Re: user-defined iterator

"Victor Bazarov" <>
Fri, 15 Jun 2007 11:02:29 -0400
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_traits' 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<SixBytes> m_list;
       class Iterator{
               const MyCont& m_cont;
               int m_index;
               Iterator(const MyCont& cnt, int index=0):m_cont(cnt),
               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);
                       return ret;
               Iterator& operator++(){//prefix?
                       return *this;
               char operator*(){
                       //...return the byte that corresponds to the
position m_

       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_back(SixBytes());

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

giving this to the compiler i get this:

3.4.1/bits/stl_iterator_base_types.h: In instantiation of

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_categoy?


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_traits' for built-in pointer types.

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