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()

上面代码是位于 https://code.woboq.org/llvm/l... 内的nullptr源码，其中

struct __nat {int __for_bool_;};
_LIBCPP_ALWAYS_INLINE _LIBCPP_CONSTEXPR nullptr_t(int __nat::*) : __lx(0) {}
_LIBCPP_ALWAYS_INLINE _LIBCPP_CONSTEXPR operator int __nat::*() const {return 0;}

有什么作用？