c++ - Check whether iterator belongs to a list

Question

Is there any way to check whether a given iterator belongs to a given list in C++?

Answer 1

The obvious but invalid approach

You can't simply iterate through the list, comparing each iterator value to your "candidate".

The C++03 Standard is vague about the validity of == applied to iterators from different containers (Mankarse's comment on Nawaz's answer links http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2948.html#446), some compilers (eg. VC++2005 debug mode) warn if you do so, but despite all that it may actually work reliably depending on your compiler/libraries - check its documentation if you don't care about portability.

The C++11 Standard is very explicit, you can't compare iterators to different containers:

§ 24.2.5 The domain of == for forward iterators is that of iterators over the same underlying sequence.

So, the answers to this question that rely on operator== are questionable now, and invalid in future.

An oft-valid approach

What you can do is iterate along the list, comparing the address of the elements (i.e. &*i) to the address of the object to which your other iterate points.

• Mankarse's comment cautions that this might not work as intended for objects providing their own operator&. You could work around this using std::addressof, or for C++03 boost's version

• Martin's comment mentions that you have to assume the candidate iterator that you're testing list membership for is safely dereferenceable - i.e. not equal to an end() iterator on the container from which it came. As Steve points out - that's a pretty reasonable precondition and shouldn't surprise anyone.

(This is fine for all Standard containers as stored elements never have the same address, but more generally user-defined containers could allow non-equal iterators to address the same value object (e.g. supporting cycles or a "flyweight pattern" style optimisation), in which case this approach would fail. Still, if you write such a container you're probably in a position to design for safe iterator comparison.)

Implementation:

template <class IteratorA, class IteratorB, class IteratorC>
inline bool range_contains(IteratorA from, const IteratorB& end,
                           const IteratorC& candidate)
{
    while (from != end)
        if (&*from++ == &*candidate)
            return true;
    return false;
}

Notes:

• This adopts the Standard library approach of accepting a range of iterator positions to search.
• The types of each iterator are allowed to vary, as there are portability issues, e.g. containers where begin() returns an iterator but end() returns a const_iterator.
• Iterators other than from are taken by const reference, as iterators can sometimes be non-trivial objects (i.e. too large to fit in a register, relatively expensive to copy). from is needed by value as it will be incremented through the range.