How do I write a range pipeline that uses temporary containers?

Question

I have a third-party function with this signature:

std::vector f(T t);

I also have an existing potentially infinite range (of the

Answer 1

Edited

Apparently, the code below violates the rule that views cannot own data they refer to. (However, I don't know if it's strictly forbidden to write something like this.)

I use ranges::view_facade to create a custom view. It holds a vector returned by f (one at a time), changing it to a range. This makes it possible to use view::join on a range of such ranges. Certainly, we can't have a random or bidirectional access to elements (but view::join itself degrades a range to an Input range), nor can we assign to them.

I copied struct MyRange from Eric Niebler's repository modifying it slightly.

#include 
#include 

using namespace ranges;

std::vector f(int i) {
    return std::vector(static_cast(i), i);
}

template
struct MyRange: ranges::view_facade> {
private:
    friend struct ranges::range_access;
    std::vector data;
    struct cursor {
    private:
        typename std::vector::const_iterator iter;
    public:
        cursor() = default;
        cursor(typename std::vector::const_iterator it) : iter(it) {}
        T const & get() const { return *iter; }
        bool equal(cursor const &that) const { return iter == that.iter; }
        void next() { ++iter; }
        // Don't need those for an InputRange:
        // void prev() { --iter; }
        // std::ptrdiff_t distance_to(cursor const &that) const { return that.iter - iter; }
        // void advance(std::ptrdiff_t n) { iter += n; }
    };
    cursor begin_cursor() const { return {data.begin()}; }
    cursor   end_cursor() const { return {data.end()}; }
public:
    MyRange() = default;
    explicit MyRange(const std::vector& v) : data(v) {}
    explicit MyRange(std::vector&& v) noexcept : data (std::move(v)) {}
};

template 
MyRange to_MyRange(std::vector && v) {
    return MyRange(std::forward>(v));
}


int main() {
    auto src = view::ints(1);        // infinite list

    auto rng = src | view::transform(f) | view::transform(to_MyRange) | view::join;

    for_each(rng | view::take(42), [](int i) {
        std::cout << i << ' ';
    });
}

// Output:
// 1 2 2 3 3 3 4 4 4 4 5 5 5 5 5 6 6 6 6 6 6 7 7 7 7 7 7 7 8 8 8 8 8 8 8 8 9 9 9 9 9 9 

Compiled with gcc 5.3.0.