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// Copyright 2013 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#ifndef BASE_STRINGS_STRING16_H_
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#define BASE_STRINGS_STRING16_H_
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// A version of std::basic_string that provides 2-byte characters even when
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// wchar_t is not implemented as a 2-byte type. You can access this class as
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// string16. We also define char16, which string16 is based upon.
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// On Windows, wchar_t is 2 bytes, and it can conveniently handle UTF-16/UCS-2
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// data. Plenty of existing code operates on strings encoded as UTF-16.
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// On many other platforms, sizeof(wchar_t) is 4 bytes by default. We can make
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// it 2 bytes by using the GCC flag -fshort-wchar. But then std::wstring fails
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// at run time, because it calls some functions (like wcslen) that come from
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// the system's native C library -- which was built with a 4-byte wchar_t!
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// It's wasteful to use 4-byte wchar_t strings to carry UTF-16 data, and it's
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// entirely improper on those systems where the encoding of wchar_t is defined
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// Here, we define string16, which is similar to std::wstring but replaces all
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// libc functions with custom, 2-byte-char compatible routines. It is capable
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// of carrying UTF-16-encoded data.
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#include "base/base_export.h"
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#include "base/basictypes.h"
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#if defined(WCHAR_T_IS_UTF16)
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typedef wchar_t char16;
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typedef std::wstring string16;
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typedef std::char_traits<wchar_t> string16_char_traits;
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#elif defined(WCHAR_T_IS_UTF32)
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typedef uint16 char16;
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// char16 versions of the functions required by string16_char_traits; these
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// are based on the wide character functions of similar names ("w" or "wcs"
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BASE_EXPORT int c16memcmp(const char16* s1, const char16* s2, size_t n);
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BASE_EXPORT size_t c16len(const char16* s);
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BASE_EXPORT const char16* c16memchr(const char16* s, char16 c, size_t n);
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BASE_EXPORT char16* c16memmove(char16* s1, const char16* s2, size_t n);
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BASE_EXPORT char16* c16memcpy(char16* s1, const char16* s2, size_t n);
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BASE_EXPORT char16* c16memset(char16* s, char16 c, size_t n);
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struct string16_char_traits {
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typedef char16 char_type;
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// int_type needs to be able to hold each possible value of char_type, and in
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// addition, the distinct value of eof().
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COMPILE_ASSERT(sizeof(int_type) > sizeof(char_type), unexpected_type_width);
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typedef std::streamoff off_type;
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typedef mbstate_t state_type;
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typedef std::fpos<state_type> pos_type;
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static void assign(char_type& c1, const char_type& c2) {
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static bool eq(const char_type& c1, const char_type& c2) {
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static bool lt(const char_type& c1, const char_type& c2) {
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static int compare(const char_type* s1, const char_type* s2, size_t n) {
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return c16memcmp(s1, s2, n);
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static size_t length(const char_type* s) {
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static const char_type* find(const char_type* s, size_t n,
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return c16memchr(s, a, n);
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static char_type* move(char_type* s1, const char_type* s2, int_type n) {
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return c16memmove(s1, s2, n);
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static char_type* copy(char_type* s1, const char_type* s2, size_t n) {
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return c16memcpy(s1, s2, n);
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static char_type* assign(char_type* s, size_t n, char_type a) {
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return c16memset(s, a, n);
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static int_type not_eof(const int_type& c) {
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return eq_int_type(c, eof()) ? 0 : c;
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static char_type to_char_type(const int_type& c) {
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static int_type to_int_type(const char_type& c) {
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static bool eq_int_type(const int_type& c1, const int_type& c2) {
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static int_type eof() {
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return static_cast<int_type>(EOF);
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typedef std::basic_string<char16, base::string16_char_traits> string16;
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BASE_EXPORT extern std::ostream& operator<<(std::ostream& out,
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const string16& str);
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// This is required by googletest to print a readable output on test failures.
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BASE_EXPORT extern void PrintTo(const string16& str, std::ostream* out);
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// The string class will be explicitly instantiated only once, in string16.cc.
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// std::basic_string<> in GNU libstdc++ contains a static data member,
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// _S_empty_rep_storage, to represent empty strings. When an operation such
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// as assignment or destruction is performed on a string, causing its existing
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// data member to be invalidated, it must not be freed if this static data
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// member is being used. Otherwise, it counts as an attempt to free static
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// (and not allocated) data, which is a memory error.
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// Generally, due to C++ template magic, _S_empty_rep_storage will be marked
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// as a coalesced symbol, meaning that the linker will combine multiple
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// instances into a single one when generating output.
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// If a string class is used by multiple shared libraries, a problem occurs.
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// Each library will get its own copy of _S_empty_rep_storage. When strings
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// are passed across a library boundary for alteration or destruction, memory
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// errors will result. GNU libstdc++ contains a configuration option,
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// --enable-fully-dynamic-string (_GLIBCXX_FULLY_DYNAMIC_STRING), which
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// disables the static data member optimization, but it's a good optimization
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// and non-STL code is generally at the mercy of the system's STL
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// configuration. Fully-dynamic strings are not the default for GNU libstdc++
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// libstdc++ itself or for the libstdc++ installations on the systems we care
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// about, such as Mac OS X and relevant flavors of Linux.
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// See also http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24196 .
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// To avoid problems, string classes need to be explicitly instantiated only
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// once, in exactly one library. All other string users see it via an "extern"
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// declaration. This is precisely how GNU libstdc++ handles
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// std::basic_string<char> (string) and std::basic_string<wchar_t> (wstring).
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// This also works around a Mac OS X linker bug in ld64-85.2.1 (Xcode 3.1.2),
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// in which the linker does not fully coalesce symbols when dead code
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// stripping is enabled. This bug causes the memory errors described above
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// to occur even when a std::basic_string<> does not cross shared library
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// boundaries, such as in statically-linked executables.
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// TODO(mark): File this bug with Apple and update this note with a bug number.
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class BASE_EXPORT std::basic_string<base::char16, base::string16_char_traits>;
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#endif // WCHAR_T_IS_UTF32
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#endif // BASE_STRINGS_STRING16_H_