Splitting a string_view using ranges

Question

I have a string containing a sequence of numbers separated with the , character. To read values from the sequence into an array I created the following code the GCC 10 refuses to compile:

#include <ranges>
#include <string_view>
#include <charconv>
#include <array>

template<std::size_t Sz>
bool to_bytes(std::array<std::uint8_t, Sz> &data, std::string_view string) {
    auto target = data.rbegin();
    for (const auto octet : string | std::views::split('.')) {
        if (target == data.rend()) {
            return false;
        }

        const auto octet_begin = octet.data();
        const auto octet_end = octet_begin + octet.size();
        const auto error = std::error_code(std::from_chars(octet_begin, octet_end, *target).ec);
        if (error) {
            return false;
        }

        ++target;
    }

    return target == data.rend();
}

Briefly speaking the compiler complains that there are no data() and size() methods available for the type of the octet variable. Do I misunderstand that type of octet should meet the contiguous_range criteria as the string_view does? It seems to be argumentative from my POV.

Answer 1

After the adoption of P2210 as a defect report against C++20, views::split can now preserve contiguity, so the above code is now valid and compiles on gcc trunk.

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Do I misunderstand that type of octet should meet the contiguous_range criteria as the string_view does?

Yes. octet is not a contiguous_range. The inner range of a split_view is never stronger than a forward_range - it is only ever a forward_range or an input_range.

This makes views::split exceedingly awkward to use for any kind of non-trivial parsing - precisely because, as you demonstrate in the question, you can't use things like from_chars (or scanf or ... ) unless you then manually produce a contiguous range out of it.