what is the preferred way to expose custom STL-style iteration?

Question

(also see Is there a good way not to hand-write all twelve required Container functions for a custom type in C++? )

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For a class such as

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Answer 1

In order to create a valid iterator, you must ensure that std::iterator_traits is valid. This means you must set the iterator category among other things.

An iterator should implement iterator(), iterator(iterator&&), iterator(iterator const&), operator==, operator !=, operator++, operator++(int), operator*, operator=, and operator->. It's also a good idea to add operator< and operator+ if you can (you can't always, e.g. linked lists.)

template 
class foo
{
public:

  using value_type = T;
  class iterator 
  { 
  public:
    using value_type = foo::value_type;
    using iterator_category = std::random_access_iterator_tag;
    // or whatever type of iterator you have...
    using pointer = value_type*;
    using reference = value_type&;
    using difference_type = std::ptrdiff_t;

    // ... 
  };

  class const_iterator 
  {
    // ... 
  };

  iterator begin() { /*...*/ }
  iterator end() { /*...*/ }

  const_iterator cbegin() const { /*...*/ }
  const_iterator cend() const { /*...*/ }

  /* ... */
};

See: http://en.cppreference.com/w/cpp/iterator/iterator_traits for more information on what you need to make a valid iterator. (Note: You also need certain properties to be a valid "container", like .size())

Ideally you should use member functions for begin and end, but it's not required... you can also overload std::begin and std::end. If you don't know how to do that, I suggest you use member functions.

You should create begin() const and end() const, but it should be an alias for cbegin(), NEVER the same as begin()!