adding an example that uses extract

[lemire]

Suppose that you want to do something of the sort...

You have a function (@the-moisrex) that can be called like so...

extract(to("key1", t.key1), to("key2", t.key2));

given...

struct my_struct {
int key1;
double key2;
}

See simdjson/simdjson#2247

Right?

Now can I do it by reflection?

That is. I have my_struct x and I want to automatically, at compile time, call...

extract(to("key1", t.key1), to("key2", t.key2));

... without having to write "key1" and so forth.

See #29

The answer is positive.

All I need is a helper function which (by reflection) turns any struct into a tuple (struct_to_tuple)... that is, something that is effectively capable of the following...

std::tuple<to<int>,to<double>> struct_to_tuple(my_struct s);

... and then I can call std::apply...

my_struct x;
std::apply([](auto&&... xs){ extract(xs...);}, struct_to_tuple(x));

[the-moisrex]

Yes this is a good idea, and we could add a .extract that accepts tuple (we already are converting it to tuple in the implementation for MSVC) and make it official, or

... or, we could directly call .extract without constructing a tuple:

namespace simdjson {

// ...

template <typename T>
consteval auto is_convertible_type() {
  for (std::meta::info field : nonstatic_data_members_of(^T)) {
    if (deserializable(type_of(field))) return false;
  }
  return true;
}

template <typename T>
  requires (is_convertible_type<T>()) // all of its fields MUST be deserializable as well
constexpr error_code tag_invoke(deserialize_tag, auto& value, T& out) {
  return [: expand_all(nonstatic_data_members_of(^T)) :] >> [&]<auto... members>{
    simdjson::ondemand::object obj;
    if (auto error = value.get_object().get(obj); error) {
      return error;
    }
    return obj.extract(to(std::meta::identifier_of(members), out.[:members:])...);
  };
}

}

[lemire]

I have some concerns regarding the performance of extract, see simdjson/simdjson#2274