編譯器: MSVC v142
所屬文件: xutility
位置(行): 377
函數(shù)名稱: _Get_unwrapped_n
函數(shù)描述:
嘗試提取迭代器的_Ptr, 如果迭代器沒有_ptr則原封不動的返回這個迭代器

源碼：

template <class _Iter, class _Diff,
    enable_if_t<
        !_Unwrappable_for_unverified_v<_Iter> //
            && ((!_Unwrappable_for_offset_v<_Iter> && is_integral_v<_Diff>) || is_same_v<_Diff, _Distance_unknown>),
        int> = 0>
_NODISCARD constexpr const _Iter& _Get_unwrapped_n(const _Iter& _It, _Diff) {
    // pass through lvalue iterator that doesn't participate in checking
    return _It;
}

?

拆分理解

• _Unwrappable_for_unverified_v

  template <class _Iter>
  _INLINE_VAR constexpr bool _Unwrappable_for_unverified_v = _Unwrappable_v<_Iter>&& _Do_unwrap_when_unverified_v<_Iter>;
  

  _Unwrappable_v 驗證模板參數(shù)是不是一個Iterator.
  _Do_unwrap_when_unverified_v 判斷是否可以執(zhí)行_Unwrapped函數(shù).

  vector<int> a {1, 2, 3, 4, 5};
  vector<int> b {};
  auto start_iter = a.begin();        // 生成一個 iterator: _Vector_iterator
  auto back_iter = back_inserter(b);  // 生成一個 iterator: back_insert_iterator
  
  // Debug 模式
  cout << _ITERATOR_DEBUG_LEVEL << endl;  // 2
  cout << _Unwrappable_for_unverified_v<decltype(start_iter)> << endl; // false
  cout << _Unwrappable_for_unverified_v<decltype(back_iter)> << endl;  // false
  
  // Release 模式
  cout << _ITERATOR_DEBUG_LEVEL << endl;  // 0
  cout << _Unwrappable_for_unverified_v<decltype(start_iter)> << endl; // true
  cout << _Unwrappable_for_unverified_v<decltype(back_iter)> << endl;  // false
  

  這個測試代碼說明，當(dāng)編譯模式是Debug時, begin和back_inserter生成的iterator都是不允許在未驗證_Verify_range的情況下調(diào)用iterator的_Unwrapped函數(shù). 當(dāng)編譯模式是Release時, begin生成的iterator允許在未驗證_Verify_range的情況下調(diào)用iterator的_Unwrapped函數(shù), 而back_inserter則不可以.
  備注: 這里說的不允許, 并不是真的不能調(diào)用, 而是做模板判斷的時候, 它會過濾掉當(dāng)前篩選.

• _Unwrappable_for_offset_v

  template <class _Iter>
  _INLINE_VAR constexpr bool _Unwrappable_for_offset_v = _Unwrappable_v<_Iter>&& _Offset_verifiable_v<_Iter>;
  

  _Unwrappable_v 驗證模板參數(shù)是不是一個Iterator.
  _Offset_verifiable_v 判斷是否可以執(zhí)行 _Verify_offset 函數(shù).

• is_integral_v

  // determine whether cv-unqualified type _Ty is integral
  template <class _Ty>
  _INLINE_VAR constexpr bool _Is_integral = _Is_any_of_v<_Ty, bool, char, signed char, unsigned char, wchar_t, 
                                                         char16_t, char32_t, short, unsigned short, int, unsigned int, 
                                                         long, unsigned long, long long, unsigned long long>;
  
  template <class _Ty>
  _INLINE_VAR constexpr bool is_integral_v = _Is_integral<remove_cv_t<_Ty>>;
  

  判斷一個type是不是一個數(shù)字類型.

• is_same_v

  template <class, class>
  _INLINE_VAR constexpr bool is_same_v = false; // determine whether arguments are the same type
  
  template <class _Ty>
  _INLINE_VAR constexpr bool is_same_v<_Ty, _Ty> = true;
  

  如果第一個類型參數(shù)和第二個類型參數(shù)相同, 則會匹配到template <class _Ty> _INLINE_VAR constexpr bool is_same_v<_Ty, _Ty> = true; 這個特化版本, 表示true.
  如果第一個類型參數(shù)和第二個類型參數(shù)不同, 則會跳過第一個匹配, 命中template <class, class> _INLINE_VAR constexpr bool is_same_v = false; 這個標(biāo)準(zhǔn)模板, 表示false.