C++ 迭代器模式实现
STL模板库中有大量迭代器实现，这些迭代器隔离了算法实现与访问接口，我们也可以编写属于自己的迭代器。STL中的迭代器均继承至一个通用迭代器接口：
template <class _Category, class _Tp, class _Distance = ptrdiff_t,
class _Pointer = _Tp*, class _Reference = _Tp&>
struct iterator {
typedef _Category iterator_category; //迭代器类型
typedef _Tp value_type;
typedef _Distance difference_type;
typedef _Pointer pointer;
typedef _Reference reference;
};
迭代器类型用于指明实例化迭代器的标签，如:输入迭代器(写)、输出迭代器(读)、随机访问迭代器等，算法可通过不同迭代器种类来实现各版本。
__distance算法就针对不同迭代器不同的实现：
template <class _InputIterator, class _Distance>
inline void __distance(_InputIterator __first, _InputIterator __last,
_Distance& __n, input_iterator_tag)
{
while (__first != __last) { ++__first; ++__n; }
}
template <class _RandomAccessIterator, class _Distance>
inline void __distance(_RandomAccessIterator __first,
_RandomAccessIterator __last,
_Distance& __n, random_access_iterator_tag)
{
__STL_REQUIRES(_RandomAccessIterator, _RandomAccessIterator);
__n += __last - __first;
}
所有的迭代器种类声明如下:
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 {};
迭代器的重要方法如下:
T& operator*() const T* operator->() const bool operator==(const iterator & __x) const bool operator!=(const iterator & __x) const iterator& operator++() iterator operator++(int)
这样就可以实现自己的迭代器了：
#include <iostream>
#include <iterator>
using namespace std;
template <typename T>
struct _A_node {
_A_node *next;
_A_node *prev;
T data;
};
template <typename T>
class A_Iterator:public iterator<forward_iterator_tag,T>
{
private:
typedef A_Iterator<T> self;
_A_node<T> *_node;
void incr()
{
_node = _node->next;
}
void decr()
{
_node = (_A_node<T>*)_node->prev;
}
public:
A_Iterator():_node(0) {}
A_Iterator(_A_node<T> *x):_node(x) {}
~A_Iterator() {}
T& operator*() const { return _node->data;}
T* operator->() const {return &(operator*());}
bool operator==(const self& __x) const {
return _node == __x._node;
}
bool operator!=(const self& __x) const {
return _node != __x._node;
}
self& operator++() { incr(); return *this;}
self operator++(int) { self __tmp = *this; incr(); return __tmp;}
self& operator--() {decr();return *this;}
self operator--(int) {self __tmp = *this; decr();return __tmp;}
};
template <typename T>
class A {
private:
typedef T Node;
_A_node<T> *pNode;
public:
typedef A_Iterator<T> iterator;
typedef _A_node<T> _Node;
A() {
pNode = new _A_node<T>;
pNode->next = pNode;
pNode->prev = pNode;
}
~A() {
//add delete node function here
delete pNode;
}
iterator begin() {return (_Node *)(pNode->next);}
iterator end() {return pNode;}
///////////////// method /////////////////////
void push(T value)
{
_A_node<T> *v = new _A_node<T>;
v->data = value;
v->next = pNode->next;
pNode->next->prev = v;
pNode->next = v;
v->prev = pNode;
}
T pop()
{
T value;
_Node *next = pNode->next;
pNode->next = next->next;
next->prev = pNode;
next->prev = next;
next->next = next;
value = next->data;
delete next;
return value;
}
};
int main(int argc, char *argv[])
{
A<int> a;
a.push(1);
a.push(2);
a.pop();
for(A<int>::iterator iter = a.begin();iter != a.end();iter++) {
*iter = 3;
std::cout << "value:" << *iter << std::endl;
}
int b = a.pop();
std::cout << (a.begin() == a.end()) << ",b:" << b << std::endl;
return 0;
}
若需增加const_iterator则需要进行重写另一个iterator，也可以通过修改iterator模板声明格式进行修改，满足其iterator格式，最终版本如下：
#include <iostream>
#include <iterator>
using namespace std;
template <typename T>
struct _A_node {
_A_node *next;
_A_node *prev;
T data;
};
template <class T,class Ref,class Ptr>
class A_Iterator:public iterator<forward_iterator_tag,T>
{
public:
typedef A_Iterator<T,T&,T*> iterator;
typedef A_Iterator<T,const T&,const T*> const_iterator;
private:
typedef A_Iterator<T,Ref,Ptr> self;
_A_node<T> *_node;
void incr()
{
_node = _node->next;
}
void decr()
{
_node = (_A_node<T>*)_node->prev;
}
public:
A_Iterator():_node(0) {}
A_Iterator(_A_node<T> *x):_node(x) {}
~A_Iterator() {}
Ref operator*() const { return _node->data;}
Ptr operator->() const {return &(operator*());}
bool operator==(const self& __x) const {
return _node == __x._node;
}
bool operator!=(const self& __x) const {
return _node != __x._node;
}
self& operator++() { incr(); return *this;}
self operator++(int) { self __tmp = *this; incr(); return __tmp;}
};
template <typename T>
class A {
private:
typedef T Node;
_A_node<T> *pNode;
public:
typedef typename A_Iterator<T,T&,T*>::iterator iterator;
typedef typename A_Iterator<T,const T&,const T*>::const_iterator const_iterator;
typedef _A_node<T> _Node;
A() {
pNode = new _A_node<T>;
pNode->next = pNode;
pNode->prev = pNode;
}
~A() {
//add delete node function here
delete pNode;
}
iterator begin() {return (_Node *)(pNode->next);}
iterator end() {return pNode;}
const_iterator const_begin() const {return (_Node *)(pNode->next);}
const_iterator const_end() const {return pNode;}
///////////////// method /////////////////////
void push(T value)
{
_A_node<T> *v = new _A_node<T>;
v->data = value;
v->next = pNode->next;
pNode->next->prev = v;
pNode->next = v;
v->prev = pNode;
}
T pop()
{
T value;
_Node *next = pNode->next;
pNode->next = next->next;
next->prev = pNode;
next->prev = next;
next->next = next;
value = next->data;
delete next;
return value;
}
};
int main(int argc, char *argv[])
{
A<int> a;
a.push(1);
a.push(2);
//a.pop();
/*
for(A<int>::iterator iter = a.begin();iter != a.end();iter++) {
*iter = 3;
std::cout << "value:" << *iter << std::endl;
}
*/
//A<int>::const_iterator iter = a.const_begin();
for(A<int>::const_iterator iter = a.const_begin();iter != a.const_end();iter++) {
//*iter = 3;
std::cout << "value:" << *iter << std::endl;
}
int b = a.pop();
std::cout << (a.begin() == a.end()) << ",b:" << b << std::endl;
return 0;
}
