Conversion operator vs constructor from given type. Which is preferable?

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?


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.


As a general rule:

There is almost <strong>never</strong> a good reason to provide <strong>implicit conversion operators</strong>; it is always <strong>preferable</strong> to use <strong>conversion constructors</strong>. With implicit conversion operators you will seldom find them invoked when you don't expect and create unnecessary confusions.

<strong>conversion constructors</strong> is also a better way since it lets you adheres you to the <strong>Principle of least Astonishment.</strong>

The only common exception to this rule is provide a conversion to a boolean type, so that objects (e.g., smart pointers) can be used in boolean contexts.


Some implementations let iterator inherit from const_iterator, and get the conversion for free.


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