Niels Lohmann
13d2b3e224
Looking up the last member of a duplicate key cannot stop at a match, so every lookup scanned the whole object (1.6 to 3.4 times slower for small objects). operator[](key), at, find, value, contains, count, and JSON pointer resolution return the first member again, as yyjson and simdjson do; materialize() and get<map>() keep the last value, as parse(). The documentation says so in the feature page and on each lookup page, and explains how to get the value parse() would give. The integer index templates and the discarded chaining of operator[] stay. Signed-off-by: Niels Lohmann <mail@nlohmann.me>
2.0 KiB
nlohmann::basic_json_view::begin
iterator begin() const noexcept;
Returns an iterator to the first element of an array, or the first member value of an object, in document order
-- the order the values appear in the source text, not sorted by key. A primitive value iterates as a range of one
element (itself); #!json null and a discarded view iterate as an empty range.
Return value
Iterator to the first element.
Exception safety
No-throw guarantee: this function never throws exceptions.
Complexity
Constant.
Notes
For an object, iteration visits every member, including all occurrences of a duplicate key -- unlike
operator[], at, find, contains, and count,
which all resolve to the first member with a given key. See the
Notes on duplicate keys of operator[].
Because objects are iterated in document order rather than sorted by key, the order seen here can differ from what
iterating the materialize()d BasicJsonType value would produce: a #!cpp basic_json object
(std::map-backed by default) sorts its keys, while a view does not.
Examples
??? example
The example below iterates a log record's members with `begin()`/[`end()`](end.md) and prints them in the order
they were written. Materializing the record into a `BasicJsonType` object and iterating that would instead print
the members sorted by key.
```cpp
--8<-- "examples/basic_json_view__begin.cpp"
```
Output:
```json
--8<-- "examples/basic_json_view__begin.output"
```
See also
- end - returns an iterator to one past the last element
- cbegin - returns a const iterator to the first element
- items - access iterator member functions in range-based for
BasicJsonType::begin- the corresponding function ofbasic_json
Version history
- Added in version 3.13.0.