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Subclass json_object so primitives shrink from 168 to 24 bytes

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The previous "every member in a struct" layout cost 168 bytes per
json_object, even for JSON_NULL / JSON_TRUE / JSON_FALSE nodes that
have no payload. Splitting json_object into a small base class plus
json_number / json_string / json_array / json_hash subclasses brings
each instance down to just the size of its actual contents:

  json_object (base, TRUE / FALSE / NULL)   24 bytes
  json_number                                48 bytes
  json_string                                48 bytes
  json_array  (empty)                        48 bytes
  json_hash   (empty)                        72 bytes

Other size wins along the way:

* Drop enable_shared_from_this<json_object> (its embedded weak_ptr
  was 16 bytes per node). json_pull now keeps an explicit
  container_stack and the parser no longer needs to resurrect a
  shared_ptr from a raw `parent` walk.
* Remove the unused `refcon` slot from the string variant.
* No virtual destructor: shared_ptr keeps the deleter from the
  original std::make_shared<json_xxx> call, so destroying a
  shared_ptr<json_object> still runs the right subclass dtor.

The base class exposes type-tagged accessors (o->string(),
o->number(), o->array(), o->keys(), o->values(), o->large_signed(),
o->large_unsigned()) that assert the type matches and downcast to
the appropriate subclass storage. All call sites were swept from
the old `o->value.X.Y` field paths to these accessors. A raw-pointer
overload of json_hash_get() replaces the few external uses of
shared_from_this() that survived in geojson-loop.cpp.

Co-authored-by: Cursor <cursoragent@cursor.com>
This commit is contained in:
Erica Fischer
2026-05-30 17:49:08 -07:00
co-authored by Cursor
parent 3da03c6075
commit f366b2c4aa
14 changed files with 424 additions and 291 deletions
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geobuf.cpp
+3 -3
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@@ -400,17 +400,17 @@ void readFeature(protozero::pbf_reader &pbf, size_t dim, double e, std::vector<s
if (o != nullptr) {
json_object_ptr min = json_hash_get(o, "minzoom");
if (min != nullptr && (min->type == JSON_NUMBER)) {
sf.tippecanoe_minzoom = integer_zoom(sst->fname, milo::dtoa_milo(min->value.number.number));
sf.tippecanoe_minzoom = integer_zoom(sst->fname, milo::dtoa_milo(min->number()));
}
json_object_ptr max = json_hash_get(o, "maxzoom");
if (max != nullptr && (max->type == JSON_NUMBER)) {
sf.tippecanoe_maxzoom = integer_zoom(sst->fname, milo::dtoa_milo(max->value.number.number));
sf.tippecanoe_maxzoom = integer_zoom(sst->fname, milo::dtoa_milo(max->number()));
}
json_object_ptr tlayer = json_hash_get(o, "layer");
if (tlayer != nullptr && (tlayer->type == JSON_STRING)) {
layername = tlayer->value.string.string;
layername = tlayer->string();
}
}
}