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api/basic_json_view/begin.md

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+# <small>nlohmann::basic_json_view::</small>begin
+
+```cpp
+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](is_discarded.md) 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[]`](operator[].md), [`at`](at.md), [`find`](find.md), and [`contains`](contains.md),
+which all resolve to the *first* member with a given key. See the
+[Notes on duplicate keys](operator[].md#notes) 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()`](materialize.md)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.
+
+## 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
+    #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"
+    
+    ```
+
+## See also
+
+- [end](end.md) - returns an iterator to one past the last element
+- [cbegin](cbegin.md) - returns a const iterator to the first element
+- [items](items.md) - access iterator member functions in range-based for
+- [`BasicJsonType::begin`](../basic_json/begin.md) - the corresponding function of `basic_json`
+
+## Version history
+
+- Added in version 3.13.0.