Niels Lohmann
6a852ab96d
* Split the json_view feature page into sub-pages
The single features/json_view.md page grew to ~600 lines covering eight
topics. Split it into features/json_view/ with an overview page and
pages for ownership and lifetime, differences from basic_json, reading
values, generic code, editing, and images.
The new differences page collects what was spread over several sections
("What is the same", "What is different", "Duplicate keys", and notes in
the conversion and serialization sections) under one summary table, and
adds the parts of the basic_json interface a view does not have. The
other pages now link to it instead of restating the rules.
Internal links across the documentation point to the new pages.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
* Docs: fix stale comments and trim repeated text in json_view pages
- fig-jsonview.js/.css: point the header comments at the new pages
- reading.md: views' strings are valid as long as the document (and a
borrowed source text), as lifetime.md says
- reading.md, generic_code.md: link to differences.md instead of
restating the member-order and repeated-key rules
- differences.md: drop the third mention of ordering comparisons, name
rbegin()/rend() in the iterator row
- exceptions.md: link editable documents to editing.md
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
---------
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
2.5 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.
Objects with 128 or more members get a hash index of their keys while parsing, so that a lookup in them takes
constant time on average.
Notes
!!! warning "Duplicate keys: the first member wins"
If the source text repeats a key, this resolves to the *first* member with it, not to the last one that
[`materialize()`](materialize.md) and `parse()` keep. See the [Notes on duplicate keys](operator[].md#notes) of
`operator[]` and [Duplicate keys](../../features/json_view/differences.md#duplicate-keys) for the reasons and for how to get
the last value.
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 keycontains- checks whether a key existsBasicJsonType::find- the corresponding function ofbasic_json
Version history
- Added in version 3.13.0.