Char16.h

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

/* Implements a UTF-16 character type. */

#ifndef mozilla_Char16_h
#define mozilla_Char16_h

#ifdef __cplusplus

/*
 * C++11 introduces a char16_t type and support for UTF-16 string and character
 * literals. C++11's char16_t is a distinct builtin type. Technically, char16_t
 * is a 16-bit code unit of a Unicode code point, not a "character".
 */

#ifdef _MSC_VER
   /*
    * C++11 says char16_t is a distinct builtin type, but Windows's yvals.h
    * typedefs char16_t as an unsigned short. We would like to alias char16_t
    * to Windows's 16-bit wchar_t so we can declare UTF-16 literals as constant
    * expressions (and pass char16_t pointers to Windows APIs). We #define
    * _CHAR16T here in order to prevent yvals.h from overriding our char16_t
    * typedefs, which we set to wchar_t for C++ code.
    *
    * In addition, #defining _CHAR16T will prevent yvals.h from defining a
    * char32_t type, so we have to undo that damage here and provide our own,
    * which is identical to the yvals.h type.
    */
#  define MOZ_UTF16_HELPER(s) L##s
#  define _CHAR16T
typedef wchar_t char16_t;
typedef unsigned int char32_t;
#else
   /* C++11 has a builtin char16_t type. */
#  define MOZ_UTF16_HELPER(s) u##s
   /**
    * This macro is used to distinguish when char16_t would be a distinct
    * typedef from wchar_t.
    */
#  define MOZ_CHAR16_IS_NOT_WCHAR
#  ifdef WIN32
#    define MOZ_USE_CHAR16_WRAPPER
#  endif
#endif

#ifdef MOZ_USE_CHAR16_WRAPPER
# include <string>
  /**
   * Win32 API extensively uses wchar_t, which is represented by a separated
   * builtin type than char16_t per spec. It's not the case for MSVC, but GCC
   * follows the spec. We want to mix wchar_t and char16_t on Windows builds.
   * This class is supposed to make it easier. It stores char16_t const pointer,
   * but provides implicit casts for wchar_t as well. On other platforms, we
   * simply use |typedef const char16_t* char16ptr_t|. Here, we want to make
   * the class as similar to this typedef, including providing some casts that
   * are allowed by the typedef.
   */
class char16ptr_t
{
private:
  const char16_t* mPtr;
  static_assert(sizeof(char16_t) == sizeof(wchar_t),
                "char16_t and wchar_t sizes differ");

public:
  char16ptr_t(const char16_t* aPtr) : mPtr(aPtr) {}
  char16ptr_t(const wchar_t* aPtr) :
    mPtr(reinterpret_cast<const char16_t*>(aPtr))
  {}

  /* Without this, nullptr assignment would be ambiguous. */
  constexpr char16ptr_t(decltype(nullptr)) : mPtr(nullptr) {}

  operator const char16_t*() const
  {
    return mPtr;
  }
  operator const wchar_t*() const
  {
    return reinterpret_cast<const wchar_t*>(mPtr);
  }
  operator const void*() const
  {
    return mPtr;
  }
  operator bool() const
  {
    return mPtr != nullptr;
  }
  operator std::wstring() const
  {
    return std::wstring(static_cast<const wchar_t*>(*this));
  }

  /* Explicit cast operators to allow things like (char16_t*)str. */
  explicit operator char16_t*() const
  {
    return const_cast<char16_t*>(mPtr);
  }
  explicit operator wchar_t*() const
  {
    return const_cast<wchar_t*>(static_cast<const wchar_t*>(*this));
  }

  /**
   * Some Windows API calls accept BYTE* but require that data actually be
   * WCHAR*.  Supporting this requires explicit operators to support the
   * requisite explicit casts.
   */
  explicit operator const char*() const
  {
    return reinterpret_cast<const char*>(mPtr);
  }
  explicit operator const unsigned char*() const
  {
    return reinterpret_cast<const unsigned char*>(mPtr);
  }
  explicit operator unsigned char*() const
  {
    return
      const_cast<unsigned char*>(reinterpret_cast<const unsigned char*>(mPtr));
  }
  explicit operator void*() const
  {
    return const_cast<char16_t*>(mPtr);
  }

  /* Some operators used on pointers. */
  char16_t operator[](size_t aIndex) const
  {
    return mPtr[aIndex];
  }
  bool operator==(const char16ptr_t& aOther) const
  {
    return mPtr == aOther.mPtr;
  }
  bool operator==(decltype(nullptr)) const
  {
    return mPtr == nullptr;
  }
  bool operator!=(const char16ptr_t& aOther) const
  {
    return mPtr != aOther.mPtr;
  }
  bool operator!=(decltype(nullptr)) const
  {
    return mPtr != nullptr;
  }
  char16ptr_t operator+(size_t aValue) const
  {
    return char16ptr_t(mPtr + aValue);
  }
  ptrdiff_t operator-(const char16ptr_t& aOther) const
  {
    return mPtr - aOther.mPtr;
  }
};

inline decltype((char*)0-(char*)0)
operator-(const char16_t* aX, const char16ptr_t aY)
{
  return aX - static_cast<const char16_t*>(aY);
}

#else

typedef const char16_t* char16ptr_t;

#endif

/*
 * Macro arguments used in concatenation or stringification won't be expanded.
 * Therefore, in order for |MOZ_UTF16(FOO)| to work as expected (which is to
 * expand |FOO| before doing whatever |MOZ_UTF16| needs to do to it) a helper
 * macro, |MOZ_UTF16_HELPER| needs to be inserted in between to allow the macro
 * argument to expand. See "3.10.6 Separate Expansion of Macro Arguments" of the
 * CPP manual for a more accurate and precise explanation.
 */
#define MOZ_UTF16(s) MOZ_UTF16_HELPER(s)

static_assert(sizeof(char16_t) == 2, "Is char16_t type 16 bits?");
static_assert(char16_t(-1) > char16_t(0), "Is char16_t type unsigned?");
static_assert(sizeof(MOZ_UTF16('A')) == 2, "Is char literal 16 bits?");
static_assert(sizeof(MOZ_UTF16("")[0]) == 2, "Is string char 16 bits?");

#endif

#endif /* mozilla_Char16_h */