michael@0: /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
michael@0: /* vim: set ts=8 sts=2 et sw=2 tw=80: */
michael@0: /* This Source Code Form is subject to the terms of the Mozilla Public
michael@0:  * License, v. 2.0. If a copy of the MPL was not distributed with this
michael@0:  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
michael@0: 
michael@0: /*
michael@0:  * Implements a workaround for compilers which do not support the C++11 nullptr
michael@0:  * constant.
michael@0:  */
michael@0: 
michael@0: #ifndef mozilla_NullPtr_h
michael@0: #define mozilla_NullPtr_h
michael@0: 
michael@0: #if defined(__clang__)
michael@0: #  if !__has_extension(cxx_nullptr)
michael@0: #    error "clang version natively supporting nullptr is required."
michael@0: #  endif
michael@0: #  define MOZ_HAVE_CXX11_NULLPTR
michael@0: #elif defined(__GNUC__)
michael@0: #  if defined(__GXX_EXPERIMENTAL_CXX0X__) || __cplusplus >= 201103L
michael@0: #    include "mozilla/Compiler.h"
michael@0: #    if MOZ_GCC_VERSION_AT_LEAST(4, 6, 0)
michael@0: #      define MOZ_HAVE_CXX11_NULLPTR
michael@0: #    endif
michael@0: #  endif
michael@0: #elif defined(_MSC_VER)
michael@0:    // The minimum supported MSVC (10, _MSC_VER 1600) supports nullptr.
michael@0: #  define MOZ_HAVE_CXX11_NULLPTR
michael@0: #endif
michael@0: 
michael@0: namespace mozilla {
michael@0: 
michael@0: /**
michael@0:  * IsNullPointer<T>::value is true iff T is the type of C++11's nullptr.  If
michael@0:  * nullptr is emulated, IsNullPointer<T>::value is false for all T.
michael@0:  *
michael@0:  * IsNullPointer is useful to give an argument the true decltype(nullptr) type.
michael@0:  * decltype(nullptr) doesn't work when nullptr is emulated.  The simplest
michael@0:  * workaround is to use template overloading and SFINAE to expose an overload
michael@0:  * only if an argument's type is decltype(nullptr).  Some examples (that assume
michael@0:  * namespace mozilla has been opened, for simplicity):
michael@0:  *
michael@0:  *   // foo(T*), foo(stuff that converts to T*), and foo(decltype(nullptr))
michael@0:  *   // (only invoked if nullptr is true nullptr, otherwise foo(T*) is invoked)
michael@0:  *   // but nothing else
michael@0:  *   void foo(T*) { }
michael@0:  *   template<typename N>
michael@0:  *   void foo(N,
michael@0:  *            typename EnableIf<IsNullPointer<N>::value, int>::Type dummy = 0)
michael@0:  *   { }
michael@0:  *
michael@0:  *   // foo(T*) *exactly* and foo(decltype(nullptr)), nothing else
michael@0:  *   void foo(T*) { }
michael@0:  *   template<typename U>
michael@0:  *   void foo(U,
michael@0:  *            typename EnableIf<!IsNullPointer<U>::value, int>::Type dummy = 0)
michael@0:  *   MOZ_DELETE;
michael@0:  *
michael@0:  * The exact details of how set up the SFINAE bits vary on a case-by-case basis.
michael@0:  * If you need help with this (and unless you've internalized way more sadmaking
michael@0:  * nullptr-emulation knowledge than you should have, you do), feel free to poke
michael@0:  * the person with blame on this comment with questions.   :-)
michael@0:  *
michael@0:  * Ideally this would be in TypeTraits.h, but C++11 omitted std::is_null_pointer
michael@0:  * (fixed in C++1y), so in the interests of easing a switch to <type_traits>,
michael@0:  * this trait lives elsewhere.
michael@0:  */
michael@0: template<typename T>
michael@0: struct IsNullPointer { static const bool value = false; };
michael@0: 
michael@0: } // namespace mozilla
michael@0: 
michael@0: /**
michael@0:  * mozilla::NullptrT is a type that's sort of like decltype(nullptr).  But it
michael@0:  * can't be identical, because emulated nullptr doesn't have a distinct type.
michael@0:  * Only with gcc 4.4/4.5, emulated nullptr is __null, and decltype(__null) is
michael@0:  * int or long.  But passing __null to an int/long parameter triggers
michael@0:  * -Werror=conversion-null errors with gcc 4.5, or (depending on subsequent use
michael@0:  * inside the overloaded function) can trigger pointer-to-integer comparison
michael@0:  * compiler errors.  So fairly often, actually, NullptrT is *not* what you want.
michael@0:  *
michael@0:  * Instead, often you should use template-based overloading in concert with
michael@0:  * SFINAE to add a nullptr overload -- see the comments by IsNullPointer.
michael@0:  *
michael@0:  * So when *should* you use NullptrT?  Thus far, the only truly good use seems
michael@0:  * to be as an argument type for operator overloads (because C++ doesn't allow
michael@0:  * operator= to have more than one argument, operator== to have more than two,
michael@0:  * &c.).  But even in such cases, it really only works if there are no other
michael@0:  * overloads of the operator that accept a pointer type.  If you want both T*
michael@0:  * and nullptr_t overloads, you'll have to wait til we drop gcc 4.4/4.5 support.
michael@0:  * (Currently b2g is the only impediment to this.)
michael@0:  */
michael@0: #ifdef MOZ_HAVE_CXX11_NULLPTR
michael@0:   // decltype does the right thing for actual nullptr.
michael@0:   namespace mozilla {
michael@0:   typedef decltype(nullptr) NullptrT;
michael@0:   template<>
michael@0:   struct IsNullPointer<decltype(nullptr)> { static const bool value = true; };
michael@0:   }
michael@0: #  undef MOZ_HAVE_CXX11_NULLPTR
michael@0: #elif MOZ_IS_GCC
michael@0: #  define nullptr __null
michael@0:   // void* sweeps up more than just nullptr, but compilers supporting true
michael@0:   // nullptr are the majority now, so they should detect mistakes.  If you're
michael@0:   // feeling paranoid, check/assert that your NullptrT equals nullptr.
michael@0:   namespace mozilla { typedef void* NullptrT; }
michael@0: #else
michael@0: #  error "No compiler support for nullptr or its emulation."
michael@0: #endif
michael@0: 
michael@0: #endif /* mozilla_NullPtr_h */