C++: no match for operator< when trying to iterate through boost::unordered_map

Question

I have the following code:

boost::unordered_map<std::string, int> map;
map["hello"]++;
map["world"]++;

for(boost::unordered_map<std::string, int>::iterator it = map.begin(); it < map.end(); it++){
    cout << map[it->first];
}

and when I try to compile I get the following error but have no idea why?

error: no match for ‘operator<’ in ‘it < map.boost::unordered::unordered_map<K, T, H, P, A>::end [with K = std::basic_string<char>, T = int, H = boost::hash<std::basic_string<char> >, P = std::equal_to<std::basic_string<char> >, A = std::allocator<std::pair<const std::basic_string<char>, int> >, boost::unordered::unordered_map<K, T, H, P, A>::iterator = boost::unordered::iterator_detail::iterator<boost::unordered::detail::ptr_node<std::pair<const std::basic_string<char>, int> >*, std::pair<const std::basic_string<char>, int> >]()

score 5 · Accepted Answer · answered Oct 24 '12 at 12:12

5

Try:

it != map.end()

as the for loop termination condition (instead of it < map.end()).

answered Oct 24 '12 at 12:12

Reunanen

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@aly As the compiler said, because no `operator<` is defined :) – sje397 Oct 24 '12 at 12:13
1

... as the map is, well, unordered. :) – Reunanen Oct 24 '12 at 12:14
Fair points, I will accept once SO lets me (there is a min time to wait) – Aly Oct 24 '12 at 12:15

score 4 · Answer 2 · answered Oct 24 '12 at 12:13

4

In case of iterators you must use != operator:

boost::unordered_map<std::string, int>::iterator it = map.begin();
for(; it != map.end(); ++it){
    cout << map[it->first];
}

You cant use < because iterator points to memory, and you cant guarantee that memory is contiguous. That's why you MUST use != compare.

answered Oct 24 '12 at 12:13

Denis Ermolin

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The Vector iterator for example defines operator< so we can use this terminating condition. I guess it depends on how the iterator is implemented – Aly Oct 24 '12 at 12:16
Honestly didnt know about it but in other hand it's logical to use it for vector but i never seen this in use. Anyway looping always use != operator – Denis Ermolin Oct 24 '12 at 12:21
It is not anything about contiguous, the simple matter is that the standard declares that iterators should be comparable with != and == which are well defined as long as the iterators belong to the same collection and are in range (including one past the end). Where as the standard does not say operator< has to be supported between iterators, so boost didn't implement it in this case. – CashCow Oct 24 '12 at 12:26
Sure i said about memory to explain why iterator use != == for compare . Not special for boost – Denis Ermolin Oct 24 '12 at 12:28

C++: no match for operator< when trying to iterate through boost::unordered_map

2 Answers2