C++标准库（六）之traits技术

traits技术

原理：利用template的参数推导机制获取传入的参数型别。

template<typename T>
struct Iter
{
    typedef T value_type;
    ....
}

template<typename T>
typename T::value_type func(T* ite)
{return *ite;}

这种程度，依旧会遇到一个问题：如果不是一个class type（比如指针，引用），就无法进行正确的参数推导。可以使用模板偏特化来处理这种情形：

template<typename T>
struct Iter<T*>
{
    typename T value_type;
};

我们需要处理的核心问题：通过traits技术如何获得iterator描述的型别？

template<typename T>
struct iterator_traits
{
    typedef typename T::iterator_category iterator_category;
    typedef typename T::value_type value_type;
    typedef typename T::difference_type difference_type;
    typedef typename T::pointer pointer;
    typedef typename T::reference reference;
};

同时，iterator_traits必须对传入的型别为pointer和pointer-to-const者设计特化版本。

template<typename T>
struct iterator_traits<T*>
{
    ...
    typedef random_access_iterator_tag iterator_category;
}

template<typename T>
struct iterator_traits<const T*>
{
    ...
    typedef random_access_iretator_tag iterator_category;
}

型别：

• value type：描述iterator指向对象的型别。
• difference type：描述两个iterator之间的距离，默认情况下使用C++内建的ptrdiff_t类型
• reference type：描述iterator所指向对象的引用
• pointer type：描述iterator所指向对象的指针
• iterator_category：描述迭代器的相应性别
  • Input Iterator：只读迭代器
  • Output Iterator：只写迭代器
  • Forward Iterator：读写迭代器
  • Bidirectional Iterator：双向移动迭代器，可读写
  • Random Access Iterator：随机移动迭代器，可读写

利用重载机制，通过在编译期就为不同版本的迭代器选择不同版本的函数：

template<typename category,typename T,typename dis=ptrdiff_t,typename poin=T*,typename ref=T&>
struct iterator
{
    typedef category iterator_category;
    typedef T value_type;
    typedef dis difference_type;
    typedef poin pointer;
    typedef ref reference;
};

struct input_iterator_tag {};
struct output_iterator_tag {};
struct forward_iterator_tag : public input_iterator_tag {};
struct bidirectional_iterator_tag : public forward_iterator_tag {};
struct random_access_iterator_tag : public bidirectional_iterator_tag {};

template <typename InputIterator,typename Distance>
inline void __advance(InputIterator& iter,Distance n,input_iterator_tag)
{
    while(n--)
        ++iter;
}

template <typename ForwardIterator,typename Distance>
inline void __advance(ForwardIterator& iter,Distance n,forward_iterator_tag)
{
    __advance(i,n,input_iterator_tag());
}

template <typename BidirectionalIterator,typename Distance>
inline void __advance(BidirectionalIterator& iter,Distance n,bidirectional_iterator_tag)
{
    if(n > 0)
        while(n--)
            ++iter;
    if(n < 0)
        while(n--)
            --iter;
}

template <typename RandomAccessIterator,typename Distance>
inline void __advance(RandomAccessIterator& iter,Distance n,random_access_iterator_tag)
{
    iter += n;
}

根据traits机制，需要使用以下 的包装器：

template <typename InputIterator,typename Distance>
inline void __advance(InputIterator& iter,Distance n)
{
    __advance(i,n,iteratir_traits<InputIterator>::iterator_category());
}

获取型别的函数：

template<typename Iterator>
inline typename iterator_traits<Iterator>::iterator_category iteratir_category(const Iterator&)
{
    typedef typename iterator_traits<Iterator>::iterator_category category;
    return category();
}

template<typename Iterator>
inline typename iterator_traits<Iterator>::distance_type* distance_type(const Iterator&)
{
    return static_cast<typename iterator_traits<Iterator>::distance_type*>(0);
}

template<typename T>
inline typename iterator_traits<Iterator>::value_type* value_type(const Iterator&)
{
    return static_cast<typename iterator_traits<Iterator>::value_type*>(0);
}

