I want to create a const_iterator that gives me access to dynamically
generated objects.
For example see the class Polygon that contains a list of points
stored as following doubles in a vector (x1, y1, x2, y2, ...). The
iterator should return only pairs of doubles to avoid possible
confusion of the user of the class Polygon. (The storage format is
needed for conversion to other libraries.)
The code for Polygon below compiles well, but the problem is using the
operator->.
inline pair<double,doubleoperator->() { return pair<double,
double>(*_points, *(_points+1)); }
If I use the -operator like this:
Polygon p;
p.addPoint(5.0, 5.0);
for(Polygon::iterator it = polygon.begin(); it != polygon.end(); +
+it) {
cout << it->first << endl;
cout << (it).second << endl;
}
I get the following compile error: result of 'operator->()' yields
non-
pointer result
The second line with the * operator gives no errors.
Is it possible to overload the operator-in the intended way?
#include <utility>
#include <vector>
using namespace std;
class Polygon{
public:
class iterator;
inline Polygon() : _points() {};
inline Polygon(unsigned size) : _points(size*2) {};
virtual ~Polygon() {};
inline void addPoint(double x, double y) {
_points.push_back(x);
_points.push_back(y);
}
inline Polygon::iterator begin() const
{ return iterator(*this, 0); }
inline Polygon::iterator end() const
{ return Polygon::iterator(*this, _points.size()); }
inline void clear() { _points.clear(); }
inline void resize(unsigned i) { _points.resize(i*2); }
inline unsigned size() const { return _points.size()/2; }
inline const double* data() const { return &_points[0]; }
class iterator : public
std::iterator<std::random_access_iterator_tag, pair<double, double
{
private:
const Polygon* _polygon;
unsigned _offset;
vector<double>::const_iterator _points;
public:
inline iterator(const Polygon& p, unsigned offset = 0) :
_polygon(&p), _offset(offset), _points(p._points.begin())
{ }
inline iterator(const iterator& i) :
_polygon(i._polygon), _offset(i._offset),
_points(i._points) { }
inline pair<double, doubleoperator*() const
{ return pair<double, double>(*_points, *(_points+1)); }
inline pair<double, doubleoperator[](unsigned i) const
{ return pair<double, double>(*(_points+(i*2)),*(_points+
(i*2)+1)); }
inline pair<double,doubleoperator->()
{
return pair<double,double>(*_points, *(_points+1));
}
inline iterator& operator++()
{ ++_points; ++_points; ++_offset; return *this; }
inline iterator operator++(int)
{ iterator p = *this; ++(*this); return p; }
inline iterator& operator=(iterator& i) {
if (this == &i) { return *this; }
_polygon = i._polygon;
_offset = i._offset;
_points = i._points;
return *this;
}
inline bool operator==(const iterator& i) const
{return _polygon == i._polygon && _offset == i._offset;}
inline bool operator<(const iterator& i) const
{ return _polygon < i._polygon || (_polygon ==i._polygon &&
_offset < i._offset); }
inline bool operator!=(const iterator& i) const { return !
(*this == i); }
inline bool operator>(const iterator& i) const { return i <
*this; }
inline bool operator<=(const iterator& i) const { return !(i <
*this); }
inline bool operator>=(const iterator& i) const { return !
(*this < i); }
};
private:
vector<double_points;
};
#endif
P.S: I use g++ (GCC) 4.1.2 20061115 (prerelease) (Debian 4.1.1-21) on
a 64bit x86 AMD machine. 9 2904
Nico wrote:
I want to create a const_iterator that gives me access to dynamically
generated objects.
[..]
Are you going to post it every few minutes? For how long?
V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask
On Thu, 06 Sep 2007 10:19:58 -0700, Nico wrote:
>I want to create a const_iterator that gives me access to dynamically generated objects. The code for Polygon below compiles well, but the problem is using the operator->.
inline pair<double,doubleoperator->() { return pair<double, double>(*_points, *(_points+1)); }
I get the following compile error: result of 'operator->()' yields non->pointer result
Define an iterator with a pointer as result type.
class iterator : public std::iterator<std::random_access_iterator_tag, pair<double, double
{
private:
inline pair<double,doubleoperator->()
// pointer and reference are defined in the base class template
pointer operator>() const;
reference operator*() const;
--
Roland Pibinger
"The best software is simple, elegant, and full of drama" - Grady Booch
Roland Pibinger wrote:
[..]
