Count matching elements after splitting

Question

I want to combine std::views::split or std::views::lazy_split with std::ranges::count but I cannot get it to work.

This is what I currently have:

std::string setting;
std::string extension;

const auto count = std::range::count(setting | std::views::split(' '), extension);

And unfortunately the MSVC error is not really helpful:

error C3889: call to object of class type 'std::ranges::_Count_fn': no matching call operator found

How can I count the elements that match extension after splitting (without having to store the parts into a container)?

Answer 1

The subranges after the split cannot be compared with string, so the constraint is not satisfied.

You can use the projection to project them into string_views, which makes them comparable to strings:

std::string setting;
std::string extension;

const auto count = std::ranges::count(
    setting | std::views::split(' '), 
    extension,
    [](auto r) { return std::string_view(r.data(), r.size()); }
);

Or with views::transform:

const auto count = std::ranges::count(
    setting | std::views::split(' ')
            | std::views::transform(
                [](auto r) { return std::string_view(r.data(), r.size()); }),
    extension
);