Is it possible to use range-based for loops with iterator ranges?

Question

Consider the usage with std::multimap, where I am given a range of iterators:

std::unordered_multimap<std::string, MyObject> mymap;

auto& range = mymap.equal_range("some_key");
for (auto& the_pair : range)
{
}

Now, the code above does not compile, but I am using it for demonstration purposes. Is it possible to use range-based for loops with a pair of iterators like this? I don't imagine it is possible directly, so I guess my question is really about whether or not there is an adapter class in STL for this use case. I could probably write my own but this seems like a common case.

Update:

If this is not a common case and STL does not provide such a proxy or adapter for this type of usage, what is required to implement them? I was thinking that I'd need to overload the free functions begin and end and also define an adapter iterator class for std::pair<T,U>. I'm just taking a conceptual stab at it, but please let me know! Thanks in advance everyone.

user4689757 · Accepted Answer · 2015-03-19T12:56:32.123

9

No, range-based for calls std::begin and std::end. There isn't any overloads for std::pair. At one point, this feature existed in a draft standard but was removed. You can write your own code. Stealing from sellibitze:

template<class Iter>
struct iter_pair_range : std::pair<Iter,Iter> {
    iter_pair_range(std::pair<Iter,Iter> const& x)
    : std::pair<Iter,Iter>(x)
    {}
    Iter begin() const {return this->first;}
    Iter end()   const {return this->second;}
};

template<class Iter>
inline iter_pair_range<Iter> as_range(std::pair<Iter,Iter> const& x)
{ return iter_pair_range<Iter>(x); }

int main() {
    std::unordered_multimap<std::string, MyObject> mymap;

    auto range = mymap.equal_range("some_key");
    for (auto& the_pair : as_range(range))
    {
    }
}

You can look at n2995 from 2009 which talks about adding ranged support to the standard library. Maybe you're better off using Boost.Range.

edited Mar 19 '15 at 12:56

answered Mar 19 '15 at 12:49

user4689757

106
3

Your `iter_pair_range` class is assuming both types in the pair are the same type which is not the case in my example. – void.pointer Mar 19 '15 at 12:51
@void.pointer what do you mean? Equal_range will always return a pair of the same type – Rado Mar 19 '15 at 12:54
1

@void.pointer The types in the pair are not the same, but the iterator-type is. It's a `multimap::const_iterator` on both occasions. In fact, a range-based for won't even work with `begin()` and `end()` returning different kinds of iterators. – JorenHeit Mar 19 '15 at 12:55
My apologies, I was looking at the wrong `pair` object! – void.pointer Mar 19 '15 at 12:57
Why was this removed from the standard? Anyone know? – Robinson Mar 19 '15 at 12:58
@Robinson Maybe because the proposal was written in MS Word. And to make it even worse: using the default color-theme ;-) – JorenHeit Mar 19 '15 at 13:04
@Robinson He provided a link near the top of his answer that might help answer your question. – void.pointer Mar 19 '15 at 13:05
Thanks I missed that. – Robinson Mar 19 '15 at 13:08

Rado · Answer 2 · 2015-03-19T12:58:03.960

3

The template-based answer is probably what you are looking for, but i would use a for_each instead with a lambda. It is cleaner and easier to digest.

edited Mar 19 '15 at 12:58

answered Mar 19 '15 at 12:49

Rado

8,634
7
31
44

I ended up using your recommendation, I didn't even consider `std::for_each`. It is much simpler. However, my question specifically asks for a solution in terms of range based for so I'm going to accept the answer with that in mind. Thanks for your reminder! – void.pointer Mar 19 '15 at 12:59

score 1 · Answer 3 · answered Apr 09 '21 at 11:00

1

With C++ 20 you can use std::ranges::subrange.

std::unordered_multimap<std::string, MyObject> mymap;

auto range = mymap.equal_range("some_key");
for (auto & [key, value] : std::ranges::subrange(range.first, range.second))
{
}

answered Apr 09 '21 at 11:00

Dumii

41
3

Is it possible to use range-based for loops with iterator ranges?

Update:

3 Answers3