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b01c238639
Generally, if you want a decltype(nullptr)-based overload, you should use SFINAE and IsNullPointer. (Examples are provided in NullPtr.h comments.) The problem is NullptrT matches far more than just __null as emulated nullptr for gcc 4.4/4.5 overloading purposes. This problem is unavoidable without true nullptr. Currently, the only valid use for NullptrT is believed to be in operator overloads. All existing nullptr-overloading code has been rewritten to use the appropriate technique for the situation, and MOZ_HAVE_CXX11_NULLPTR is no longer an API. --HG-- extra : rebase_source : 01abfcb66ae569db7b04a7b53f5cd5fd8151bffd
113 lines
4.5 KiB
C++
113 lines
4.5 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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/*
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* Implements a workaround for compilers which do not support the C++11 nullptr
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* constant.
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*/
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#ifndef mozilla_NullPtr_h
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#define mozilla_NullPtr_h
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#if defined(__clang__)
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# if !__has_extension(cxx_nullptr)
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# error "clang version natively supporting nullptr is required."
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# endif
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# define MOZ_HAVE_CXX11_NULLPTR
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#elif defined(__GNUC__)
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# if defined(__GXX_EXPERIMENTAL_CXX0X__) || __cplusplus >= 201103L
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# include "mozilla/Compiler.h"
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# if MOZ_GCC_VERSION_AT_LEAST(4, 6, 0)
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# define MOZ_HAVE_CXX11_NULLPTR
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# endif
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# endif
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#elif defined(_MSC_VER)
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// The minimum supported MSVC (10, _MSC_VER 1600) supports nullptr.
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# define MOZ_HAVE_CXX11_NULLPTR
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#endif
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namespace mozilla {
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/**
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* IsNullPointer<T>::value is true iff T is the type of C++11's nullptr. If
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* nullptr is emulated, IsNullPointer<T>::value is false for all T.
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*
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* IsNullPointer is useful to give an argument the true decltype(nullptr) type.
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* decltype(nullptr) doesn't work when nullptr is emulated. The simplest
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* workaround is to use template overloading and SFINAE to expose an overload
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* only if an argument's type is decltype(nullptr). Some examples (that assume
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* namespace mozilla has been opened, for simplicity):
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*
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* // foo(T*), foo(stuff that converts to T*), and foo(decltype(nullptr))
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* // (only invoked if nullptr is true nullptr, otherwise foo(T*) is invoked)
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* // but nothing else
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* void foo(T*) { }
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* template<typename N>
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* void foo(N,
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* typename EnableIf<IsNullPointer<N>::value, int>::Type dummy = 0)
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* { }
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*
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* // foo(T*) *exactly* and foo(decltype(nullptr)), nothing else
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* void foo(T*) { }
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* template<typename U>
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* void foo(U,
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* typename EnableIf<!IsNullPointer<U>::value, int>::Type dummy = 0)
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* MOZ_DELETE;
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*
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* The exact details of how set up the SFINAE bits vary on a case-by-case basis.
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* If you need help with this (and unless you've internalized way more sadmaking
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* nullptr-emulation knowledge than you should have, you do), feel free to poke
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* the person with blame on this comment with questions. :-)
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*
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* Ideally this would be in TypeTraits.h, but C++11 omitted std::is_null_pointer
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* (fixed in C++1y), so in the interests of easing a switch to <type_traits>,
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* this trait lives elsewhere.
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*/
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template<typename T>
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struct IsNullPointer { static const bool value = false; };
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} // namespace mozilla
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/**
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* mozilla::NullptrT is a type that's sort of like decltype(nullptr). But it
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* can't be identical, because emulated nullptr doesn't have a distinct type.
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* Only with gcc 4.4/4.5, emulated nullptr is __null, and decltype(__null) is
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* int or long. But passing __null to an int/long parameter triggers
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* -Werror=conversion-null errors with gcc 4.5, or (depending on subsequent use
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* inside the overloaded function) can trigger pointer-to-integer comparison
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* compiler errors. So fairly often, actually, NullptrT is *not* what you want.
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*
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* Instead, often you should use template-based overloading in concert with
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* SFINAE to add a nullptr overload -- see the comments by IsNullPointer.
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*
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* So when *should* you use NullptrT? Thus far, the only truly good use seems
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* to be as an argument type for operator overloads (because C++ doesn't allow
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* operator= to have more than one argument, operator== to have more than two,
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* &c.). But even in such cases, it really only works if there are no other
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* overloads of the operator that accept a pointer type. If you want both T*
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* and nullptr_t overloads, you'll have to wait til we drop gcc 4.4/4.5 support.
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* (Currently b2g is the only impediment to this.)
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*/
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#ifdef MOZ_HAVE_CXX11_NULLPTR
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// decltype does the right thing for actual nullptr.
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namespace mozilla {
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typedef decltype(nullptr) NullptrT;
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template<>
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struct IsNullPointer<decltype(nullptr)> { static const bool value = true; };
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}
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# undef MOZ_HAVE_CXX11_NULLPTR
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#elif MOZ_IS_GCC
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# define nullptr __null
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// void* sweeps up more than just nullptr, but compilers supporting true
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// nullptr are the majority now, so they should detect mistakes. If you're
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// feeling paranoid, check/assert that your NullptrT equals nullptr.
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namespace mozilla { typedef void* NullptrT; }
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#else
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# error "No compiler support for nullptr or its emulation."
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#endif
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#endif /* mozilla_NullPtr_h */
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