diff -r 000000000000 -r 6474c204b198 mfbt/NullPtr.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mfbt/NullPtr.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,112 @@
+/* -*- 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 */