wt/json

mirror of https://github.com/nlohmann/json.git synced 2026-10-01 13:34:14 +07:00

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Niels Lohmann 5a9f59a688 Document element access, lookup, and iteration of json_view

- API pages for operator[], at, front, back, find, contains, count,
  begin, end, cbegin, cend, items, and type_name of basic_json_view,
  linked both ways with the basic_json pages
- the feature page and size() describe document order and duplicate
  keys
- the examples show when the view helps: reading a few fields of a large
  text, probing optional members, and members in source order

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

2026-09-29 14:02:50 +02:00

2.1 KiB

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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.

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.

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