How to make transform view for the nested structure data?

Question

How can I map nested structure data with std::views::transform?

In the code below the transformation for as_v1 works just fine and allows copying the data.

How can I do the same for the data in the nested structure NS?

#include <algorithm>
#include <iostream>
#include <ranges>
#include <vector>

struct S
{
    float v1;
    float v2;

    struct SN {
        float v3;
    };

    SN sn;
};

int main() {
    std::vector<S> aos{ { 1.0f, 2.0f, { 3.0f } } };
    std::vector<float> soa(aos.size());

    // Would work just fine for the:
    // auto as_v1 = aos | std::views::transform(&S::v1);

    // How could I manage a nested structure's member:
    // This, of course, doesn't compile because of types inconsistency
    auto as_v3 = aos | std::views::transform(offsetof(S, sn.v3));

    std::ranges::copy(as_v3, soa.begin());
    std::cout << "soa value:" << soa[0] << std::endl;
}

I understand that S::SN::v3 won't work since we need the offset of the member and it will be relative to the sn object and it seems that offsetof should do the job, but I can't figure out how to make type conversion.

Is is doable?

Demo

Answer 1

You are trying to access the member of a member.

This will do it.

auto as_v3 = aos | std::views::transform(&S::sn) | std::views::transform(&S::SN::v3);