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- 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>
1.7 KiB
1.7 KiB
nlohmann::basic_json_view::contains
bool contains(string_view_t key) const;
bool contains(const char* key) const;
bool contains(const string_t& key) const;
Checks whether the value is an object with a member with key key.
Parameters
key(in)- key value to check its existence
Return value
#!cpp true if the value is an object and has a member with key key, #!cpp false otherwise.
Exception safety
No-throw guarantee: this function never throws exceptions.
Complexity
Linear in the number of members: as for ordered_json, members are compared one after
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.
Notes
This method always returns #!cpp false when the value is not an object -- including a discarded
view.
!!! info "Postconditions"
If `#!cpp v.contains(key)` returns `#!cpp true`, then `#!cpp v[key]` is not [discarded](is_discarded.md).
Examples
??? example
The example below counts how many of a batch of records carry an optional `retry_of` field, using `contains()`
to check without ever materializing a single record of the batch.
```cpp
--8<-- "examples/basic_json_view__contains.cpp"
```
Output:
```json
--8<-- "examples/basic_json_view__contains.output"
```
See also
- find - find a value in an object
- count - returns the number of occurrences of a key
BasicJsonType::contains- the corresponding function ofbasic_json
Version history
- Added in version 3.13.0.