Connecting Tech Pros Worldwide Forums | Help | Site Map
bytes > topic > c > answers

problem overloading the ++ operator of an iterator

Newbie
  
Join Date: Sep 2007
Location: Denmark
Posts: 20
#1: Oct 22 '07
Hi,
I have the weirdest problem, and I can not see what is going wrong.
I have made a 2d container class, and am implementing an iterator for that class. However, the ++ operator is behaving very strange. The implementation looks like this (there is a lot of code, but I have only included it all for consistency. The problem is quite small and local):


Expand|Select|Wrap|Line Numbers
  1.  
  2. template<typename T> class Row<T> : public std::vector<T> {};
  3.  
  4. template<typename T> class Grid<T> : public std::vector<Row<T> >
  5. {
  6. public:
  7.  
  8.        class iterator;
  9.  
  10.         iterator
  11.         begin()
  12.         {
  13.             typename std::vector<Row<T> >::iterator it;
  14.             it = this->std::vector<Row<T> >::begin();
  15.             typename Row<T>::iterator ri;
  16.             ri = it->begin();
  17.             iterator ret(it,ri);
  18.             return ret;
  19.         }
  20.  
  21.  
  22.         iterator
  23.         end()
  24.         {
  25.             typename std::vector<Row<T> >::iterator it;
  26.             it = this->std::vector<Row<T> >::end()-1; 
  27.             typename Row<T>::iterator ri;
  28.             ri = it->end();
  29.             iterator ret(it,ri);
  30.             return ret;
  31.         }
  32. };
  33.  
  34. template<typename T>
  35.     class Grid<T>::iterator : public Row<T>::iterator
  36.         {
  37.         private:
  38.             typedef typename std::vector<Row<T> >::iterator row_iterator;
  39.             typedef typename Row<T>::iterator cell_iterator;
  40.  
  41.             row_iterator row_iter;
  42.             cell_iterator cell_iter;
  43.  
  44.         public:
  45.             iterator(){}
  46.             iterator(row_iterator ri, cell_iterator ci) : row_iter(ri), cell_iter(ci) {}
  47.             iterator(const iterator& rhs) : row_iter(rhs.row_iter), cell_iter(rhs.cell_iter) {}
  48.  
  49.             iterator
  50.             operator=(const iterator& rhs)
  51.             {
  52.                 if(this = &rhs) return *this;
  53.  
  54.                 cell_iter = rhs.cell_iter;
  55.                 row_iter = rhs.row_iter;
  56.                 return *this;
  57.             }
  58.  
  59.             T&
  60.             operator*()
  61.             {
  62.                 return *cell_iter;
  63.             }
  64.  
  65.  
  66.             iterator&
  67.             operator++() //prefix   //this is where the problem is!
  68.             {
  69.                 cell_iter++;
  70.                 if (cell_iter == row_iter->end())
  71.                 {
  72.                     std::cout << "end of line" << std::endl;
  73.                     ++row_iter;
  74.                     cell_iter == row_iter->begin();
  75.                 }
  76.                 return *this;
  77.             }
  78.  
  79.  
  80.             bool
  81.             operator==(const iterator& rhs)
  82.             {
  83.                 return (cell_iter == rhs.cell_iter && row_iter == rhs.row_iter);
  84.             }
  85.  
  86.             bool
  87.             operator!=(const iterator& rhs)
  88.             {
  89.                 return (!*this == rhs);
  90.             }
  91.  
  92.  
  93.             void
  94.             test()
  95.             {
  96.                 cell_iter = row_iter->begin();
  97.                 while (cell_iter != row_iter->end()) std::cout << *cell_iter++;
  98.                 ++row_iter;
  99.                 cell_iter = row_iter->begin();
  100.                 std::cout << std::endl;
  101.                 while (cell_iter != row_iter->end()) std::cout << *cell_iter++;
  102.                 std::cout << std::endl << std::endl;
  103.             }
  104.         };
I have added all the code for consistency, but the problem is where I have indicated by my comment.

It should all compile without problems, but has an unwanted behaviour when i test it, e.g.

