Declaring the value_type when implementing an iterator

Question

I am implementing an iterator for a custom container in c++11

I get the following error:

no type named 'value_type' in 'struct std::iterator_traits<PixBuffer<float>::seq_read_iterator>'
    typedef typename iterator_traits<_OI>::value_type _ValueTypeO;

I have read before the answer here: How to implement an STL-style iterator and avoid common pitfalls?

It states that I can:

specialize std::iterator_traits<youriterator> : This is not the preferred method according to the author, for code readability, as this type declaration is not nested in the class
Put the same typedefs in the iterator itself : I did this but I still have the error above
Inherit from std::iterator : will this fix my problem, and what is the proper way to use this method?

This interator works as a back_inserter. The current class definition is:

template<typename T>
class PixBuffer
{
public:
    friend class seq_read_iterator;

    /* container class implementation removed from here */

public:
    class seq_read_iterator
    {
    public:
        typedef T value_type;

        seq_read_iterator(PixBuffer & pb);
        seq_read_iterator & operator*(T val);
        seq_read_iterator & operator++();
        seq_read_iterator operator++(int);
        seq_read_iterator & operator=(T val);

        /* iterator class implementation removed from here */
    };


    seq_read_iterator seqReadIterator() { return seq_read_iterator(*this); }
};

Thanks in advance

"_Put the same typedefs in the iterator itself : I did this but I still have the error above_". It seems like you did it wrongly. Show relevant parts an maybe we could help. — Revolver_Ocelot, Jan 22 '16 at 10:21
Regarding "I thought this was forbidden by the standard": `[namespace.std]/1` "A program may add a template specialization for any standard library template to namespace `std` only if the declaration depends on a user-defined type and the specialization meets the standard library requirements for the original template and is not explicitly prohibited." — TartanLlama, Jan 22 '16 at 10:23
@TartanLlama : you are right, this is valid. I would prefer another method for better code organization, though, if possible. — galinette, Jan 22 '16 at 10:27
The current trend seems to be option 2, add the typedefs to the iterator. That's what the standard document does in later drafts. — Bo Persson, Jan 22 '16 at 10:35

score 2 · Accepted Answer · answered Jan 22 '16 at 10:37

All 3 posted ways to make iterator work are legal.

Preferred way is to add required type aliases to the class itself. Inheriting from std::iterator is just a quick way to do that.

Specializing std::iterator_traits is usually used, when you already have a non-conforming iterator, which code you cannot change.

Your code does not work because you did not add all 5 required type aliases. Before C++11 it would just explode and complain that it cannot instantiate iterator_traits. Now it just creates empty trait class:

http://en.cppreference.com/w/cpp/iterator/iterator_traits

If Iterator does not have the five member types difference_type, value_type, pointer, reference, and iterator_category, then this template has no member types (std::iterator_traits is SFINAE-friendly)

Thanks! The gcc error message is quite misleading then – galinette Jan 22 '16 at 11:13 — galinette, Jan 22 '16 at 11:13

Declaring the value_type when implementing an iterator

1 Answers1