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Fix key types convertible to std::string_view breaking lookups (#5689)

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Since #4958, a key type implicitly convertible to std::string_view was
accepted by is_usable_as_basic_json_key_type without checking that the
object's comparator can actually compare object_t::key_type with that
key type. The key was then forwarded unchanged to the underlying map,
so const operator[], at, find, count, contains, erase and value failed
to compile (a hard error inside <map>) for a key convertible only to
std::string_view, and value() rejected such keys outright. For keys
convertible to both std::string and std::string_view, the KeyType&&
templates now won overload resolution over the object_t::key_type
overloads and then failed the same way, a regression from 3.12.0. Only
the non-const operator[] worked, because it uses emplace(), which
constructs a std::string from the key explicitly. ordered_json was not
affected, since ordered_map checks comparability itself.

Add a trait, is_string_view_convertible_key_type, that recognizes a key
type that is convertible to std::string_view but not directly
comparable with the object's key type, provided std::string_view itself
is comparable with it. at(), operator[], find(), count(), contains(),
erase() and value() now route such keys through a new lookup_key()
helper that converts them to std::string_view before they reach the
object, matching how the object's transparent comparator already
supports std::string_view lookups.

Fixes #5663.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-10-04 11:46:32 +02:00
committed by GitHub
parent 0d01d6ae90
commit b730946432
10 changed files with 243 additions and 36 deletions
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docs/mkdocs/docs/api/basic_json/contains.md
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@@ -131,7 +131,9 @@ Logarithmic in the size of the JSON object.
## Version history
1. Added in version 3.11.0.
2. Added in version 3.6.0. Extended template `KeyType` to support comparable types in version 3.11.0.
2. Added in version 3.6.0. Extended template `KeyType` to support comparable types in version 3.11.0. Fixed in
version 3.13.0 to consistently accept `std::string_view`-convertible keys, as already supported by
[`operator[]`](operator[].md), [`at`](at.md), [`value`](value.md), and other lookup functions.
3. Added in version 3.7.0.
4. Deleted overloads for integral key types added in version 3.13.0 to reject such calls at compile time instead of
causing undefined behavior at runtime.