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create objects for tokens that cannot be array indices (#5357)

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When operator[](json_pointer) traverses a level that does not exist yet,
the null value is turned into an array or an object depending on the
reference token. The check only tested whether all characters are digits,
so tokens that can never be a valid array index selected an array and
then failed:

- "01" (and any other token with a leading '0') threw parse_error.106
- the empty token threw out_of_range.404

Both tokens are valid object keys, and both work when the level already
exists as an object, so creating the level changed the outcome. Test the
token against the RFC 6901, Sect. 4 grammar for array indices instead, so
that such tokens create an object. This only affects pointers that threw
before.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-08-04 08:51:54 +02:00
parent ad94fb01cc
commit db1d0983e6
4 changed files with 91 additions and 14 deletions
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docs/mkdocs/docs/api/basic_json/operator[].md
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@@ -128,10 +128,13 @@ Strong exception safety: if an exception occurs, the original value stays intact
When the JSON pointer traverses intermediate levels that don't exist at all yet (not just a missing
leaf), each missing level is created as an array or an object depending on whether the corresponding
pointer token parses as a non-negative integer: a numeric token creates an array, a non-numeric token
creates an object. For example, on an initially `#!json null` value, `/foo/0/0/0` creates nested arrays,
while `/foo/one/one/one` creates nested objects. This is not specified by the JSON Pointer RFC; it is
this library's own, intentional disambiguation rule. See also [JSON Pointer](../../features/json_pointer.md).
pointer token is a valid array index: a token that is a nonempty sequence of digits without a leading
`0` (or the token `-`) creates an array, and every other token creates an object. For example, on an
initially `#!json null` value, `/foo/0/0/0` creates nested arrays, while `/foo/one/one/one` creates
nested objects. Tokens such as `01` or the empty token cannot be array indices (cf. RFC 6901, Sect. 4)
and therefore create objects, just as they would if the level already existed as an object. This is not
specified by the JSON Pointer RFC; it is this library's own, intentional disambiguation rule. See also
[JSON Pointer](../../features/json_pointer.md).
## Examples