Recently, I want to know how nullptr works. In http://www.stroustrup.com/N1488-nullptr.pdf , I found the code.(Sorry, I don't have 10 reputations, so I can't post images here, you can follow the link above , the code is at page 3.)
The code is cool. And I searched the keyword nullptr in Woboq again, I found the source code in LLVM is different from the above, I copy them below.
struct _LIBCPP_TEMPLATE_VIS nullptr_t
{
void* __lx;
struct __nat {int __for_bool_;};
_LIBCPP_ALWAYS_INLINE _LIBCPP_CONSTEXPR nullptr_t() : __lx(0) {}
_LIBCPP_ALWAYS_INLINE _LIBCPP_CONSTEXPR nullptr_t(int __nat::*) : __lx(0) {}
_LIBCPP_ALWAYS_INLINE _LIBCPP_CONSTEXPR operator int __nat::*() const {return 0;}
template <class _Tp>
_LIBCPP_ALWAYS_INLINE _LIBCPP_CONSTEXPR
operator _Tp* () const {return 0;}
template <class _Tp, class _Up>
_LIBCPP_ALWAYS_INLINE
operator _Tp _Up::* () const {return 0;}
friend _LIBCPP_ALWAYS_INLINE _LIBCPP_CONSTEXPR bool operator==(nullptr_t, nullptr_t) {return true;}
friend _LIBCPP_ALWAYS_INLINE _LIBCPP_CONSTEXPR bool operator!=(nullptr_t, nullptr_t) {return false;}
};
inline _LIBCPP_ALWAYS_INLINE _LIBCPP_CONSTEXPR nullptr_t __get_nullptr_t() {return nullptr_t(0);}
#define nullptr _VSTD::__get_nullptr_t()
The most diference is, it defines struct __nat and two functions
_LIBCPP_ALWAYS_INLINE _LIBCPP_CONSTEXPR nullptr_t(int __nat::*) : __lx(0) {}
_LIBCPP_ALWAYS_INLINE _LIBCPP_CONSTEXPR operator int __nat::*() const {return 0;}
I thought it for a long time, and I still don't understand why LLVM implemented it like this. Can someone give me any advice?