2.6 KiB
2.6 KiB
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 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 |