c; 199 return result; 200 } 201}; 202} // namespace std 203 204#endif // WCHAR_T_IS_UTF32 205 206#endif // BASE_STRINGS_STRING16_H_ 207