gecko-dev/mfbt/NullPtr.h
Jeff Walden 5d3f01c937 Bug 953296 - Implement mozilla::NullptrT as a typedef to use to accept nullptr values. Also add mozilla::IsNullPointer<T>, a trait for detecting *only* true nullptr (emulated nullptr [__null] used by gcc 4.4/4.5 isn't true nullptr). r=ehsan
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
2014-01-02 17:27:41 -06:00

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4.5 KiB
C++

/* -*- 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 */