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Return the first member of duplicate keys from lookups again

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Looking up the last member of a duplicate key cannot stop at a match, so
every lookup scanned the whole object (1.6 to 3.4 times slower for small
objects). operator[](key), at, find, value, contains, count, and JSON
pointer resolution return the first member again, as yyjson and simdjson
do; materialize() and get<map>() keep the last value, as parse().

The documentation says so in the feature page and on each lookup page,
and explains how to get the value parse() would give. The integer index
templates and the discarded chaining of operator[] stay.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-10-09 23:20:51 +02:00
parent 9acc902781
commit 13d2b3e224
15 changed files with 153 additions and 80 deletions
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13
docs/mkdocs/docs/api/basic_json_view/at.md
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@@ -15,7 +15,7 @@ basic_json_view at(IntegerType idx) const;
basic_json_view at(const json_pointer& ptr) const;
```
1. Returns the value of the object member with key `key` -- the last one, should the key occur more than once (see
1. Returns the value of the object member with key `key` -- the first one, should the key occur more than once (see
[Notes on duplicate keys](operator[].md#notes)).
2. Returns the array element at index `idx`. The template accepts every integer type except `#!cpp bool` and
`#!cpp std::size_t` and forwards to the `size_type` overload, as for [`operator[]`](operator[].md); a negative
@@ -35,7 +35,7 @@ basic_json_view at(const json_pointer& ptr) const;
## Return value
1. the value of the last member with key `key`
1. the value of the first member with key `key`
2. the element at index `idx`
3. the value `ptr` resolves to, starting at this value
@@ -75,7 +75,7 @@ None of these exceptions carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics
## Complexity
1. Linear in the number of members: as for [`ordered_json`](../ordered_json.md), members are compared one after
another, in document order, scanning all of them, since the last match is wanted. Each comparison first checks the
another, in document order, stopping at the first match. Each comparison first checks the
key's length -- already known from the index, without reading the key bytes -- before comparing its content.
2. Linear in `idx`: elements are skipped one at a time from the first one, since they are not a fixed size in the
index (unlike `BasicJsonType`'s array, which is random-access).
@@ -84,6 +84,13 @@ None of these exceptions carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics
## Notes
!!! warning "Duplicate keys: the first member wins"
If the source text repeats a key, this resolves to the *first* member with it, not to the last one that
[`materialize()`](materialize.md) and `parse()` keep. See the [Notes on duplicate keys](operator[].md#notes) of
`operator[]` and [Duplicate keys](../../features/json_view.md#duplicate-keys) for the reasons and for how to get
the last value.
Unlike [`operator[]`](operator[].md), which returns a [discarded](is_discarded.md) view for a missing key or an
out-of-range index, `at` always throws -- exactly as `BasicJsonType::at` does, and with the same messages, so
existing error handling written against `BasicJsonType::at` keeps working unchanged when switched to a view. This