mirror of
https://github.com/mozilla/gecko-dev.git
synced 2024-12-28 11:28:38 +00:00
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 -*- */
|
|
/* 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 */
|