How to correctly implement custom iterators and const_iterators?

Question

Choose type of iterator which fits your container: input, output, forward etc.
Use base iterator classes from standard library. For example, std::iterator with random_access_iterator_tag.These base classes define all type definitions required by STL and do other work.
To avoid code duplication iterator class should be a template class and be parametrized by “value type”, “pointer type”, “reference type” or all of them (depends on implementation). For example:
```
// iterator class is parametrized by pointer type
template <typename PointerType> class MyIterator {
    // iterator class definition goes here
};

typedef MyIterator<int*> iterator_type;
typedef MyIterator<const int*> const_iterator_type;
```
Notice iterator_type and const_iterator_type type definitions: they are types for your non-const and const iterators.