Niels Lohmann
08dcdaf9b8
- 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>
2.0 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.