Add a type in the natvis template for detail::json_default_base

[sakuntalle]

With version 3.12.0 the underlying type for a nlohmann::json object has become nlohmann::json_abi_diag_v3_12_0::detail::json_default_base and this type does not have a Type definition in the corresponding natvis template and generated natvis file.

This PR updates the Natvis jinja template to add all the types derived from json_default_base. Also the template is rendered to produce the nlohmann_json.natvis

fixes #4972

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• If applicable, an existing issue is referenced.
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[coveralls]

coverage: 99.191%. remained the same
when pulling 78ede22 on sakuntalle:develop
into a6a92c1 on nlohmann:develop.

[gregmarr]

m_data is not a member of json_default_base. Why is this useful?

[sakuntalle]

Hey Greg!
I understand your concern and I'll have to admit I'm not versed into the library internal data structures.
But what I did notice with the VS version added in the linked issue can be seen in this screenshot

So VS sees the nlohmann:json object obj type as a json default_base object which most likely is just a polymorphic type. That's probably the reason why m_data can be found in the object.
And after I've added the natvis type I was suggesting in this PR VS did manage to show properly the content of it as in the screenshot below

[gregmarr]

That's interesting. What is json_t?

[sakuntalle]

oh yeah, sorry, that's just a typedef to nlohmann::json we use in our codebase

[sakuntalle]

Hey guys,

I've dig deeper into this situation and I've learned the followings:
In Visual Studio 2022, when inspecting nlohmann::json variables, the debugger often shows them as the empty base type nlohmann::detail::json_default_base. This happens because VS displays objects according to their static type, not their dynamic type — even though the runtime object is a full nlohmann::json.
Without a visualizer for json_default_base, the debugger cannot interpret the derived JSON contents and only shows raw memory or {...}.

Adding a Natvis for json_default_base ensures that the debugger displays JSON values based on their actual type (object, array, string, etc.) and allows convenient expansion. Although json_default_base is empty in C++, this visualizer is required to bridge the gap between VS’s static-type view and the actual runtime data.

I've tried a few workarounds to make it take the dynamic type instead but I failed to produce a working natvis file.
Maybe somebody with more Natvis experience would be able to crack this, I dunno :)

[gregmarr]

That's definitely not their static type, because if that were their static type, you wouldn't be able to do anything with them. The static type of obj is nlohmann::json. I wonder if it's being confused by the inline namespace or the alias, and that's what it's looking up in natvis. It is finding the base type, though.

[gregmarr]

This might help track down the issue: https://stackoverflow.com/questions/74713426/debugging-visual-studio-code-natvis-based-custom-view-objects