Extraction of templates and generic programming-02-iterator extraction example of fixed extraction technology

stay stl The types of iterators in can be divided into the following 5 Categories:

1. Output iterator
   struct output_iterator_tag;
2. Input iterator
   struct input_iterator_tag;
3. Forward iterator
   struct forward_iterator_tag;
4. Bidirectional iterator
   struct bidirectional_iterator_tag;
5. Random access iterator
   struct random_access_iterator_tag;
   stay stl Different containers in often provide different types of iterators , Such as vector Containers provide random access iterators , and list The container provides a bidirectional iterator . Look at the code below ：

void _display_category(random_access_iterator_tag mytag)
{
    
	cout << "random_access_iterator_tag\n";
}

void _display_category(bidirectional_iterator_tag mytag)
{
    
	cout << "bidirectional_iterator_tag\n";
}

void _display_category(forward_iterator_tag mytag)
{
    
	cout << "forward_iterator_tag\n";
}

void _display_category(output_iterator_tag mytag)
{
    
	cout << "output_iterator_tag\n";
}

void _display_category(input_iterator_tag mytag)
{
    
	cout << "input_iterator_tag\n";
}



template<typename T>
void display_category(T iter)
{
    

	typename iterator_traits<T>::iterator_category cagy;
	_display_category(cagy);
}


int main()
{
    
	display_category(vector<int>::iterator());
	display_category(list<int>::iterator());
	system("pause");
	return 0;
}

result ：
random_access_iterator_tag
bidirectional_iterator_tag
And it turns out that ,vector The iterator type corresponding to the container is random_access_iterator_tag,list The container corresponds to bidirectional_iterator_tag.
Upper acquisition vector,list Iterator type of container , It is achieved by fixing the extraction class template . In order to deepen the use of fixed extraction template , Here we use list,vector For example , Take a look at how these two containers let our programmers get the corresponding iterator types with the help of fixed extraction templates .

class _List_My_iterator
{
    
public:
	using iterator_category = bidirectional_iterator_tag;

};

template<class T>
class list_My
{
    
public:
	using iterator = _List_My_iterator;
};


class _Vector_My_iterator
{
    
public:
	using iterator_category = random_access_iterator_tag;

};

template<class T>
class vector_My
{
    
public:
	using iterator = _Vector_My_iterator;
};

template<typename IterT>
struct iterator_traits_My
{
    
	using iterator_category = typename IterT::iterator_category;

};
template<typename T>
void display_category(T iter)
{
    
	typename iterator_traits_My<T>::iterator_category cagy;
	_display_category(cagy);
}


int main()
{
    
	display_category(vector_My<int>::iterator());
	display_category(list_My<int>::iterator());
	system("pause");
	return 0;
}

（1） When executed display_category(vector_My<int>::iterator()); when , The first thing to do is create a _Vector_My_iterator Class object , So at this time display_category() Template parameters corresponding to the function template T To be inferred as _Vector_My_iterator type .
（2） Then enter the display_category(), face typename iterator_traits_My<T>::iterator_category cagy; sentence . This code line is an extractor , because T To be inferred as _Vector_My_iterator, So changing careers is equivalent to iterator_traits_My<_Vector_My_iterator>::iterator_category cagy;
（3） In extractor iterator_traits_My in ,T Type is _Vector_My_iterator, therefore using iterator_category = _Vector_My_iterator::iterator_category;
（4） therefore typename iterator_traits_My<T>::iterator_category cagy; amount to _Vector_My_iterator::iterator_category cagy; That is to say random_access_iterator_tag cagy;
result ：
random_access_iterator_tag
bidirectional_iterator_tag