base/string16.h

   1 // Copyright (c) 2012 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 #pragma once
   8
   9 // WHAT:
  10 // A version of std::basic_string that provides 2-byte characters even when
  11 // wchar_t is not implemented as a 2-byte type. You can access this class as
  12 // string16. We also define char16, which string16 is based upon.
  13 //
  14 // WHY:
  15 // On Windows, wchar_t is 2 bytes, and it can conveniently handle UTF-16/UCS-2
  16 // data. Plenty of existing code operates on strings encoded as UTF-16.
  17 //
  18 // On many other platforms, sizeof(wchar_t) is 4 bytes by default. We can make
  19 // it 2 bytes by using the GCC flag -fshort-wchar. But then std::wstring fails
  20 // at run time, because it calls some functions (like wcslen) that come from
  21 // the system's native C library -- which was built with a 4-byte wchar_t!
  22 // It's wasteful to use 4-byte wchar_t strings to carry UTF-16 data, and it's
  23 // entirely improper on those systems where the encoding of wchar_t is defined
  24 // as UTF-32.
  25 //
  26 // Here, we define string16, which is similar to std::wstring but replaces all
  27 // libc functions with custom, 2-byte-char compatible routines. It is capable
  28 // of carrying UTF-16-encoded data.
  29
  30 #include <stdio.h>
  31 #include <string>
  32
  33 #include "base/base_export.h"
  34 #include "base/basictypes.h"
  35
  36 #if defined(WCHAR_T_IS_UTF16)
  37
  38 typedef wchar_t char16;
  39 typedef std::wstring string16;
  40
  41 #elif defined(WCHAR_T_IS_UTF32)
  42
  43 typedef uint16 char16;
  44
  45 namespace base {
  46
  47 // char16 versions of the functions required by string16_char_traits; these
  48 // are based on the wide character functions of similar names ("w" or "wcs"
  49 // instead of "c16").
  50 BASE_EXPORT int c16memcmp(const char16* s1, const char16* s2, size_t n);
  51 BASE_EXPORT size_t c16len(const char16* s);
  52 BASE_EXPORT const char16* c16memchr(const char16* s, char16 c, size_t n);
  53 BASE_EXPORT char16* c16memmove(char16* s1, const char16* s2, size_t n);
  54 BASE_EXPORT char16* c16memcpy(char16* s1, const char16* s2, size_t n);
  55 BASE_EXPORT char16* c16memset(char16* s, char16 c, size_t n);
  56
  57 struct string16_char_traits {
  58   typedef char16 char_type;
  59   typedef int int_type;
  60
  61   // int_type needs to be able to hold each possible value of char_type, and in
  62   // addition, the distinct value of eof().
  63   COMPILE_ASSERT(sizeof(int_type) > sizeof(char_type), unexpected_type_width);
  64
  65   typedef std::streamoff off_type;
  66   typedef mbstate_t state_type;
  67   typedef std::fpos<state_type> pos_type;
  68
  69   static void assign(char_type& c1, const char_type& c2) {
  70     c1 = c2;
  71   }
  72
  73   static bool eq(const char_type& c1, const char_type& c2) {
  74     return c1 == c2;
  75   }
  76   static bool lt(const char_type& c1, const char_type& c2) {
  77     return c1 < c2;
  78   }
  79
  80   static int compare(const char_type* s1, const char_type* s2, size_t n) {
  81     return c16memcmp(s1, s2, n);
  82   }
  83
  84   static size_t length(const char_type* s) {
  85     return c16len(s);
  86   }
  87
  88   static const char_type* find(const char_type* s, size_t n,
  89                                const char_type& a) {
  90     return c16memchr(s, a, n);
  91   }
  92
  93   static char_type* move(char_type* s1, const char_type* s2, int_type n) {
  94     return c16memmove(s1, s2, n);
  95   }
  96
  97   static char_type* copy(char_type* s1, const char_type* s2, size_t n) {
  98     return c16memcpy(s1, s2, n);
  99   }
 100
 101   static char_type* assign(char_type* s, size_t n, char_type a) {
 102     return c16memset(s, a, n);
 103   }
 104
 105   static int_type not_eof(const int_type& c) {
 106     return eq_int_type(c, eof()) ? 0 : c;
 107   }
 108
 109   static char_type to_char_type(const int_type& c) {
 110     return char_type(c);
 111   }
 112
 113   static int_type to_int_type(const char_type& c) {
 114     return int_type(c);
 115   }
 116
 117   static bool eq_int_type(const int_type& c1, const int_type& c2) {
 118     return c1 == c2;
 119   }
 120
 121   static int_type eof() {
 122     return static_cast<int_type>(EOF);
 123   }
 124 };
 125
 126 }  // namespace base
 127
 128 // The string class will be explicitly instantiated only once, in string16.cc.
 129 //
 130 // std::basic_string<> in GNU libstdc++ contains a static data member,
 131 // _S_empty_rep_storage, to represent empty strings.  When an operation such
 132 // as assignment or destruction is performed on a string, causing its existing
 133 // data member to be invalidated, it must not be freed if this static data
 134 // member is being used.  Otherwise, it counts as an attempt to free static
 135 // (and not allocated) data, which is a memory error.
 136 //
 137 // Generally, due to C++ template magic, _S_empty_rep_storage will be marked
 138 // as a coalesced symbol, meaning that the linker will combine multiple
 139 // instances into a single one when generating output.
 140 //
 141 // If a string class is used by multiple shared libraries, a problem occurs.
 142 // Each library will get its own copy of _S_empty_rep_storage.  When strings
 143 // are passed across a library boundary for alteration or destruction, memory
 144 // errors will result.  GNU libstdc++ contains a configuration option,
 145 // --enable-fully-dynamic-string (_GLIBCXX_FULLY_DYNAMIC_STRING), which
 146 // disables the static data member optimization, but it's a good optimization
 147 // and non-STL code is generally at the mercy of the system's STL
 148 // configuration.  Fully-dynamic strings are not the default for GNU libstdc++
 149 // libstdc++ itself or for the libstdc++ installations on the systems we care
 150 // about, such as Mac OS X and relevant flavors of Linux.
 151 //
 152 // See also http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24196 .
 153 //
 154 // To avoid problems, string classes need to be explicitly instantiated only
 155 // once, in exactly one library.  All other string users see it via an "extern"
 156 // declaration.  This is precisely how GNU libstdc++ handles
 157 // std::basic_string<char> (string) and std::basic_string<wchar_t> (wstring).
 158 //
 159 // This also works around a Mac OS X linker bug in ld64-85.2.1 (Xcode 3.1.2),
 160 // in which the linker does not fully coalesce symbols when dead code
 161 // stripping is enabled.  This bug causes the memory errors described above
 162 // to occur even when a std::basic_string<> does not cross shared library
 163 // boundaries, such as in statically-linked executables.
 164 //
 165 // TODO(mark): File this bug with Apple and update this note with a bug number.
 166
 167 extern template
 168 class BASE_EXPORT std::basic_string<char16, base::string16_char_traits>;
 169
 170 typedef std::basic_string<char16, base::string16_char_traits> string16;
 171
 172 namespace base {
 173 BASE_EXPORT extern std::ostream& operator<<(std::ostream& out,
 174                                             const string16& str);
 175
 176 // This is required by googletest to print a readable output on test failures.
 177 BASE_EXPORT extern void PrintTo(const string16& str, std::ostream* out);
 178 }
 179
 180 #endif  // WCHAR_T_IS_UTF32
 181
 182 #endif  // BASE_STRING16_H_