5a9f59a688
- 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>
2.1 KiB
nlohmann::basic_json_view::count
size_type count(string_view_t key) const;
size_type count(const char* key) const;
size_type count(const string_t& key) const;
Returns #!cpp 1 if the value is an object with a member with key key, #!cpp 0 otherwise.
Parameters
key(in)- key value of the element to count
Return value
#!cpp 1 if the value is an object and has a member with key key, #!cpp 0 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 0 when the value is not an object -- including a discarded
view.
Unlike BasicJsonType::count(), whose return value can in principle exceed #!cpp 1 for
an ObjectType that allows multiple entries per key, count() here never does: it is exactly
contains() as #!cpp 0/#!cpp 1. This holds even if the source text has a duplicate key -- see the
Notes on duplicate keys of operator[] -- because a #!cpp count() > 1 result would require
counting every member with a matching key, not just finding the first one.
Examples
??? example
The example below validates that every transaction of a batch carries a mandatory `amount` field, using
`count()` before deciding whether to materialize a transaction at all.
```cpp
--8<-- "examples/basic_json_view__count.cpp"
```
Output:
```json
--8<-- "examples/basic_json_view__count.output"
```
See also
- find - find a value in an object
- contains - checks whether a key exists
BasicJsonType::count- the corresponding function ofbasic_json
Version history
- Added in version 3.13.0.