string16.h revision 731df977c0511bca2206b5f333555b1205ff1f43 
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#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/basictypes.h"
34
35#if defined(WCHAR_T_IS_UTF16)
36
37typedef wchar_t char16;
38typedef std::wstring string16;
39
40#elif defined(WCHAR_T_IS_UTF32)
41
42typedef uint16 char16;
43
44namespace base {
45
46// char16 versions of the functions required by string16_char_traits; these
47// are based on the wide character functions of similar names ("w" or "wcs"
48// instead of "c16").
49int c16memcmp(const char16* s1, const char16* s2, size_t n);
50size_t c16len(const char16* s);
51const char16* c16memchr(const char16* s, char16 c, size_t n);
52char16* c16memmove(char16* s1, const char16* s2, size_t n);
53char16* c16memcpy(char16* s1, const char16* s2, size_t n);
54char16* c16memset(char16* s, char16 c, size_t n);
55
56struct string16_char_traits {
57  typedef char16 char_type;
58  typedef int int_type;
59
60  // int_type needs to be able to hold each possible value of char_type, and in
61  // addition, the distinct value of eof().
62  COMPILE_ASSERT(sizeof(int_type) > sizeof(char_type), unexpected_type_width);
63
64  typedef std::streamoff off_type;
65  typedef mbstate_t state_type;
66  typedef std::fpos<state_type> pos_type;
67
68  static void assign(char_type& c1, const char_type& c2) {
69    c1 = c2;
70  }
71
72  static bool eq(const char_type& c1, const char_type& c2) {
73    return c1 == c2;
74  }
75  static bool lt(const char_type& c1, const char_type& c2) {
76    return c1 < c2;
77  }
78
79  static int compare(const char_type* s1, const char_type* s2, size_t n) {
80    return c16memcmp(s1, s2, n);
81  }
82
83  static size_t length(const char_type* s) {
84    return c16len(s);
85  }
86
87  static const char_type* find(const char_type* s, size_t n,
88                               const char_type& a) {
89    return c16memchr(s, a, n);
90  }
91
92  static char_type* move(char_type* s1, const char_type* s2, int_type n) {
93    return c16memmove(s1, s2, n);
94  }
95
96  static char_type* copy(char_type* s1, const char_type* s2, size_t n) {
97    return c16memcpy(s1, s2, n);
98  }
99
100  static char_type* assign(char_type* s, size_t n, char_type a) {
101    return c16memset(s, a, n);
102  }
103
104  static int_type not_eof(const int_type& c) {
105    return eq_int_type(c, eof()) ? 0 : c;
106  }
107
108  static char_type to_char_type(const int_type& c) {
109    return char_type(c);
110  }
111
112  static int_type to_int_type(const char_type& c) {
113    return int_type(c);
114  }
115
116  static bool eq_int_type(const int_type& c1, const int_type& c2) {
117    return c1 == c2;
118  }
119
120  static int_type eof() {
121    return static_cast<int_type>(EOF);
122  }
123};
124
125}  // namespace base
126
127// The string class will be explicitly instantiated only once, in string16.cc.
128//
129// std::basic_string<> in GNU libstdc++ contains a static data member,
130// _S_empty_rep_storage, to represent empty strings.  When an operation such
131// as assignment or destruction is performed on a string, causing its existing
132// data member to be invalidated, it must not be freed if this static data
133// member is being used.  Otherwise, it counts as an attempt to free static
134// (and not allocated) data, which is a memory error.
135//
136// Generally, due to C++ template magic, _S_empty_rep_storage will be marked
137// as a coalesced symbol, meaning that the linker will combine multiple
138// instances into a single one when generating output.
139//
140// If a string class is used by multiple shared libraries, a problem occurs.
141// Each library will get its own copy of _S_empty_rep_storage.  When strings
142// are passed across a library boundary for alteration or destruction, memory
143// errors will result.  GNU libstdc++ contains a configuration option,
144// --enable-fully-dynamic-string (_GLIBCXX_FULLY_DYNAMIC_STRING), which
145// disables the static data member optimization, but it's a good optimization
146// and non-STL code is generally at the mercy of the system's STL
147// configuration.  Fully-dynamic strings are not the default for GNU libstdc++
148// libstdc++ itself or for the libstdc++ installations on the systems we care
149// about, such as Mac OS X and relevant flavors of Linux.
150//
151// See also http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24196 .
152//
153// To avoid problems, string classes need to be explicitly instantiated only
154// once, in exactly one library.  All other string users see it via an "extern"
155// declaration.  This is precisely how GNU libstdc++ handles
156// std::basic_string<char> (string) and std::basic_string<wchar_t> (wstring).
157//
158// This also works around a Mac OS X linker bug in ld64-85.2.1 (Xcode 3.1.2),
159// in which the linker does not fully coalesce symbols when dead code
160// stripping is enabled.  This bug causes the memory errors described above
161// to occur even when a std::basic_string<> does not cross shared library
162// boundaries, such as in statically-linked executables.
163//
164// TODO(mark): File this bug with Apple and update this note with a bug number.
165
166extern template class std::basic_string<char16, base::string16_char_traits>;
167
168typedef std::basic_string<char16, base::string16_char_traits> string16;
169
170namespace base {
171extern std::ostream& operator<<(std::ostream& out, const string16& str);
172}
173
174#endif  // WCHAR_T_IS_UTF32
175
176#endif  // BASE_STRING16_H_
177