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), and a null value or a discarded view iterates 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, 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 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.

Examples¶

Example

The example below iterates a log record's members with begin()/end() 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.

#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:

time="10:00:01"
level="info"
message="started"

Version history¶

Added in version 3.13.0 unreleased.