浅析iterator

andalousie

声明一下，这不是在实现<vector>库，而是基于C++的vector给出一个iterator方法。vc6.0/g++编译通过。第一个版本：

1. #include <iostream>
   
2. #include <vector>
   
3. using namespace std;
   
4. 
   
5. template <class Object>
   
6. class VectorIterator;
   
7. 
   
8. // Same as the vector, but has a getIterator method.
   
9. // No extra data, no overridden methods, so non-virtual
   
10. // destructor in original vector is OK!
    
11. template <class Object>
    
12. class MyVector : public vector<Object>
    
13. {
    
14.   public:
    
15.     explicit MyVector( int size = 0 )
    
16.       : vector<Object>( size ) { }
    
17. 
    
18.     VectorIterator<Object> getIterator( ) const
    
19.       { return VectorIterator<Object>( this ); }
    
20. };
    
21. 
    
22. // A passive iterator class. Steps through its MyVector.
    
23. template <class Object>
    
24. class VectorIterator
    
25. {
    
26.   public:
    
27.     VectorIterator( const MyVector<Object> *v )
    
28.       : owner( v ), count( 0 ) { }
    
29. 
    
30.     bool hasNext( ) const
    
31.       { return count != owner->size( ); }
    
32. 
    
33.     const Object & next( )
    
34.       { return (*owner)[ count++ ]; }
    
35. 
    
36.   private:
    
37.     const MyVector<Object> *owner;
    
38.     int count;
    
39. };
    
40. 
    
41. int main( )
    
42. {
    
43.     MyVector<int> v;
    
44. 
    
45.     v.push_back( 3 );
    
46.     v.push_back( 2 );
    
47. 
    
48.     cout << "Vector contents: " << endl;
    
49. 
    
50.     VectorIterator<int> itr = v.getIterator( );
    
51.     while( itr.hasNext( ) )
    
52.         cout << itr.next( ) << endl;
    
53. 
    
54.     return 0;
    
55. }

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注意，这里我们的itr不再表现为指针的样子，类似Java的方法。
第二个版本：
class MyVector内容不做改变，增加VectorIterator的方法。

1. ...
   
2. // A passive iterator class. Steps through its MyVector.
   
3. template <class Object>
   
4. class VectorIterator
   
5. {
   
6.   public:
   
7.     VectorIterator( const MyVector<Object> *v )
   
8.       : owner( v ), count( 0 ) { }
   
9. 
   
10.     void reset( )
    
11.       { count = 0; }
    
12. 
    
13.     bool isValid( ) const
    
14.       { return count < owner->size( ); }
    
15. 
    
16.     void advance( )
    
17.       { count++; }
    
18. 
    
19.     const Object & retrieve( ) const
    
20.       { return (*owner)[ count ]; }
    
21. 
    
22.   private:
    
23.     const MyVector<Object> *owner;
    
24.     int count;
    
25. };
    
26. 
    
27. 
    
28. int main( )
    
29. {
    
30.     MyVector<int> v;
    
31. 
    
32.     v.push_back( 3 );
    
33.     v.push_back( 2 );
    
34.     VectorIterator<int> itr = v.getIterator( );
    
35. 
    
36.     for( int i = 0; i < 2; i++ )
    
37.     {
    
38.         cout << "Vector contents: " << endl;
    
39.         for( itr.reset( ); itr.isValid( ); itr.advance( ) )
    
40.             cout << itr.retrieve( ) << endl;
    
41.     }
    
42. 
    
43.     return 0;
    
44. }

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第三个版本：基于继承的迭代器

1. ...
   
2. template <class Object>
   
3. class Iterator;
   
4. 
   
5. template <class Object>
   
6. class VectorIterator;
   
7. 
   
8. // Same as the vector, but has a getIterator method.
   
9. // No extra data, no overridden methods, so non-virtual
   
10. // destructor in original vector is OK!
    
11. template <class Object>
    
12. class MyVector : public vector<Object>
    
13. {
    
14.   public:
    
15.     explicit MyVector( int size = 0 )
    
16.       : vector<Object>( size ) { }
    
17. 
    
18.     Iterator<Object> *getIterator( ) const
    
19.       { return new VectorIterator<Object>( this ); }
    
20. };
    
21. 
    
22. 
    
23. // A passive iterator class protocol. Steps through its container.
    
24. template <class Object>
    
25. class Iterator
    
26. {
    
27.   public:
    
28.     virtual ~Iterator( ) { }
    
29. 
    
30.     virtual bool hasNext( ) const = 0;
    
31.     virtual const Object & next( ) = 0;
    
32. };
    
33. 
    
34. // A concrete implementation of the iterator.
    
35. // Could have been nested inside of MyVector!
    
36. template <class Object>
    
37. class VectorIterator : public Iterator<Object>
    
38. {
    
39.   public:
    
40.     VectorIterator( const MyVector<Object> *v )
    
41.       : owner( v ), count( 0 ) { }
    
42. 
    
43.     bool hasNext( ) const
    
44.       { return count != owner->size( ); }
    
45. 
    
46.     const Object & next( )
    
47.       { return (*owner)[ count++ ]; }
    
48. 
    
49.   private:
    
50.     const MyVector<Object> *owner;
    
51.     int count;
    
52. };
    
53. 
    
54. 
    
55. int main( )
    
56. {
    
57.     MyVector<int> v;
    
58. 
    
59.     v.push_back( 3 );
    
60.     v.push_back( 2 );
    
61. 
    
62.     cout << "Vector contents: " << endl;
    
63. 
    
64.     Iterator<int> *itr = v.getIterator( );
    
65.     while( itr->hasNext( ) )
    
66.         cout << itr->next( ) << endl;
    
67. 
    
68.     delete itr;
    
69. 
    
70.     return 0;
    
71. }

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缺点：由于*itr是动态分配的，所以需要delete。
大家如果不放心，用auto_ptr也可以。
就是36行改成

1.     auto_ptr<Iterator<Object> > getIterator( ) const
   
2.       { return auto_ptr<Iterator<Object> >(new VectorIterator<Object>( this )); }

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82行改成

1.     auto_ptr<Iterator<int> > itr = v.getIterator( );

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就可以了。