How to pass a smart iterator to a function that accepts a classic iterator?

Question

I'm trying to familiarize with the ranges-v3 library, that will be part of the C++20 standard. To do so, I'm trying to refactor some toy code by replacing (where suitable) classic iterators and algorithms with the new available constructs. In this particular example, I can't figure out how to pass the returned iterator of a call to ranges::min_element (which replaced a call to std::min_element) to another function of mine which accepts a classic iterator as parameter.

I've searched inside the documentation of the library looking for some sort of functions like smartIt2classicIt with no success.

Here is a minimal example

void f(std::vector<int>& v, std::vector<int>::iterator it); // old function that I want to reuse
auto predicate = [](int i){ return true; }; // check function

std::vector<int> v;
// auto min_el = std::min_element(...); // old code
auto filtered_range = v | ranges::view::filter(predicate); // to avoid a dangling iterator
auto min_el = ranges::min_element(filtered_range);

f(v, min_el); // pass min_el to f: doesn't compile with the new code

At first I expected that the result of ranges::min_element was implicitly convertible to a classic iterator, but I was wrong: the compiler returns a long error saying that cannot convert a ranges::basic_iterator bla bla bla to a std::vector bla bla bla iterator. Given this error I deduce that ranges::min_element indeed returns some sort of iterator, but how to use it the old way?

I see three possible solutions:

1. Change the way min_el is passed to f
2. Change the type of the second argument of f (possibly in a backward-compatible way)
3. Change both things

but I can't figure out how to implement any of them. Maybe there is another solution? I also see another possible source of troubles, since the returned iterator probably refers to filtered_range instead of v... Any help is welcome!

Answer 1

You can convert an adapted iterator to its base iterator via the base() member function.

For example:

std::vector<int>::const_iterator foo(std::vector<int> const& v) {
    auto filtered = v | ranges::view::filter([](int i){return i > 5;});
    auto min_el = ranges::min_element(filtered);
    return min_el.base();
}

Note that this only removes one layer of wrapping, it doesn't drop to the bottom. So if you had another adapter then you'd need another base():

std::vector<int>::const_iterator foo(std::vector<int> const& v) {
    auto filtered = v | ranges::view::filter([](int i){return i > 5;})
                      | ranges::view::transform([](int i){ return i * i; });
    auto min_el = ranges::min_element(filtered);
    return min_el.base().base();
}