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

So is this just missing from those libraries or is converting to a std::map not really intended to be allowed?

std::map to std::vector:

According to the description of [range.utility.conv.to], this

map<int, double> m;
auto f = ranges::to<vector>(m);

will invoke the following overload

template<template<class...> class C, input_­range R, class... Args>
  constexpr auto to(R&& r, Args&&... args); 

Let DEDUCE_­EXPR be defined as follows:

• C(declval<R>(), declval<Args>()...) if that is a valid expression,
• otherwise, C(from_­range, declval<R>(), declval<Args>()...) if that is a valid expression,
• otherwise, C(declval<input-iterator>(), declval<input-iterator>(), declval<Args>()...) if that is a valid expression,
• otherwise, the program is ill-formed.

Returns: to<decltype(DEDUCE_­EXPR)>(std​::​forward<R>(r), std​::​forward<Args>(args)...).

Where C is vector, R is map<int, double>&, and Args... is empty parameter pack. Note that C++23 introduces the following range version constructor for vector

template<container-compatible-range<T> R>
  constexpr vector(from_range_t, R&& rg, const Allocator& = Allocator());

Effects: Constructs a vector object with the elements of the range rg, using the specified allocator.

and the following CTAD

template<ranges::input_­range R, class Allocator = allocator<ranges::range_value_t<R>>>
  vector(from_range_t, R&&, Allocator = Allocator())
    -> vector<ranges::range_value_t<R>, Allocator>;

So C(from_range, declval<R>()) is a valid expression, and the type of DEDUCE_EXPR will be vector<pair<const int, double>>, which will further invoke

to<vector<pair<const int, double>>>(m);

which will construct a vector with value_type of pair<const int, double> through the range version constructor.

So ranges::to<vector>(m) in C++23 is basically equivalent to

map<int, double> m;
vector f(m.begin(), m.end());

---

The reason range-v3 fails is that its internal implementation detects that vector<pair<const int, double>> is reservable, so it will first default construct the vector and call v.reserve() to pre-allocate the memory, and then copy the map by calling v.assign(), but since pair<const int, double> is not copy assignable, so compilation fails.

_{I suspect this is an implementation bug of range-v3, since it would compile if the vector's reserve() function didn't exist, and this optimized overload doesn't seem to constrain that the value_type must be copy-assignable.}

std::vector to std::map:

And for the following

auto g = ranges::to<map>(f);

Since std::map also has the following constructors and corresponding CATD in in C++23

template<container-compatible-range<value_type> R>
  map(from_range_t, R&& rg, const Compare& comp = Compare(), const Allocator& = Allocator());

So following the same rules of the game, we will get a map<int, double>.

Answer 2

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.