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 <stdio.h>
30#include <string>
31
32#include "base/base_export.h"
33#include "base/basictypes.h"
34
35#if defined(WCHAR_T_IS_UTF16)
36
37namespace base {
38
39typedef wchar_t char16;
40typedef std::wstring string16;
41typedef std::char_traits<wchar_t> string16_char_traits;
42
43}  // namespace base
44
45#elif defined(WCHAR_T_IS_UTF32)
46
47namespace base {
48
49typedef uint16 char16;
50
51// char16 versions of the functions required by string16_char_traits; these
52// are based on the wide character functions of similar names ("w" or "wcs"
53// instead of "c16").
54BASE_EXPORT int c16memcmp(const char16* s1, const char16* s2, size_t n);
55BASE_EXPORT size_t c16len(const char16* s);
56BASE_EXPORT const char16* c16memchr(const char16* s, char16 c, size_t n);
57BASE_EXPORT char16* c16memmove(char16* s1, const char16* s2, size_t n);
58BASE_EXPORT char16* c16memcpy(char16* s1, const char16* s2, size_t n);
59BASE_EXPORT char16* c16memset(char16* s, char16 c, size_t n);
60
61struct string16_char_traits {
62  typedef char16 char_type;
63  typedef int int_type;
64
65  // int_type needs to be able to hold each possible value of char_type, and in
66  // addition, the distinct value of eof().
67  COMPILE_ASSERT(sizeof(int_type) > sizeof(char_type), unexpected_type_width);
68
69  typedef std::streamoff off_type;
70  typedef mbstate_t state_type;
71  typedef std::fpos<state_type> pos_type;
72
73  static void assign(char_type& c1, const char_type& c2) {
74    c1 = c2;
75  }
76
77  static bool eq(const char_type& c1, const char_type& c2) {
78    return c1 == c2;
79  }
80  static bool lt(const char_type& c1, const char_type& c2) {
81    return c1 < c2;
82  }
83
84  static int compare(const char_type* s1, const char_type* s2, size_t n) {
85    return c16memcmp(s1, s2, n);
86  }
87
88  static size_t length(const char_type* s) {
89    return c16len(s);
90  }
91
92  static const char_type* find(const char_type* s, size_t n,
93                               const char_type& a) {
94    return c16memchr(s, a, n);
95  }
96
97  static char_type* move(char_type* s1, const char_type* s2, int_type n) {
98    return c16memmove(s1, s2, n);
99  }
100
101  static char_type* copy(char_type* s1, const char_type* s2, size_t n) {
102    return c16memcpy(s1, s2, n);
103  }
104
105  static char_type* assign(char_type* s, size_t n, char_type a) {
106    return c16memset(s, a, n);
107  }
108
109  static int_type not_eof(const int_type& c) {
110    return eq_int_type(c, eof()) ? 0 : c;
111  }
112
113  static char_type to_char_type(const int_type& c) {
114    return char_type(c);
115  }
116
117  static int_type to_int_type(const char_type& c) {
118    return int_type(c);
119  }
120
121  static bool eq_int_type(const int_type& c1, const int_type& c2) {
122    return c1 == c2;
123  }
124
125  static int_type eof() {
126    return static_cast<int_type>(EOF);
127  }
128};
129
130typedef std::basic_string<char16, base::string16_char_traits> string16;
131
132BASE_EXPORT extern std::ostream& operator<<(std::ostream& out,
133                                            const string16& str);
134
135// This is required by googletest to print a readable output on test failures.
136BASE_EXPORT extern void PrintTo(const string16& str, std::ostream* out);
137
138}  // namespace base
139
140// The string class will be explicitly instantiated only once, in string16.cc.
141//
142// std::basic_string<> in GNU libstdc++ contains a static data member,
143// _S_empty_rep_storage, to represent empty strings.  When an operation such
144// as assignment or destruction is performed on a string, causing its existing
145// data member to be invalidated, it must not be freed if this static data
146// member is being used.  Otherwise, it counts as an attempt to free static
147// (and not allocated) data, which is a memory error.
148//
149// Generally, due to C++ template magic, _S_empty_rep_storage will be marked
150// as a coalesced symbol, meaning that the linker will combine multiple
151// instances into a single one when generating output.
152//
153// If a string class is used by multiple shared libraries, a problem occurs.
154// Each library will get its own copy of _S_empty_rep_storage.  When strings
155// are passed across a library boundary for alteration or destruction, memory
156// errors will result.  GNU libstdc++ contains a configuration option,
157// --enable-fully-dynamic-string (_GLIBCXX_FULLY_DYNAMIC_STRING), which
158// disables the static data member optimization, but it's a good optimization
159// and non-STL code is generally at the mercy of the system's STL
160// configuration.  Fully-dynamic strings are not the default for GNU libstdc++
161// libstdc++ itself or for the libstdc++ installations on the systems we care
162// about, such as Mac OS X and relevant flavors of Linux.
163//
164// See also http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24196 .
165//
166// To avoid problems, string classes need to be explicitly instantiated only
167// once, in exactly one library.  All other string users see it via an "extern"
168// declaration.  This is precisely how GNU libstdc++ handles
169// std::basic_string<char> (string) and std::basic_string<wchar_t> (wstring).
170//
171// This also works around a Mac OS X linker bug in ld64-85.2.1 (Xcode 3.1.2),
172// in which the linker does not fully coalesce symbols when dead code
173// stripping is enabled.  This bug causes the memory errors described above
174// to occur even when a std::basic_string<> does not cross shared library
175// boundaries, such as in statically-linked executables.
176//
177// TODO(mark): File this bug with Apple and update this note with a bug number.
178
179extern template
180class BASE_EXPORT std::basic_string<base::char16, base::string16_char_traits>;
181
182#endif  // WCHAR_T_IS_UTF32
183
184#endif  // BASE_STRINGS_STRING16_H_
185