user-defined iterator

[vasili]

hello All,

I have a simple issue.
I defined a custom container, that encloses a std::list, which in turn
holds objects that are a simple abstraction of a six position array.

Now, i would like to serialize the whole newly-defined container, in
order to copy the contents to another array. So i thought to define an
iterator which represented a "pointer" to the container's data. But,
when i feed these iterators to std::copy the compiler complains about
a lot of types which are defined when a std::iterator is instanced.

the code:

//i leave all unnecessary stuff out just to be clear
#include <list>

using namespace std;

class SixBytes{
public: //i don't trash the example with any accessor methods
char m_data[6];
};

class MyCont{
list<SixBytesm_ list;
public:
class Iterator{
const MyCont& m_cont;
int m_index;
public:
Iterator(const MyCont& cnt, int index=0):m_cont (cnt),
m_index(index){ }
Iterator operator++(int) {//postfix? just placed this
and the following methods to be "complete" //
w.r.t. the requirements of the std::copy algorithm and to this example
Iterator ret(*this);
m_index++;
return ret;
}
Iterator& operator++(){//prefix?
m_index++;
return *this;
}
char operator*(){
//...return the byte that corresponds to the
position m_
index
}
};

inline Iterator begin() const{ return Iterator(*this) ; }//the
start of the serialization
inline const Iterator end() const { return Iterator(*this,
m_list.size()*6 ); }//it's end
MyCont(){ m_list.push_bac k(SixBytes());
m_list.push_bac k(SixBytes());}
};

int main(){
char data[13];
data[0] = 2;
MyCont m;
std::copy(m.beg in(), m.end(), data+1);
}//example ends here

giving this to the compiler i get this:

/usr/lib/gcc/i586-mandrake-linux-gnu/3.4.1/../../../../include/c++/
3.4.1/bits/stl_iterator_ba se_types.h: In instantiation of
`std::iterator_ traits<MyCont:: Iterator>':
/usr/lib/gcc/i586-mandrake-linux-gnu/3.4.1/../../../../include/c++/
3.4.1/bits/stl_algobase.h: 305: instantiated from `_OutputIterato r
std::__copy_ni2 (_InputIterator , _InputIterator, _OutputIterator ,
__false_type) [with _InputIterator = MyCont::Iterato r, _OutputIterator
= char*]'
/usr/lib/gcc/i586-mandrake-linux-gnu/3.4.1/../../../../include/c++/
3.4.1/bits/stl_algobase.h: 327: instantiated from `_OutputIterato r
std::__copy_ni1 (_InputIterator , _InputIterator, _OutputIterator ,
__false_type) [with _InputIterator = MyCont::Iterato r, _OutputIterator
= char*]'
/usr/lib/gcc/i586-mandrake-linux-gnu/3.4.1/../../../../include/c++/
3.4.1/bits/stl_algobase.h: 358: instantiated from `_OutputIterato r
std::copy(_Inpu tIterator, _InputIterator, _OutputIterator ) [with
_InputIterator = MyCont::Iterato r, _OutputIterator = char*]'
container.cpp:4 1: instantiated from here
/usr/lib/gcc/i586-mandrake-linux-gnu/3.4.1/../../../../include/c++/
3.4.1/bits/stl_iterator_ba se_types.h:129: error: no type named
`iterator_categ ory' in `class MyCont::Iterato r'

....more ot those errors about value_type and all such things.

Do i have to provide my implementation with these types?
If yes, what about iterator_catego y?
What about the simple char* parameters given to the std::copy
algorithm ? why are they ok?

thank you all,
vasilis.

[Victor Bazarov]

vasili wrote:

I have a simple issue.

:-)

I defined a custom container, that encloses a std::list, which in turn
holds objects that are a simple abstraction of a six position array.

Now, i would like to serialize the whole newly-defined container, in
order to copy the contents to another array. So i thought to define an
iterator which represented a "pointer" to the container's data. But,
when i feed these iterators to std::copy the compiler complains about
a lot of types which are defined when a std::iterator is instanced.

