More use of move semantics in deserialization

[SylvainCorlay]

Short version

For certain cases, where copying things is an issue (as the one described below), I would like to favor examples similar to move_only_type over the creation of a from_json function, but specializations of adl_serializer does not seem to work with containers of types for which the specialization exist.

Ideally, I would like to have a "return by value" version of from_json. The STL container of those types would be built by emplacing the values returned by from_json into the container.

Long version / context

We currently use your library (vendoring the json header for now) in xeus, a native C++ implementation of the Jupyter protocol.

In this work, some of the classes that we deserialize from json have a value semantics and implement the RAII pattern. Basically, constructor and destructors amount to acquiring and freeing the resources. Any non-move copy acquires a new version of the resource (with a different id). Move constructor and assignment transfers the resource to the moved-to object.

This is a context in which being able to move things to a container would make a lot of sense...

[theodelrieu]

Hello, do you have a small example that fails to compile?

If I understand you correctly, the bug is in the library-provided conversions?

[SylvainCorlay]

I just want to avoid copies when filling the e.g. an std::vector<my_type>, using e.g. emplace_back.

[theodelrieu]

The following code works fine, there is no copy involved:

#include <iostream>
#include <json.hpp>

struct test {
  test() = default;
  test(test const &) = delete;
  test(test &&) { std::cout << "move" << std::endl; };

  test &operator=(test const &) = delete;
  test &operator=(test &&) = default;
  ~test() = default;
};

namespace nlohmann {
template <> struct adl_serializer<::test> {
  static ::test from_json(json const &) { return {}; }
};
}

int main(int argc, char const *argv[]) {
  auto j = nlohmann::json{1, 2, 3};
  auto v = j.get<std::vector<test>>();
  v.emplace_back(nlohmann::json(1));
}

I think I misunderstand your issue, feel free to post some code :)

[SylvainCorlay]

Hum, it seems that you have well understood that issue, and your snippet works well. Issue must be on my side (I am tracking assignments and seeing a few). Closing, thanks for your reply.