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), and a #!json null value or a discarded view iterates as an empty range.
Iterator to the first element.
No-throw guarantee: this function never throws exceptions.
Constant.
For an object, iteration visits every member, including all occurrences of a duplicate key -- unlike operator[], at, find, and contains, 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.
An ordered_json value also keeps document order, but it keeps only one entry per key: as parse() does, it keeps the last value, at the position of the key's first occurrence. The view, in contrast, visits every member, including the duplicates.
??? 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
#include <iostream>
#include <nlohmann/json_view.hpp>
using json_document = nlohmann::json_document;
int main()
{
// a log record: the fields matter in the order they were written, e.g.
// to reproduce the record as it was logged. A nlohmann::json object
// sorts its keys, so materializing and iterating it would instead print
// them alphabetically ("level", "message", "time")
json_document record = json_document::parse(R"({"time": "10:00:01", "level": "info", "message": "started"})");
for (auto it = record.root().begin(); it != record.root().end(); ++it)
{
std::cout << it.key() << '=' << it->materialize().dump() << '\n';
}
}
```
Output:
```json
time="10:00:01"
level="info"
message="started"
```
BasicJsonType::begin - the corresponding function of basic_json