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 /* -*- 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 workaround for compilers which do not support the C++11 nullptr

  * constant.

  */

 #ifndef mozilla_NullPtr_h

 #define mozilla_NullPtr_h

 #if defined(__clang__)

 #  if !__has_extension(cxx_nullptr)

 #    error "clang version natively supporting nullptr is required."

 #  endif

 #  define MOZ_HAVE_CXX11_NULLPTR

 #elif defined(__GNUC__)

 #  if defined(__GXX_EXPERIMENTAL_CXX0X__) || __cplusplus >= 201103L

 #    include "mozilla/Compiler.h"

 #    if MOZ_GCC_VERSION_AT_LEAST(4, 6, 0)

 #      define MOZ_HAVE_CXX11_NULLPTR

 #    endif

 #  endif

 #elif defined(_MSC_VER)

    // The minimum supported MSVC (10, _MSC_VER 1600) supports nullptr.

 #  define MOZ_HAVE_CXX11_NULLPTR

 #endif

 namespace mozilla {

 /**

  * IsNullPointer<T>::value is true iff T is the type of C++11's nullptr.  If

  * nullptr is emulated, IsNullPointer<T>::value is false for all T.

  *

  * IsNullPointer is useful to give an argument the true decltype(nullptr) type.

  * decltype(nullptr) doesn't work when nullptr is emulated.  The simplest

  * workaround is to use template overloading and SFINAE to expose an overload

  * only if an argument's type is decltype(nullptr).  Some examples (that assume

  * namespace mozilla has been opened, for simplicity):

  *

  *   // foo(T*), foo(stuff that converts to T*), and foo(decltype(nullptr))

  *   // (only invoked if nullptr is true nullptr, otherwise foo(T*) is invoked)

  *   // but nothing else

  *   void foo(T*) { }

  *   template<typename N>

  *   void foo(N,

  *            typename EnableIf<IsNullPointer<N>::value, int>::Type dummy = 0)

  *   { }

  *

  *   // foo(T*) *exactly* and foo(decltype(nullptr)), nothing else

  *   void foo(T*) { }

  *   template<typename U>

  *   void foo(U,

  *            typename EnableIf<!IsNullPointer<U>::value, int>::Type dummy = 0)

  *   MOZ_DELETE;

  *

  * The exact details of how set up the SFINAE bits vary on a case-by-case basis.

  * If you need help with this (and unless you've internalized way more sadmaking

  * nullptr-emulation knowledge than you should have, you do), feel free to poke

  * the person with blame on this comment with questions.   :-)

  *

  * Ideally this would be in TypeTraits.h, but C++11 omitted std::is_null_pointer

  * (fixed in C++1y), so in the interests of easing a switch to <type_traits>,

  * this trait lives elsewhere.

  */

 template<typename T>

 struct IsNullPointer { static const bool value = false; };

 } // namespace mozilla

 /**

  * mozilla::NullptrT is a type that's sort of like decltype(nullptr).  But it

  * can't be identical, because emulated nullptr doesn't have a distinct type.

  * Only with gcc 4.4/4.5, emulated nullptr is __null, and decltype(__null) is

  * int or long.  But passing __null to an int/long parameter triggers

  * -Werror=conversion-null errors with gcc 4.5, or (depending on subsequent use

  * inside the overloaded function) can trigger pointer-to-integer comparison

  * compiler errors.  So fairly often, actually, NullptrT is *not* what you want.

  *

  * Instead, often you should use template-based overloading in concert with

  * SFINAE to add a nullptr overload -- see the comments by IsNullPointer.

  *

  * So when *should* you use NullptrT?  Thus far, the only truly good use seems

  * to be as an argument type for operator overloads (because C++ doesn't allow

  * operator= to have more than one argument, operator== to have more than two,

  * &c.).  But even in such cases, it really only works if there are no other

  * overloads of the operator that accept a pointer type.  If you want both T*

  * and nullptr_t overloads, you'll have to wait til we drop gcc 4.4/4.5 support.

  * (Currently b2g is the only impediment to this.)

  */

 #ifdef MOZ_HAVE_CXX11_NULLPTR

   // decltype does the right thing for actual nullptr.

   namespace mozilla {

   typedef decltype(nullptr) NullptrT;

   template<>

   struct IsNullPointer<decltype(nullptr)> { static const bool value = true; };

   }

 #  undef MOZ_HAVE_CXX11_NULLPTR

 #elif MOZ_IS_GCC

 #  define nullptr __null

   // void* sweeps up more than just nullptr, but compilers supporting true

   // nullptr are the majority now, so they should detect mistakes.  If you're

   // feeling paranoid, check/assert that your NullptrT equals nullptr.

   namespace mozilla { typedef void* NullptrT; }

 #else

 #  error "No compiler support for nullptr or its emulation."

 #endif

 #endif /* mozilla_NullPtr_h */