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Niels Lohmann b730946432 Fix key types convertible to std::string_view breaking lookups (#5689 )

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>

2026-10-04 11:46:32 +02:00

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nlohmann::basic_json::count

// (1)
size_type count(const typename object_t::key_type& key) const;

// (2)
template<typename KeyType>
size_type count(KeyType&& key) const;

Returns the number of elements with key key. If ObjectType is the default std::map type, the return value will always be 0 (key was not found) or 1 (key was found).
See 1. This overload is only available if KeyType is comparable with #!cpp typename object_t::key_type and #!cpp typename object_comparator_t::is_transparent denotes a type.

Template parameters

KeyType: A type for an object key other than json_pointer that is comparable with string_t using object_comparator_t. This can also be a string view (C++17).

Parameters

key (in): key value of the element to count.

Return value

Number of elements with key key. If the JSON value is not an object, the return value will be 0.

Exception safety

Strong exception safety: if an exception occurs, the original value stays intact.

Complexity

Logarithmic in the size of the JSON object.

Notes

This method always returns 0 when executed on a JSON type that is not an object.
Calling this function with an integer argument (for example, #!cpp count(0)) does not compile: such an argument would otherwise implicitly convert to a null #!cpp const char* and, from there, cause undefined behavior when constructing a #!cpp std::string for the object key. To check for an array element instead, use at, operator[], or compare against size.

Examples

??? example "Example: (1) count number of elements"

The example shows how `count()` is used.

```cpp
--8<-- "examples/count__object_t_key_type.cpp"
```

Output:

```json
--8<-- "examples/count__object_t_key_type.output"
```

??? example "Example: (2) count number of elements using string_view"

The example shows how `count()` is used.

```cpp
--8<-- "examples/count__keytype.c++17.cpp"
```

Output:

```json
--8<-- "examples/count__keytype.c++17.output"
```

Version history

Added in version 3.11.0.
Added in version 1.0.0. Changed parameter key type to KeyType&& in version 3.11.0. Fixed in version 3.13.0 to consistently accept std::string_view-convertible keys, as already supported by operator[], at, value, and other lookup functions.
Deleted overload for integral key types added in version 3.13.0 to reject such calls at compile time instead of causing undefined behavior at runtime.

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