How to implement an STL-style iterator and avoid common pitfalls?

Question

I made a collection for which I want to provide an STL-style, random-access iterator. I was searching around for an example implementation of an iterator but I didn\'t find

Answer 1

And now a keys iterator for range-based for loop.

template
class keys_it
{
    typename C::const_iterator it_;
public:
    using key_type        = typename C::key_type;
    using pointer         = typename C::key_type*;
    using difference_type = std::ptrdiff_t;

    keys_it(const typename C::const_iterator & it) : it_(it) {}

    keys_it         operator++(int               ) /* postfix */ { return it_++         ; }
    keys_it&        operator++(                  ) /*  prefix */ { ++it_; return *this  ; }
    const key_type& operator* (                  ) const         { return it_->first    ; }
    const key_type& operator->(                  ) const         { return it_->first    ; }
    keys_it         operator+ (difference_type v ) const         { return it_ + v       ; }
    bool            operator==(const keys_it& rhs) const         { return it_ == rhs.it_; }
    bool            operator!=(const keys_it& rhs) const         { return it_ != rhs.it_; }
};

template
class keys_impl
{
    const C & c;
public:
    keys_impl(const C & container) : c(container) {}
    const keys_it begin() const { return keys_it(std::begin(c)); }
    const keys_it end  () const { return keys_it(std::end  (c)); }
};

template
keys_impl keys(const C & container) { return keys_impl(container); }

Usage:

std::map my_map;
// fill my_map
for (const std::string & k : keys(my_map))
{
    // do things
}

That's what i was looking for. But nobody had it, it seems.

You get my OCD code alignment as a bonus.

As an exercise, write your own for values(my_map)