Niels Lohmann
77acd4563c
Give basic_json_view the read-only access functions of basic_json: operator[] and at() with keys and indices, front()/back(), find(), contains(), count(), begin()/end() and cbegin()/cend(), items() with structured bindings from C++17 on, and type_name(). Exceptions have the ids and messages of the const functions of basic_json. Where basic_json has undefined behavior the view answers safely: operator[] with a missing key or an out-of-range index returns a discarded view, and front()/back() of an empty container throw invalid_iterator.214. Objects are iterated in document order, and all members are visited; duplicate-key lookups find the first member (as yyjson and simdjson do), while parse(), materialize(), and the map conversions keep the last value, as parse() does. Keys of up to 16 bytes are compared with two overlapping loads. Add value conversions: get<T>()/get_to() for arithmetic types, bool, nullptr_t, strings (std::basic_string copied, string_view_t without a copy), BasicJsonType, views, std::vector, and maps with string keys; get_string() for the string without a copy; number_token() for the number exactly as written in the source; value() with keys and JSON pointers; and operator[]/at()/ contains() with JSON pointers. Everything else, including types with from_json(), goes through materialize() of that subtree. get<T>() of arithmetic types is inlined down to the conversion, so reading an integer needs no call. detail::json_pointer_access exposes a pointer's reference tokens to code outside basic_json. Signed-off-by: Niels Lohmann <mail@nlohmann.me>
2.0 KiB
nlohmann::basic_json_view::find
iterator find(string_view_t key) const;
iterator find(const char* key) const;
iterator find(const string_t& key) const;
Finds a member with key key -- the first one, should the key occur more than once (see
Notes on duplicate keys). If the value is not an object, or no member has this key,
end() is returned.
Parameters
key(in)- key value of the element to search for
Return value
An iterator to the member with key key, or end() if there is none or the value is not an object.
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
Unlike BasicJsonType::find, which always returns #!cpp end() for a non-object type, this
also does so for a discarded view -- there is no separate "invalid" iterator to return.
Examples
??? example
The example below scans a batch of events for those that carry an optional `user_id` field, using `find()`
instead of [`operator[]`](operator[].md) (which would throw for the events that are not objects at all) or
[`contains()`](contains.md) followed by a second lookup. Events without a match are never materialized.
```cpp
--8<-- "examples/basic_json_view__find.cpp"
```
Output:
```json
--8<-- "examples/basic_json_view__find.output"
```
See also
- count - returns the number of occurrences of a key
- contains - checks whether a key exists
BasicJsonType::find- the corresponding function ofbasic_json
Version history
- Added in version 3.13.0.