Since you want to use standard algorithm, it _may_ require that you
specialize 'iterator_trait s' for your custom iterator.

>
the code:

//i leave all unnecessary stuff out just to be clear
#include <list>

using namespace std;

class SixBytes{
public: //i don't trash the example with any accessor methods
char m_data[6];
};

class MyCont{
list<SixBytesm_ list;
public:
class Iterator{
const MyCont& m_cont;
int m_index;
public:
Iterator(const MyCont& cnt, int index=0):m_cont (cnt),
m_index(index){ }
Iterator operator++(int) {//postfix? just placed this
and the following methods to be "complete" //
w.r.t. the requirements of the std::copy algorithm and to this example
Iterator ret(*this);
m_index++;
return ret;
}
Iterator& operator++(){//prefix?
m_index++;
return *this;
}
char operator*(){
//...return the byte that corresponds to the
position m_
index
}
};

inline Iterator begin() const{ return Iterator(*this) ; }//the
start of the serialization
inline const Iterator end() const { return Iterator(*this,
m_list.size()*6 ); }//it's end
MyCont(){ m_list.push_bac k(SixBytes());
m_list.push_bac k(SixBytes());}
};

int main(){
char data[13];
data[0] = 2;
MyCont m;
std::copy(m.beg in(), m.end(), data+1);
}//example ends here

giving this to the compiler i get this:

/usr/lib/gcc/i586-mandrake-linux-gnu/3.4.1/../../../../include/c++/
3.4.1/bits/stl_iterator_ba se_types.h: In instantiation of
`std::iterator_ traits<MyCont:: Iterator>':

^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^ ^^^^^^^^
That's the implicit specialisation the compiler attempts. And fails.

[..]

Do i have to provide my implementation with these types?

Yes, if you want your code to compile.

If yes, what about iterator_catego y?

Yes

What about the simple char* parameters given to the std::copy
algorithm ? why are they ok?

Yes, because the standard library most likely already contains the
specialisation of 'itetator_trait s' for built-in pointer types.

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[Roland Pibinger]

On Fri, 15 Jun 2007 07:18:55 -0700, vasili wrote:

>I have a simple issue.

If it were simple it wouldn't be C++.

>I defined a custom container, that encloses a std::list, which in turn
holds objects that are a simple abstraction of a six position array.

AFAICS, you want to iterate over the list _and_ the 'six position
array' with a new iterator (not just re-use the
list<SixBytes>: :iterator). In this case you need to define your own
iterator with all required typedefs and functions. Look for 'custom
STL iterator', e.g.
http://www.stanford.edu/class/cs107l...-Iterators.pdf
http://www.oonumerics.org/tmpw00/becker.html
--
Roland Pibinger
"The best software is simple, elegant, and full of drama" - Grady Booch

[James Kanze]

On Jun 15, 5:02 pm, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:

vasili wrote:

I have a simple issue.

:-)

It takes a complicated language to solve complicated
problems:-).

I defined a custom container, that encloses a std::list, which in turn
holds objects that are a simple abstraction of a six position array.

Now, i would like to serialize the whole newly-defined container, in
order to copy the contents to another array. So i thought to define an
iterator which represented a "pointer" to the container's data. But,
when i feed these iterators to std::copy the compiler complains about
a lot of types which are defined when a std::iterator is instanced.

Since you want to use standard algorithm, it _may_ require that you
specialize 'iterator_trait s' for your custom iterator.

He must do something to ensure that iterator_traits <Iterator>
contains the proper typedefs. The generic implementation of
this template supposes that there are corresponding typedef's in
the Iterator class; the standard library also contains a partial
specialization for pointers (since pointers obviously don't
contain the necessary typedef's). He can thus either provide a
custom specialization, with the necessary typedef's, or put the
typedef's in his class. The latter is the classical solution,
and the standard offers a class template, std::iterator, to help
here. All he has to do is have his iterator derive (publicly)
from the appropriate instantiation of std::iterator, and it
should suffice.

--
James Kanze (Gabi Software) email: ja*********@gma il.com
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