memory - libcxx源码

libcxx/include/__memory/pointer_traits.h中有两个常用的指针类型转换的模板

1. rebind_pointer
   接受两个模板参数 From和To，将From类型的指针转换为To类型的指针

__rebind_pointer<From,To>::type 

pointer_traits<From>中，成员rebind<To>的别名

2. pointer_traits
   大致的行为是，将rebind<T>声明为指向T的指针
   有一个partial spec和一个主模板

template <class _Ptr>
struct _LIBCPP_TEMPLATE_VIS pointer_traits
{
    typedef _Ptr                                                     pointer;
    typedef typename __pointer_traits_element_type<pointer>::type    element_type;
    typedef typename __pointer_traits_difference_type<pointer>::type difference_type;

#ifndef _LIBCPP_CXX03_LANG
    template <class _Up> using rebind = typename __pointer_traits_rebind<pointer, _Up>::type;
#else
    template <class _Up> struct rebind
        {typedef typename __pointer_traits_rebind<pointer, _Up>::type other;};
#endif // _LIBCPP_CXX03_LANG

private:
    struct __nat {};
public:
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    static pointer pointer_to(typename conditional<is_void<element_type>::value,
                                           __nat, element_type>::type& __r)
        {return pointer::pointer_to(__r);}
};

template <class _Tp>
struct _LIBCPP_TEMPLATE_VIS pointer_traits<_Tp*>
{
    typedef _Tp*      pointer;
    typedef _Tp       element_type;
    typedef ptrdiff_t difference_type;

#ifndef _LIBCPP_CXX03_LANG
    template <class _Up> using rebind = _Up*;
#else
    template <class _Up> struct rebind {typedef _Up* other;};
#endif

private:
    struct __nat {};
public:
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    static pointer pointer_to(typename conditional<is_void<element_type>::value,
                                      __nat, element_type>::type& __r) _NOEXCEPT
        {return _VSTD::addressof(__r);}
};

template <class _From, class _To>
struct __rebind_pointer {
#ifndef _LIBCPP_CXX03_LANG
    typedef typename pointer_traits<_From>::template rebind<_To>        type;
#else
    typedef typename pointer_traits<_From>::template rebind<_To>::other type;
#endif
};

The template disambiguator for dependent names
https://en.cppreference.com/w/cpp/language/dependent_name.html#:~:text=The template disambiguator for dependent names

alias template
https://en.cppreference.com/w/cpp/language/type_alias.html