static_cast literal 0 to other types in STL

Question

Which type can 0 be cast to? pointers, numeric vars. Any others? Will the following cast be safe?

ps: an excerpt from STL implementation of iterator

template <class Iterator>
inline typename iterator_traits<Iterator>::difference_type*
distance_type(const Iterator&) {
  return static_cast<typename iterator_traits<Iterator>::difference_type*>(0);
}

template <class Iterator>
inline typename iterator_traits<Iterator>::value_type*
value_type(const Iterator&) {
  return static_cast<typename iterator_traits<Iterator>::value_type*>(0);
}

Well, it will certainly not if `value_type` is chosen accordingly. `0` can be cast to pointers and numerics, that's it. — filmor, Sep 10 '13 at 08:18
Both those casts are to pointers, and pointers can be NULL, so yes. — Jonathan Potter, Sep 10 '13 at 08:21
@zoujyjs: More precisely: Does the specialization for your own case about which you are worried compile? — Kerrek SB, Sep 11 '13 at 09:43

score 1 · Accepted Answer · answered Sep 10 '13 at 08:18

All kinds of pointers including pointer-to-function, pointer-to-member, pointer-to-member-function; arithmetic types; anything with a 1-arg int constructor; anything with a 1-arg constructor taking a type that 0 can be implicitly converted to. Probably something else I haven't thought of.

Assuming that iterator_traits resolves to std::iterator_traits, then typename iterator_traits<Iterator>::difference_type* is certainly a pointer-to-object type, except perhaps in a program that has an incorrect specialization of iterator_traits. So yes, the cast works.

If you messed up and defined difference_type or value_type as a reference type, or not a type at all, then you'd have problems with that code.

`void`, too. I think "arithmetic" plus `void` together are the _fundamental types_. — MSalters, Sep 10 '13 at 12:29

static_cast literal 0 to other types in STL

1 Answers1