// pointer and reference are defined in the base class template
pointer operator>() const;
I am guessing you meant
pointer operator->() const;
reference operator*() const;
V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask
On Thu, 6 Sep 2007 17:03:17 -0400, "Victor Bazarov" wrote:
>Roland Pibinger wrote:
>[..] // pointer and reference are defined in the base class template pointer operator>() const;
I am guessing you meant
pointer operator->() const;
Yes (the clipboard ate the hyphen).
Define an iterator with a pointer as result type.
I can't return a pointer, because I have no pair-object on the heap!
If I would
return new pair<double,double>(*_points, *(_points+1));
I would create a memory leak!
So I want just to return a copy of my generated pair that will be
deleted when out of scope of the calling function.
>I am guessing you meant
pointer operator->() const;
Of course. But that's not the point.
On 2007-09-07 09:30, Nico wrote:
>Define an iterator with a pointer as result type.
I can't return a pointer, because I have no pair-object on the heap!
If I would
return new pair<double,double>(*_points, *(_points+1));
I would create a memory leak!
So I want just to return a copy of my generated pair that will be
deleted when out of scope of the calling function.
I think returning a smart pointer would work.
--
Erik Wikström
Nico wrote:
>Define an iterator with a pointer as result type.
I can't return a pointer, because I have no pair-object on the heap!
If I would
return new pair<double,double>(*_points, *(_points+1));
I would create a memory leak!
So I want just to return a copy of my generated pair that will be
deleted when out of scope of the calling function.
[snip]
Is there a reason why you have a data member
std::vector<double>
where x-coordinates and y-coordinates are stored alternatingly? If you used
std::vector< std::pair< double, double
instead, the problem would just go away and you could use the iterator type
provided by std::vector.
Alternatively you would have to construct a temporary proxy object that
fakes the pair< double, double reference you want to return. If you want
an iterator and not just a const_iterator, said proxy object would (a)
internally construct a pair<double,doublefrom the values in the vector,
(b) convert silently into a pair<double,double>& on demand, and (c) upon
destruction write back the possibly changed coordinates into the vector
(for this reason, the proxy also needs to remember the indices of the
original values).
Best
Kai-Uwe Bux
On Fri, 07 Sep 2007 09:12:57 -0700, Kai-Uwe Bux wrote:
>Alternatively you would have to construct a temporary proxy object that fakes the pair< double, double reference you want to return. If you want an iterator and not just a const_iterator, said proxy object would (a) internally construct a pair<double,doublefrom the values in the vector, (b) convert silently into a pair<double,double>& on demand, and (c) upon destruction write back the possibly changed coordinates into the vector (for this reason, the proxy also needs to remember the indices of the original values).
A member in Polygon::iterator would be easier:
// not tested
class Polygon{
class iterator {
pair<double, doublecurrent;
public:
pointer operator->() const {
current = pair<double,double>(*_points, *(_points+1));
return ¤t;
}
// ...
};
};
--
Roland Pibinger
"The best software is simple, elegant, and full of drama" - Grady Booch
Roland Pibinger wrote:
On Fri, 07 Sep 2007 09:12:57 -0700, Kai-Uwe Bux wrote:
>>Alternatively you would have to construct a temporary proxy object that fakes the pair< double, double reference you want to return. If you want an iterator and not just a const_iterator, said proxy object would (a) internally construct a pair<double,doublefrom the values in the vector, (b) convert silently into a pair<double,double>& on demand, and (c) upon destruction write back the possibly changed coordinates into the vector (for this reason, the proxy also needs to remember the indices of the original values).
A member in Polygon::iterator would be easier:
// not tested
class Polygon{
class iterator {
pair<double, doublecurrent;
public:
pointer operator->() const {
current = pair<double,double>(*_points, *(_points+1));
return ¤t;
}
// ...
};
};
That would work for a const_iterator. I do not see, however, how that
mechanism deals with uses of iterator that would need to write to the
container, like
iter->first = 0.2;
Best
Kai-Uwe Bux This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
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