总结

设计适当的型别，是迭代器的责任。
设计适当的迭代器，是容器的责任。

__type_traits

traits技术弥补了C++语言本身的不足，但是traits技术不仅存在于Iterator之中，也存在于STL之中。iterator_traits负责萃取迭代器特性，__type_traits则负责萃取型别参数。此处的型别参数是指：这个型别是否具有non-trivial default ctor？是否具有non-trivial copy ctor？是否具有non-travial assignment operator？是否具有non-trravial dctor？如果没有的话，我们可以使用memcpy()，memmove()采取最有效的操作。否则的话，需要根据ctor，copy，dctor等进行复杂的操作。
可以使用如下类似与iterator_traits的方式：

__type_traits<T>::has_trivial_default_constructor
__type_traits<T>::has_trivial_copy_constructor
__type_traits<T>::has_trivial_assignment_operator
__type_traits<T>::has_trivial_destructor
__type_traits<T>::is_POD_type

struct __true_type {};
struct __false_type {};

template<typename T>
struct __type_traits
{
    typedef __false_type has_trivial_default_constructor;
    typedef __false_type has_trivial_copy_constructor;
    typedef __false_type has_trivial_assignment_operator
    typedef __false_type has_trivial_destructor;
    typedef __false_type is_POD_type; 
}

STL为所有的内嵌性别定义最保守的值，然后再为体统提供类型（char，long，int，double）等设计适当的特化版本。

__STL_TEMPLATE_NULL struct __type_traits<bool> {
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

#endif /* __STL_NO_BOOL */

__STL_TEMPLATE_NULL struct __type_traits<char> {
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

__STL_TEMPLATE_NULL struct __type_traits<signed char> {
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

__STL_TEMPLATE_NULL struct __type_traits<unsigned char> {
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

#ifdef __STL_HAS_WCHAR_T

__STL_TEMPLATE_NULL struct __type_traits<wchar_t> {
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

#endif /* __STL_HAS_WCHAR_T */

__STL_TEMPLATE_NULL struct __type_traits<short> {
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

__STL_TEMPLATE_NULL struct __type_traits<unsigned short> {
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

__STL_TEMPLATE_NULL struct __type_traits<int> {
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

__STL_TEMPLATE_NULL struct __type_traits<unsigned int> {
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

__STL_TEMPLATE_NULL struct __type_traits<long> {
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

__STL_TEMPLATE_NULL struct __type_traits<unsigned long> {
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

#ifdef __STL_LONG_LONG

__STL_TEMPLATE_NULL struct __type_traits<long long> {
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

__STL_TEMPLATE_NULL struct __type_traits<unsigned long long> {
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

#endif /* __STL_LONG_LONG */

__STL_TEMPLATE_NULL struct __type_traits<float> {
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

__STL_TEMPLATE_NULL struct __type_traits<double> {
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

__STL_TEMPLATE_NULL struct __type_traits<long double> {
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

#ifdef __STL_CLASS_PARTIAL_SPECIALIZATION

template <class _Tp>
struct __type_traits<_Tp*> {
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

#else /* __STL_CLASS_PARTIAL_SPECIALIZATION */

__STL_TEMPLATE_NULL struct __type_traits<char*> {
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

__STL_TEMPLATE_NULL struct __type_traits<signed char*> {
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

__STL_TEMPLATE_NULL struct __type_traits<unsigned char*> {
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

__STL_TEMPLATE_NULL struct __type_traits<const char*> {
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

__STL_TEMPLATE_NULL struct __type_traits<const signed char*> {
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};

__STL_TEMPLATE_NULL struct __type_traits<const unsigned char*> {
   typedef __true_type    has_trivial_default_constructor;
   typedef __true_type    has_trivial_copy_constructor;
   typedef __true_type    has_trivial_assignment_operator;
   typedef __true_type    has_trivial_destructor;
   typedef __true_type    is_POD_type;
};