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Keep the last of duplicate keys in the object hash index

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Lookups in objects with 128 members or more now return the last member of
a repeated key, like the linear search of smaller objects and like
materialize() and basic_json::parse(). build_object_index let the first
occurrence win, so the same text gave different results depending on the
size of the object.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-10-09 17:13:26 +02:00
parent fdeda3eb66
commit 15344777e6
4 changed files with 111 additions and 17 deletions
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14
docs/mkdocs/docs/api/basic_json_view/operator[].md
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@@ -111,14 +111,12 @@ document.
If the source text has an object with a duplicate key, `#!cpp operator[]` (and [`at`](at.md), [`find`](find.md),
[`contains`](contains.md), [`count`](count.md), [`value`](value.md), and JSON pointer resolution) all resolve to
the *first* member with that key, because a lookup can stop as soon as it finds a match. This is different from
[`materialize()`](materialize.md) (and [`BasicJsonType::parse()`](../basic_json/parse.md)), which replay every
member in order and so keep the *last* value for a repeated key, so `#!cpp v["a"]` and
`#!cpp v.materialize()["a"]` can differ. To get the value `parse()` would give, use
[`materialize()`](materialize.md) or iterate the members with [`items()`](items.md) and keep the last match.
[`begin()`](begin.md)/[`end()`](end.md) and [`items()`](items.md) iterate over *all* members, including
duplicates, in document order. See [Duplicate keys](../../features/json_view.md#duplicate-keys) and
[`size()`](size.md#notes).
the *last* member with that key. This is the member [`materialize()`](materialize.md) (and
[`BasicJsonType::parse()`](../basic_json/parse.md)) keeps, so a lookup in the view and in the materialized value
agree. [`begin()`](begin.md)/[`end()`](end.md) and [`items()`](items.md) iterate over *all* members, including
duplicates, in document order. A lookup in an object without a hash index scans all members for this: it cannot
stop at the first match. The hash index of a larger object (128 members or more) leads to the last member of a key
as well. See the example below and [`size()`](size.md#notes).
!!! info "JSON pointer resolution"