Does std::ranges::to allow converting to a std::map?

Question

In the std::ranges::to paper wg21.link/p1206 ths overview section has the following

//Supports converting associative container to sequence containers
auto f = ranges::to<vector>(m);

However I can't find where the detail of converting to a std::map is descibed in the rest of the paper. I tried range-v3 and Sy Brand's implementation of ranges::to in https://github.com/TartanLlama/ranges and neither of them compiles code converting a range to a std::map. So is this just missing from those libraries or is converting to a std::map not really intended to be allowed?

Answer 1

Does std::ranges::to allow converting to a std::map?

Yes.

I tried range-v3 and Sy Brand's implementation of ranges::to in https://github.com/TartanLlama/ranges and neither of them compiles code converting a range to a std::map

I haven't tried Sy's implementation, and it looks like range-v3's implementation is weirdly broken:

#include <map>
#include <vector>
#include <range/v3/range/conversion.hpp>

int main() {
    std::vector<std::pair<int, int>> v = {{1, 2}, {3, 4}};

    // this works (explicit)
    // m1 is a std::map<int, int>
    auto m1 = ranges::to<std::map<int, int>>(v);

    // this works (deduced with a pipe)
    // m2 is a std::map<int, int>
    auto m2 = v | ranges::to<std::map>();

    // but this does not (deduced, direct call)
    auto m3 = ranges::to<std::map>(v);
}

The issue is that the class template direct call version in range-v3 for some reason specifically tries to instantiate C<range_value_t<R>> (which would be std::map<std::pair<int, int>> in this case, clearly wrong) even though there is a metafunction here already that does the right thing and would deduce std::map<int, int> (used by the pipe version).

The ranges::to in the standard library specifies these two to do the same (correct) thing, so this will work in C++23. This is just an easy bug fix in range-v3.