C++笔记——Traits

Traits

1. iterator_traits —— “迭代器的特性(型别)萃取机”

标准迭代器都包括 iterator_category, value_type, difference_type, pointer, reference 五个内嵌型别(五个typedef) (设计适当的相应型别, 是迭代器的责任。而设计适当的迭代器，则是容器的责任。)

以下代码摘自gcc v8.1.0

/**
*  @brief  Traits class for iterators.
*
*  This class does nothing but define nested typedefs.  The general
*  version simply @a forwards the nested typedefs from the Iterator
*  argument.  Specialized versions for pointers and pointers-to-const
*  provide tighter, more correct semantics.
*/
template<typename _Iterator>
  struct iterator_traits
  : public __iterator_traits<_Iterator> { };


/// Partial specialization for pointer types. 针对 指针类型 的偏特化版本
template<typename _Tp>
  struct iterator_traits<_Tp*>
  {
    typedef random_access_iterator_tag  iterator_category;
    typedef _Tp                         value_type;
    typedef ptrdiff_t                   difference_type;
    typedef _Tp*                        pointer;
    typedef _Tp&                        reference;
  };

/// Partial specialization for const pointer types. 针对 const指针 的偏特化版本
template<typename _Tp>
  struct iterator_traits<const _Tp*>
  {
    typedef random_access_iterator_tag  iterator_category;
    typedef _Tp                         value_type;
    typedef ptrdiff_t                   difference_type;
    typedef const _Tp*                  pointer;
    typedef const _Tp&                  reference;
  };

上面代码中的 __iterator_traits 为：

template<typename _Iterator, typename = __void_t<> >
  struct __iterator_traits { };

template<typename _Iterator>
  struct __iterator_traits< _Iterator,
                            __void_t< typename _Iterator::iterator_category,
                                      typename _Iterator::value_type,
                                      typename _Iterator::difference_type,
                                      typename _Iterator::pointer,
                                      typename _Iterator::reference > >
  {
    typedef typename _Iterator::iterator_category iterator_category;
    typedef typename _Iterator::value_type        value_type;
    typedef typename _Iterator::difference_type   difference_type;
    typedef typename _Iterator::pointer           pointer;
    typedef typename _Iterator::reference         reference;
  };

2. 用于确定类型、修改类型限定等：

列出去除cv限定符的代码如下：

/// remove_const
template<typename _Tp>
  struct remove_const
  { typedef _Tp     type; };

template<typename _Tp>
  struct remove_const<_Tp const>
  { typedef _Tp     type; };

/// remove_volatile
template<typename _Tp>
  struct remove_volatile
  { typedef _Tp     type; };

template<typename _Tp>
  struct remove_volatile<_Tp volatile>
  { typedef _Tp     type; };

/// remove_cv
template<typename _Tp>
  struct remove_cv
  {
    typedef typename
    remove_const<typename remove_volatile<_Tp>::type>::type     type;
  };

3. 详细讲解

TODO…