1.1 --- /dev/null Thu Jan 01 00:00:00 1970 +0000
1.2 +++ b/mfbt/NullPtr.h Wed Dec 31 06:09:35 2014 +0100
1.3 @@ -0,0 +1,112 @@
1.4 +/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
1.5 +/* vim: set ts=8 sts=2 et sw=2 tw=80: */
1.6 +/* This Source Code Form is subject to the terms of the Mozilla Public
1.7 + * License, v. 2.0. If a copy of the MPL was not distributed with this
1.8 + * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
1.9 +
1.10 +/*
1.11 + * Implements a workaround for compilers which do not support the C++11 nullptr
1.12 + * constant.
1.13 + */
1.14 +
1.15 +#ifndef mozilla_NullPtr_h
1.16 +#define mozilla_NullPtr_h
1.17 +
1.18 +#if defined(__clang__)
1.19 +# if !__has_extension(cxx_nullptr)
1.20 +# error "clang version natively supporting nullptr is required."
1.21 +# endif
1.22 +# define MOZ_HAVE_CXX11_NULLPTR
1.23 +#elif defined(__GNUC__)
1.24 +# if defined(__GXX_EXPERIMENTAL_CXX0X__) || __cplusplus >= 201103L
1.25 +# include "mozilla/Compiler.h"
1.26 +# if MOZ_GCC_VERSION_AT_LEAST(4, 6, 0)
1.27 +# define MOZ_HAVE_CXX11_NULLPTR
1.28 +# endif
1.29 +# endif
1.30 +#elif defined(_MSC_VER)
1.31 + // The minimum supported MSVC (10, _MSC_VER 1600) supports nullptr.
1.32 +# define MOZ_HAVE_CXX11_NULLPTR
1.33 +#endif
1.34 +
1.35 +namespace mozilla {
1.36 +
1.37 +/**
1.38 + * IsNullPointer<T>::value is true iff T is the type of C++11's nullptr. If
1.39 + * nullptr is emulated, IsNullPointer<T>::value is false for all T.
1.40 + *
1.41 + * IsNullPointer is useful to give an argument the true decltype(nullptr) type.
1.42 + * decltype(nullptr) doesn't work when nullptr is emulated. The simplest
1.43 + * workaround is to use template overloading and SFINAE to expose an overload
1.44 + * only if an argument's type is decltype(nullptr). Some examples (that assume
1.45 + * namespace mozilla has been opened, for simplicity):
1.46 + *
1.47 + * // foo(T*), foo(stuff that converts to T*), and foo(decltype(nullptr))
1.48 + * // (only invoked if nullptr is true nullptr, otherwise foo(T*) is invoked)
1.49 + * // but nothing else
1.50 + * void foo(T*) { }
1.51 + * template<typename N>
1.52 + * void foo(N,
1.53 + * typename EnableIf<IsNullPointer<N>::value, int>::Type dummy = 0)
1.54 + * { }
1.55 + *
1.56 + * // foo(T*) *exactly* and foo(decltype(nullptr)), nothing else
1.57 + * void foo(T*) { }
1.58 + * template<typename U>
1.59 + * void foo(U,
1.60 + * typename EnableIf<!IsNullPointer<U>::value, int>::Type dummy = 0)
1.61 + * MOZ_DELETE;
1.62 + *
1.63 + * The exact details of how set up the SFINAE bits vary on a case-by-case basis.
1.64 + * If you need help with this (and unless you've internalized way more sadmaking
1.65 + * nullptr-emulation knowledge than you should have, you do), feel free to poke
1.66 + * the person with blame on this comment with questions. :-)
1.67 + *
1.68 + * Ideally this would be in TypeTraits.h, but C++11 omitted std::is_null_pointer
1.69 + * (fixed in C++1y), so in the interests of easing a switch to <type_traits>,
1.70 + * this trait lives elsewhere.
1.71 + */
1.72 +template<typename T>
1.73 +struct IsNullPointer { static const bool value = false; };
1.74 +
1.75 +} // namespace mozilla
1.76 +
1.77 +/**
1.78 + * mozilla::NullptrT is a type that's sort of like decltype(nullptr). But it
1.79 + * can't be identical, because emulated nullptr doesn't have a distinct type.
1.80 + * Only with gcc 4.4/4.5, emulated nullptr is __null, and decltype(__null) is
1.81 + * int or long. But passing __null to an int/long parameter triggers
1.82 + * -Werror=conversion-null errors with gcc 4.5, or (depending on subsequent use
1.83 + * inside the overloaded function) can trigger pointer-to-integer comparison
1.84 + * compiler errors. So fairly often, actually, NullptrT is *not* what you want.
1.85 + *
1.86 + * Instead, often you should use template-based overloading in concert with
1.87 + * SFINAE to add a nullptr overload -- see the comments by IsNullPointer.
1.88 + *
1.89 + * So when *should* you use NullptrT? Thus far, the only truly good use seems
1.90 + * to be as an argument type for operator overloads (because C++ doesn't allow
1.91 + * operator= to have more than one argument, operator== to have more than two,
1.92 + * &c.). But even in such cases, it really only works if there are no other
1.93 + * overloads of the operator that accept a pointer type. If you want both T*
1.94 + * and nullptr_t overloads, you'll have to wait til we drop gcc 4.4/4.5 support.
1.95 + * (Currently b2g is the only impediment to this.)
1.96 + */
1.97 +#ifdef MOZ_HAVE_CXX11_NULLPTR
1.98 + // decltype does the right thing for actual nullptr.
1.99 + namespace mozilla {
1.100 + typedef decltype(nullptr) NullptrT;
1.101 + template<>
1.102 + struct IsNullPointer<decltype(nullptr)> { static const bool value = true; };
1.103 + }
1.104 +# undef MOZ_HAVE_CXX11_NULLPTR
1.105 +#elif MOZ_IS_GCC
1.106 +# define nullptr __null
1.107 + // void* sweeps up more than just nullptr, but compilers supporting true
1.108 + // nullptr are the majority now, so they should detect mistakes. If you're
1.109 + // feeling paranoid, check/assert that your NullptrT equals nullptr.
1.110 + namespace mozilla { typedef void* NullptrT; }
1.111 +#else
1.112 +# error "No compiler support for nullptr or its emulation."
1.113 +#endif
1.114 +
1.115 +#endif /* mozilla_NullPtr_h */
