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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.

 #ifndef BASE_STRINGS_STRING16_H_

 #define BASE_STRINGS_STRING16_H_

 // 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 <stdio.h>

 #include <string>

 #include "base/base_export.h"

 #include "base/basictypes.h"

 #if defined(WCHAR_T_IS_UTF16)

 namespace base {

 typedef wchar_t char16;

 typedef std::wstring string16;

 typedef std::char_traits<wchar_t> string16_char_traits;

 }  // namespace base

 #elif defined(WCHAR_T_IS_UTF32)

 namespace base {

 typedef uint16 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);

 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().

   COMPILE_ASSERT(sizeof(int_type) > sizeof(char_type), unexpected_type_width);

   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, int_type 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);

   }

 };

 typedef std::basic_string<char16, base::string16_char_traits> string16;

 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 base

 // The string class will be explicitly instantiated only once, in string16.cc.

 //

 // 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 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24196 .

 //

 // 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 std::basic_string<base::char16, base::string16_char_traits>;

 #endif  // WCHAR_T_IS_UTF32

 // TODO(brettw) update users of string16 to use the namespace and remove

 // this "using".

 using base::char16;

 using base::string16;

 #endif  // BASE_STRINGS_STRING16_H_