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doc/mkdocs/docs/features/element_access/checked_access.md

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+# Checked access: at
+
+## Overview
+
+The `#!cpp at()` member function performs checked access; that is, it returns a reference to the desired value if it exists and throws a [`basic_json::out_of_range` exception](../../home/exceptions.md#out-of-range) otherwise.
+
+??? example
+
+    Consider the following JSON value:
+    
+    ```json
+    {
+        "name": "Mary Smith",
+        "age": 42,
+        "hobbies": ["hiking", "reading"]
+    }
+    ```
+    
+    Assume the value is parsed to a `json` variable `j`.
+
+    | expression | value |
+    | ---------- | ----- |
+    | `#!cpp j`  | `#!json {"name": "Mary Smith", "age": 42, "hobbies": ["hiking", "reading"]}` |
+    | `#!cpp j.at("name")`  | `#!json "Mary Smith"` |
+    | `#!cpp j.at("age")`  | `#!json 42` |
+    | `#!cpp j.at("hobbies")`  | `#!json ["hiking", "reading"]` |
+    | `#!cpp j.at("hobbies").at(0)`  | `#!json "hiking"` |
+    | `#!cpp j.at("hobbies").at(1)`  | `#!json "reading"` |
+
+The return value is a reference, so it can be modify the original value.
+
+??? example
+
+    ```cpp
+    j.at("name") = "John Smith";
+    ```
+    
+    This code produces the following JSON value:
+    
+    ```json
+    {
+        "name": "John Smith",
+        "age": 42,
+        "hobbies": ["hiking", "reading"]
+    }
+    ```
+
+When accessing an invalid index (i.e., and index greater than or equal to the array size) or the passed object key is non-existing, an exception is thrown.
+
+??? example
+
+    ```cpp
+    j.at("hobbies").at(3) = "cooking";
+    ```
+    
+    This code produces the following exception:
+    
+    ```
+    [json.exception.out_of_range.401] array index 3 is out of range
+    ```
+
+## Notes
+
+
+!!! failure "Exceptions"
+
+    - `at` can only be used with objects (with a string argument) or with arrays (with a numeric argument). For other types, a [`basic_json::type_error`](../../home/exceptions.md#jsonexceptiontype_error304) is thrown.
+    - [`basic_json::out_of_range` exception](../../home/exceptions.md#out-of-range) exceptions are thrown if the provided key is not found in an object or the provided index is invalid.
+
+## Summary
+
+| scenario | non-const value | const value |
+| -------- | ------------- | ----------- |
+| access to existing object key | reference to existing value is returned | const reference to existing value is returned |
+| access to valid array index | reference to existing value is returned | const reference to existing value is returned |
+| access to non-existing object key | `basic_json::out_of_range` exception is thrown | `basic_json::out_of_range` exception is thrown |
+| access to invalid array index | `basic_json::out_of_range` exception is thrown | `basic_json::out_of_range` exception is thrown |