Add with_*_t alias templates to create basic_json types with changed template parameters

[nlohmann]

Adds member alias templates to basic_json that create a basic_json type with one of its template parameters replaced, so you don't have to spell out all eleven:

using my_json = nlohmann::json::with_string_t<my_string>;
// instead of
using my_json = nlohmann::basic_json<std::map, std::vector, my_string, bool, std::int64_t, std::uint64_t,
                                     double, std::allocator, nlohmann::adl_serializer, std::vector<std::uint8_t>>;

Aliases: with_object_t, with_array_t, with_string_t, with_boolean_t, with_integers_t<I, U>, with_float_t, with_allocator_t, with_json_serializer_t, with_binary_t, with_base_class_t. Together they cover all template parameters. They are members, so they also work from ordered_json or any other specialization and keep its other parameters.

This takes over #3898 by @barcode, which went stale. Their commit is included with them as author. On top of it, following the review in #3898:

• with_changed_*_t is renamed to the shorter with_*_t.
• The separate signed and unsigned integer aliases are merged into with_integers_t<I, U> (@gregmarr's suggestion), because the two are usually changed together.
• New documentation page api/basic_json/with_t.md with an example, linked from the basic_json index and mkdocs.yml.
• std::is_same tests for every alias in unit-udt.cpp, including one that checks ordered_json::with_string_t keeps ordered_map.
• Tests and examples that spelled out the full parameter list now use the aliases.

Supersedes #3898. Thanks @barcode!

Public API

Only additions: ten new public member alias templates on basic_json. Nothing existing changes. No breaking changes.

🤖 Generated with Claude Code

[gregmarr]

Seems weird that this isn't indented. Intentional?

[nlohmann]

Not intentional, thanks — the entry ended up at the top level of nav. Fixed in 8ab8fa9.

(Written by Claude Code.)

[gregmarr]

Because this is a nested template type, these can chain. This is not obvious and should probably be called out with an example. Also, that it can start with ordered_json as well.

  using my_json = nlohmann::json::with_integers_t<int, unsigned int>::with_float_t<float>;

I did verify that nlohmann::json::with_object_t<nlohmann::ordered_map> and nlohmann::ordered_json are the same type, as least as far as typeid().name() is concerned. That's what I would expect.

[nlohmann]

Good point, thanks for checking. 8ab8fa9 adds a paragraph to with_t.md explaining that the aliases are members of every basic_json specialization (including ordered_json), so they can be chained:

using my_json = nlohmann::json::with_integers_t<int, unsigned int>::with_float_t<float>;

It also notes that the order of chained aliases does not matter and that json::with_object_t<nlohmann::ordered_map> is ordered_json. These are now checked with std::is_same in unit-udt.cpp (both chaining orders, and json ↔ ordered_json in both directions).

(Written by Claude Code.)

[gregmarr]

@nlohmann Would you want to update the tests and examples to use this new functionality? If so, would you prefer nlohmann::basic_json::with_... or nlohmann::json::with_... and would you want to change ...basic_json<my_object_type to ...json::with_object_t<my_object_type>.

In looking at this, I found a few places where only integer_t was changed, and a few places that all three numbers were changed. There might be a case to have with_integer_t and with_numbers_t as well.

[nlohmann]

Thanks! Yes, done in #5787. It uses nlohmann::json::with_… (basic_json would need <>), and that includes the short basic_json<my_object_type> forms. I left out with_numbers_t / with_integer_t for now: the five int32/uint32/float sites read fine as json::with_integers_t<std::int32_t, std::uint32_t>::with_float_t<float>, and the signed-only ones as with_integers_t<std::int32_t, std::uint64_t>. If more call sites show up, we can still add a with_numbers_t.

(Written by Claude Code on behalf of @nlohmann.)

[gregmarr]

I left out with_numbers_t / with_integer_t for now

Sounds good.