Expand|Select|Wrap|Line Numbers
  1. #include<iostream>
  2.  
  3. #include "Grid.h"
  4.  
  5. int main()
  6. {
  7.    Grid<int> grid(3,3);
  8.  
  9.     for (int i=0; i != 3; ++i)
  10.     {
  11.         for (int j=0; j!= 3; ++j)
  12.         {
  13.             grid[i][j]=i*3 + j + 1;
  14.         }
  15.     }
  16.  
  17.    std::cout << *(grid.begin()) << std::endl;  
  18.    std::cout << *(--grid.end()) << std::endl; 
  19.  
  20.    Grid<int>::iterator iter3 = grid.begin();  
  21.    iter3.test();                                 // this behaves as expected!
  22.  
  23.    Grid<int>::iterator iter = grid.begin();
  24.  
  25.     for (int i= 0; i!=15; ++i)
  26.     {
  27.         std::cout << *iter << "  ";  // this does not!
  28.         ++iter;
  29.     }
  30. }
when I conduct iter3.test() i get the behaviour i want - it writes
123
456

when i use the ++ operator below, it writes
1 2 3 end of line
## ## ## 4 5 6 end of line
## ## ## 7 8 9 end of line
where ## indicates some undefined number.
I would gladly appreciate any help you could give me, as I have now been looking at this problem for days.
Thanks

Moderator
  
Join Date: Mar 2007
Location: North Bend Washington USA
Posts: 5,370
#2: Oct 22 '07

re: problem overloading the ++ operator of an iterator

I think on line 75 that you need to return cell_iter that you have been incrementing rather than this.
Newbie
  
Join Date: Sep 2007
Location: Denmark
Posts: 20
#3: Oct 22 '07

re: problem overloading the ++ operator of an iterator

Quote:

Originally Posted by weaknessforcats

I think on line 75 that you need to return cell_iter that you have been incrementing rather than this.

Thanks, but I do not see how that could work?
There is an implicit this->cell_iter++ in line 68. My iterator contains two other iterators:
Expand|Select|Wrap|Line Numbers
  1. Grid<T>::iterator : public Row<T>::iterator
  2. {
  3. typename std::vector<Row<T> >::iterator row_iter;
  4. typename Row<T>::iterator cell_iter;
  5. //...
  6. };
row_iter points at a row in the Grid, and cell_iter moves in that row. My intention with the increment operator is that when cell_iter reaches the end of its row, row_iter should point to the next row, and cell_iter to the first element of that row.
Moderator
  
Join Date: Mar 2007
Location: North Bend Washington USA
Posts: 5,370
#4: Oct 22 '07

re: problem overloading the ++ operator of an iterator

Sorry. I was off base on my comment.

However, what about:
Quote:

Originally Posted by mkborregaard

++row_iter;
cell_iter == row_iter->begin();

When row_iter == row_iter.end(), cell_iter is set back to row_iter.begin().

Maybe that should be:
Expand|Select|Wrap|Line Numbers
  1. if (++row_iter == row_iter.end()
  2. {
  3.     cell_iter = row_iter.end();
  4. }
  5. else
  6. {
  7.      cell_iter = row_iter.begin();
  8. }
  9. return *this;
  10.  
Newbie
  
Join Date: Sep 2007
Location: Denmark
Posts: 20
#5: Oct 22 '07

re: problem overloading the ++ operator of an iterator

Yes!
Finally it works :) thanks!
Michael
Reply

« previous question | next question »

Similar C / C++ bytes

/bytes/development

/bytes/it

Forums
Visit our community forums for general discussions and latest on Bytes

Our staff and community areas have moved to bytes.com/forums/

/bytes/about

We are a network of experts and professionals in IT and software development that help one another with answers to tough questions and share insights. Get the best answers to your questions from over 226,449 network members.

dev questions:
algorithms | asp | asp.net | c/c++ | coldfusion | c# | flash | html/css | java | javascript |
mobile dev | .net | perl | php | python | ruby | software dev | vb.net | visual basic | xml
it questions:
access | apache | careers | db2 | iis | macosx | mysql | networking | oracle | postgresql | sql server | unix | windows

About Bytes | Copyright 1995-2009 BYTES. All rights Reserved