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// Copyright 2013 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
// WHAT:
// A version of std::basic_string that provides 2-byte characters even when
// wchar_t is not implemented as a 2-byte type. You can access this class as
// string16. We also define char16, which string16 is based upon.
// WHY:
// On Windows, wchar_t is 2 bytes, and it can conveniently handle UTF-16/UCS-2
// data. Plenty of existing code operates on strings encoded as UTF-16.
// On many other platforms, sizeof(wchar_t) is 4 bytes by default. We can make
// it 2 bytes by using the GCC flag -fshort-wchar. But then std::wstring fails
// at run time, because it calls some functions (like wcslen) that come from
// the system's native C library -- which was built with a 4-byte wchar_t!
// It's wasteful to use 4-byte wchar_t strings to carry UTF-16 data, and it's
// entirely improper on those systems where the encoding of wchar_t is defined
// as UTF-32.
// Here, we define string16, which is similar to std::wstring but replaces all
// libc functions with custom, 2-byte-char compatible routines. It is capable
// of carrying UTF-16-encoded data.
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <functional>
#include <string>
#include "base/base_export.h"
#include "build/build_config.h"
#if defined(WCHAR_T_IS_UTF16)
// Define a macro for wrapping construction of char16 arrays and string16s from
// a literal string. This indirection allows for an easier migration of
// base::char16 to char16_t on platforms where WCHAR_T_IS_UTF16, as only a one
// character change to the macro will be necessary.
// This macro does not exist when WCHAR_T_IS_UTF32, as it is currently not
// possible to create a char array form a literal in this case.
// TODO( Remove this macro once base::char16 is
// char16_t on all platforms.
#define STRING16_LITERAL(x) L##x
namespace base {
typedef wchar_t char16;
typedef std::wstring string16;
} // namespace base
#elif defined(WCHAR_T_IS_UTF32)
#include <wchar.h> // for mbstate_t
namespace base {
typedef uint16_t char16;
// char16 versions of the functions required by string16_char_traits; these
// are based on the wide character functions of similar names ("w" or "wcs"
// instead of "c16").
BASE_EXPORT int c16memcmp(const char16* s1, const char16* s2, size_t n);
BASE_EXPORT size_t c16len(const char16* s);
BASE_EXPORT const char16* c16memchr(const char16* s, char16 c, size_t n);
BASE_EXPORT char16* c16memmove(char16* s1, const char16* s2, size_t n);
BASE_EXPORT char16* c16memcpy(char16* s1, const char16* s2, size_t n);
BASE_EXPORT char16* c16memset(char16* s, char16 c, size_t n);
// This namespace contains the implementation of base::string16 along with
// things that need to be found via argument-dependent lookup from a
// base::string16.
namespace string16_internals {
struct string16_char_traits {
typedef char16 char_type;
typedef int int_type;
// int_type needs to be able to hold each possible value of char_type, and in
// addition, the distinct value of eof().
static_assert(sizeof(int_type) > sizeof(char_type),
"int must be larger than 16 bits wide");
typedef std::streamoff off_type;
typedef mbstate_t state_type;
typedef std::fpos<state_type> pos_type;
static void assign(char_type& c1, const char_type& c2) {
c1 = c2;
static bool eq(const char_type& c1, const char_type& c2) {
return c1 == c2;
static bool lt(const char_type& c1, const char_type& c2) {
return c1 < c2;
static int compare(const char_type* s1, const char_type* s2, size_t n) {
return c16memcmp(s1, s2, n);
static size_t length(const char_type* s) {
return c16len(s);
static const char_type* find(const char_type* s, size_t n,
const char_type& a) {
return c16memchr(s, a, n);
static char_type* move(char_type* s1, const char_type* s2, size_t n) {
return c16memmove(s1, s2, n);
static char_type* copy(char_type* s1, const char_type* s2, size_t n) {
return c16memcpy(s1, s2, n);
static char_type* assign(char_type* s, size_t n, char_type a) {
return c16memset(s, a, n);
static int_type not_eof(const int_type& c) {
return eq_int_type(c, eof()) ? 0 : c;
static char_type to_char_type(const int_type& c) {
return char_type(c);
static int_type to_int_type(const char_type& c) {
return int_type(c);
static bool eq_int_type(const int_type& c1, const int_type& c2) {
return c1 == c2;
static int_type eof() {
return static_cast<int_type>(EOF);
} // namespace string16_internals
typedef std::basic_string<char16,
namespace string16_internals {
BASE_EXPORT extern std::ostream& operator<<(std::ostream& out,
const string16& str);
// This is required by googletest to print a readable output on test failures.
BASE_EXPORT extern void PrintTo(const string16& str, std::ostream* out);
} // namespace string16_internals
} // namespace base
// The string class will be explicitly instantiated only once, in
// std::basic_string<> in GNU libstdc++ contains a static data member,
// _S_empty_rep_storage, to represent empty strings. When an operation such
// as assignment or destruction is performed on a string, causing its existing
// data member to be invalidated, it must not be freed if this static data
// member is being used. Otherwise, it counts as an attempt to free static
// (and not allocated) data, which is a memory error.
// Generally, due to C++ template magic, _S_empty_rep_storage will be marked
// as a coalesced symbol, meaning that the linker will combine multiple
// instances into a single one when generating output.
// If a string class is used by multiple shared libraries, a problem occurs.
// Each library will get its own copy of _S_empty_rep_storage. When strings
// are passed across a library boundary for alteration or destruction, memory
// errors will result. GNU libstdc++ contains a configuration option,
// --enable-fully-dynamic-string (_GLIBCXX_FULLY_DYNAMIC_STRING), which
// disables the static data member optimization, but it's a good optimization
// and non-STL code is generally at the mercy of the system's STL
// configuration. Fully-dynamic strings are not the default for GNU libstdc++
// libstdc++ itself or for the libstdc++ installations on the systems we care
// about, such as Mac OS X and relevant flavors of Linux.
// See also .
// To avoid problems, string classes need to be explicitly instantiated only
// once, in exactly one library. All other string users see it via an "extern"
// declaration. This is precisely how GNU libstdc++ handles
// std::basic_string<char> (string) and std::basic_string<wchar_t> (wstring).
// This also works around a Mac OS X linker bug in ld64-85.2.1 (Xcode 3.1.2),
// in which the linker does not fully coalesce symbols when dead code
// stripping is enabled. This bug causes the memory errors described above
// to occur even when a std::basic_string<> does not cross shared library
// boundaries, such as in statically-linked executables.
// TODO(mark): File this bug with Apple and update this note with a bug number.
extern template class BASE_EXPORT
// Specialize std::hash for base::string16. Although the style guide forbids
// this in general, it is necessary for consistency with WCHAR_T_IS_UTF16
// platforms, where base::string16 is a type alias for std::wstring.
namespace std {
template <>
struct hash<base::string16> {
std::size_t operator()(const base::string16& s) const {
std::size_t result = 0;
for (base::char16 c : s)
result = (result * 131) + c;
return result;
} // namespace std
#endif // WCHAR_T_IS_UTF32