C++/is_integral_v 实现解读 折叠表达式

C++ 标准17 折叠表达式的学习

源码如下：其中调用了  _Is_any_of_v

_EXPORT_STD template <class _Ty>
_INLINE_VAR constexpr bool is_integral_v = _Is_any_of_v<remove_cv_t<_Ty>, bool, char, signed char, unsigned char,
    wchar_t,
#ifdef __cpp_char8_t
    char8_t,
#endif // __cpp_char8_t
    char16_t, char32_t, short, unsigned short, int, unsigned int, long, unsigned long, long long, unsigned long long>;

 

其中：_Is_any_of_v 调用：

template <class _Ty, class... _Types>
_INLINE_VAR constexpr bool _Is_any_of_v = // true if and only if _Ty is in _Types
    disjunction_v<is_same<_Ty, _Types>...>;

 

disjunction_v 实现代码：

template <bool _First_value, class _First, class... _Rest>
struct _Disjunction { // handle true trait or last trait
    using type = _First;
};

template <class _False, class _Next, class... _Rest>
struct _Disjunction<false, _False, _Next, _Rest...> { // first trait is false, try the next trait
    using type = typename _Disjunction<_Next::value, _Next, _Rest...>::type;
};

_EXPORT_STD template <class... _Traits>
struct disjunction : false_type {}; // If _Traits is empty, false_type

template <class _First, class... _Rest>
struct disjunction<_First, _Rest...> : _Disjunction<_First::value, _First, _Rest...>::type {
    // the first true trait in _Traits, or the last trait if none are true
};

主要使用了偏特化，与折叠表达式特性

修改如下：

template<bool bHas, class First, class... Rest>
struct hasInclude2
{
    using type = First;
};
/// 这里明确递归使用，偏特化处理
/// 理解的难点是偏特化
template<class First, class Next, class... Rest>
struct hasInclude2<false, First, Next, Rest...> : hasInclude2<Next::value, Next, Rest...>{};

template<class First,class... Rest>
struct hasInclude1 : hasInclude2<First::value,First,Rest...>{ };


template<class T, class... Rest>
struct hasInclude : hasInclude1<std::is_same<T, Rest>...>{};

 

bool bChar = hasInclude<char,char,int,long>::type::value;