Article 47: Please use traits class to represent type information

In STL's algorithms, our hope is often to perform different, more efficient operations based on different types of iterators:

1Template<typename Itert, TypeName distt>2 voidAdvance (itert iter, Distt dis)3 {4     if(ITER isa random access iterator)5ITER + =dis;6     Else{7         if(Dis >=0){8              while(dis--)9iter++;Ten}Else{ One              while(dis++) Aiter--; -         } -     } the};

This technique of identifying the iterator category is called Iterator_traits, and the first requirement for each user-defined iterator type should be a TypeDef, similar to the following:

1 template<...>2class  list{3public:4      class  iterator{5        typedef typename BIDIRECTIONAL_ITERATOR_TAG Iterator_category; 6         ... 7     }; 8 };

This allows the iterator_traits to extract the type of the iterator through the extraction technique:

1 template<typename itert>2struct  iterator_traits3{  4    typedef typename Itert::iterator_category iterator_category; 5     ... 6 };

The above technique may not work for pointers carcass, because pointers cannot be built to specify iteartor_catagory; This problem can only be solved by the specificity of the template:

1 tempalte<typename itert>2struct iterator_traits<itert *>3{  4    typedef random_access_iterator_tag iterator_category; 5     ... 6 };

With Iterator_traits technology, it is possible to achieve an efficient iteration. But it is not actually implemented through the form of if Iterator is a random_access_iterator, but by overloading the function:

1Template<typename Itert, TypeName distt>2Doadvance (Itert iter, Distt D, Std::random_access_iterator_tag)//the tag here is often first extracted by iterator_traits;3 {4ITER + =D;5 }6Template<typename Itert, TypeName distt>7 doadvance (Itert iter, Distt D, Std::bidirectional_iterator_tag)8 {9     if(Dis >=0){Ten          while(dis--) Oneiter++; A}Else -          while(dis++) -iter--; the } -Tempalte<typename Itert, TypeName distt> -Doadvance (Itert iter, Distt D, Std::input_iterator_tag)//note the iterator version here - { +     if(D <0) -         ThrowStd::out_of_range ("negetive distance!"); +     Else{ A          while(d--) + +iter; at     } -}

The last version of Doadvanced is actually a input_iterator version, but since Forward_iterator_tag is inherited from Input_iterator_tag, So the doadvance here can also handle the forward iterator. By the different versions above, iterator can actually write the implementation version of the advance:

1 template<typename Itert, TypeName distt>2doadvance (Itert iter, Distt D)3 {4    5         // Note that the TypeName6 } is used here

So, a traits class is actually equivalent to:Create a set of H-overloaded functions or function templates that differ only in their traits parameters. Create a control function or template function to invoke the above template function and pass the traits information.
 Summary:Trait_class makes the information about the type available at compile time, which is done with the exception of template and template. combined with the overloaded mechanism, Traits_class may be able to perform if-else tests on the type at compile time.

Article 47: Please use traits class to represent type information