STL Traits编程技法

 1 //class type的“内嵌型别”
 2 template <class _Iterator>
 3 struct iterator_traits {
 4   typedef typename _Iterator::iterator_category iterator_category;
 5   typedef typename _Iterator::value_type        value_type;
 6   typedef typename _Iterator::difference_type   difference_type;
 7   typedef typename _Iterator::pointer           pointer;
 8   typedef typename _Iterator::reference         reference;
 9 };
10 
11 //对原生指针的偏特化
12 template <class _Tp>
13 struct iterator_traits<_Tp*> {
14   typedef random_access_iterator_tag iterator_category;
15   typedef _Tp                         value_type;
16   typedef ptrdiff_t                   difference_type;
17   typedef _Tp*                        pointer;
18   typedef _Tp&                        reference;
19 };
20 
21 //对const指针的偏特化
22 template <class _Tp>
23 struct iterator_traits<const _Tp*> {
24   typedef random_access_iterator_tag iterator_category;
25   typedef _Tp                         value_type;
26   typedef ptrdiff_t                   difference_type;
27   typedef const _Tp*                  pointer;
28   typedef const _Tp&                  reference;
29 };

 1 //用于input_iterator的版本
 2 template <class _InputIter, class _Distance>
 3 inline void __advance(_InputIter& __i, _Distance __n, input_iterator_tag) {
 4   while (__n--) ++__i;
 5 }
 6 
 7 //用于 bidirectional_iterator的版本
 8 template <class _BidirectionalIterator, class _Distance>
 9 inline void __advance(_BidirectionalIterator& __i, _Distance __n, 
10                       bidirectional_iterator_tag) {
11   __STL_REQUIRES(_BidirectionalIterator, _BidirectionalIterator);
12   if (__n >= 0)
13     while (__n--) ++__i;
14   else
15     while (__n++) --__i;
16 }
17 
18 //用于 random_access_iterator的版本
19 template <class _RandomAccessIterator, class _Distance>
20 inline void __advance(_RandomAccessIterator& __i, _Distance __n, 
21                       random_access_iterator_tag) {
22   __STL_REQUIRES(_RandomAccessIterator, _RandomAccessIterator);
23   __i += __n;
24 }
25 
26 //advance方法的对外接口
27 template <class _InputIterator, class _Distance>
28 inline void advance(_InputIterator& __i, _Distance __n) {
29   __STL_REQUIRES(_InputIterator, _InputIterator);
30   __advance(__i, __n, iterator_category(__i));
31 }

1 struct __true_type {
2 };
3 
4 struct __false_type {
5 };

 1 template <class _Tp>
 2 struct __type_traits { 
 3    typedef __true_type     this_dummy_member_must_be_first;
 4                    /* Do not remove this member. It informs a compiler which
 5                       automatically specializes __type_traits that this
 6                       __type_traits template is special. It just makes sure that
 7                       things work if an implementation is using a template
 8                       called __type_traits for something unrelated. */
 9 
10    /* The following restrictions should be observed for the sake of
11       compilers which automatically produce type specific specializations 
12       of this class:
13           - You may reorder the members below if you wish
14           - You may remove any of the members below if you wish
15           - You must not rename members without making the corresponding
16             name change in the compiler
17           - Members you add will be treated like regular members unless
18             you add the appropriate support in the compiler. */
19  
20 
21    typedef __false_type    has_trivial_default_constructor;
22    typedef __false_type    has_trivial_copy_constructor;
23    typedef __false_type    has_trivial_assignment_operator;
24    typedef __false_type    has_trivial_destructor;
25    typedef __false_type    is_POD_type;
26 };