Question

I’m defining iterator types for my container and of course I want iterator to be convertible to const_iterator. But I’m not sure which is better/preferable:

Conversion operator in iterator

class iterator
{
    operator const_iterator();
};

or non-explicit constructor in const_iterator

class iterator
{
    // implementation
    friend class iterator; // hard to avoid this
};

class const_iterator
{
    const_iterator(iterator const &);
};

Are there any guidelines which way is better?

Answer 1

You should write a casting operator only if it is possible to return the desired type without constructing a new object, or if its return is a simple data type. For example, if it were to return a const reference to a property within the class, then you should write it as a casting operator.

In all other cases, you should just write the appropriate constructor.