Custom STL iterator implementation error - c++

Question

I'm attempting to write, for a proof of concept, an STL like iterator that will iterate over a string by skipping every other character. However, I'm encountering many different strange C++ errors I'm having trouble understanding.
My code is:
#include<iostream>
#include<string>
using std::string;
class TestIterator : public std::iterator<std::forward_iterator_tag, string> {
private:
string::iterator _it;
public:
TestIterator() {}
string& operator++() {
return _it + 2;
}
string& operator=(const string& other) {
_it = other;
}
};
int main(int argc, char** argv) {
string a("123045678");
TestIterator start = a.begin();
TestIterator end = a.end();
string b(start, end);
std::cout << b << std::endl;
return 0;
}
When I compile it I get:
% g++ -std=gnu++0x test.cpp -o test
test.cpp: In member function ‘std::string& TestIterator::operator++()’:
test.cpp:14:16: error: invalid initialization of non-const reference of type ‘std::string& {aka std::basic_string<char>&}’ from an rvalue of type ‘__gnu_cxx::__normal_iterator<char*, std::basic_string<char> >’
return _it + 2;
^
test.cpp: In member function ‘std::string& TestIterator::operator=(const string&)’:
test.cpp:18:9: error: no match for ‘operator=’ (operand types are ‘std::basic_string<char>::iterator {aka __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >}’ and ‘const string {aka const std::basic_string<char>}’)
_it = other;
^
test.cpp:18:9: note: candidates are:
In file included from /usr/include/c++/4.8/bits/stl_algobase.h:67:0,
from /usr/include/c++/4.8/bits/char_traits.h:39,
from /usr/include/c++/4.8/ios:40,
from /usr/include/c++/4.8/ostream:38,
from /usr/include/c++/4.8/iostream:39,
from test.cpp:1:
/usr/include/c++/4.8/bits/stl_iterator.h:708:11: note: __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >& __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >::operator=(const __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >&)
class __normal_iterator
^
/usr/include/c++/4.8/bits/stl_iterator.h:708:11: note: no known conversion for argument 1 from ‘const string {aka const std::basic_string<char>}’ to ‘const __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >&’
/usr/include/c++/4.8/bits/stl_iterator.h:708:11: note: __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >& __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >::operator=(__gnu_cxx::__normal_iterator<char*, std::basic_string<char> >&&)
/usr/include/c++/4.8/bits/stl_iterator.h:708:11: note: no known conversion for argument 1 from ‘const string {aka const std::basic_string<char>}’ to ‘__gnu_cxx::__normal_iterator<char*, std::basic_string<char> >&&’
test.cpp: In function ‘int main(int, char**)’:
test.cpp:24:32: error: conversion from ‘std::basic_string<char>::iterator {aka __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >}’ to non-scalar type ‘TestIterator’ requested
TestIterator start = a.begin();
^
test.cpp:25:28: error: conversion from ‘std::basic_string<char>::iterator {aka __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >}’ to non-scalar type ‘TestIterator’ requested
TestIterator end = a.end();
^
In file included from /usr/include/c++/4.8/string:53:0,
from /usr/include/c++/4.8/bits/locale_classes.h:40,
from /usr/include/c++/4.8/bits/ios_base.h:41,
from /usr/include/c++/4.8/ios:42,
from /usr/include/c++/4.8/ostream:38,
from /usr/include/c++/4.8/iostream:39,
from test.cpp:1:
/usr/include/c++/4.8/bits/basic_string.tcc: In instantiation of ‘static _CharT* std::basic_string<_CharT, _Traits, _Alloc>::_S_construct(_InIterator, _InIterator, const _Alloc&, std::forward_iterator_tag) [with _FwdIterator = TestIterator; _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’:
/usr/include/c++/4.8/bits/basic_string.h:1725:56: required from ‘static _CharT* std::basic_string<_CharT, _Traits, _Alloc>::_S_construct_aux(_InIterator, _InIterator, const _Alloc&, std::__false_type) [with _InIterator = TestIterator; _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’
/usr/include/c++/4.8/bits/basic_string.h:1746:58: required from ‘static _CharT* std::basic_string<_CharT, _Traits, _Alloc>::_S_construct(_InIterator, _InIterator, const _Alloc&) [with _InIterator = TestIterator; _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’
/usr/include/c++/4.8/bits/basic_string.tcc:229:49: required from ‘std::basic_string<_CharT, _Traits, _Alloc>::basic_string(_InputIterator, _InputIterator, const _Alloc&) [with _InputIterator = TestIterator; _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’
test.cpp:26:22: required from here
/usr/include/c++/4.8/bits/basic_string.tcc:128:12: error: no match for ‘operator==’ (operand types are ‘TestIterator’ and ‘TestIterator’)
if (__beg == __end && __a == _Alloc())
^
/usr/include/c++/4.8/bits/basic_string.tcc:128:12: note: candidates are:
...
Is there any other way to do something like this (that is, provide a new iterator over the string class) without storing iterators inside my class that will act as an iterator? I've piece together this class from many different code listings online, so the semantics may not be perfect.
Any help on resolving this iterator issue would be much appreciated.

Answer 1

STL iterators are created by container objects, not constructed from the containers themselves:
int main(int argc, char** argv) {
string a("abcdefghijk");
TestIterator start = a.begin(); //<-----------
TestIterator stop = a.end(); //<-----------
string b(start, stop); //<-----------
std::cout << b << std::endl;
return 0;
}
Therefore, your new iterators must be able to be constructed from string::iterator which string::begin() returns.
STL iterators do not have begin() and end() themselves; that's the job of the container.
operator++() should return a reference to the iterator being incremented, not a reference to the container it points to.