json/docs/mkdocs/docs/features/element_access/unchecked_access.md

Unchecked access: operator[]

Overview

Elements in a JSON object and a JSON array can be accessed via operator[] similar to a #!cpp std::map and a #!cpp std::vector, respectively.

??? example "Example: read access"

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["name"]`       | `#!json "Mary Smith"`                                                        |
| `#!cpp j["age"]`        | `#!json 42`                                                                  |
| `#!cpp j["hobbies"]`    | `#!json ["hiking", "reading"]`                                               |
| `#!cpp j["hobbies"][0]` | `#!json "hiking"`                                                            |
| `#!cpp j["hobbies"][1]` | `#!json "reading"`                                                           |

The return value is a reference, so it can modify the original value. In case the passed object key is non-existing, a #!json null value is inserted which can immediately be overwritten.

??? example "Example: write access"

```cpp
j["name"] = "John Smith";
j["maidenName"] = "Jones";
```

This code produces the following JSON value:

```json
{
    "name": "John Smith",
    "maidenName": "Jones",
    "age": 42,
    "hobbies": ["hiking", "reading"]
}
```

When accessing an invalid index (i.e., an index greater than or equal to the array size), the JSON array is resized such that the passed index is the new maximal index. Intermediate values are filled with #!json null.

??? example "Example: filling up arrays with #!json null values"

```cpp
j["hobbies"][0] = "running";
j["hobbies"][3] = "cooking";
```

This code produces the following JSON value:

```json
{
    "name": "John Smith",
    "maidenName": "Jones",
    "age": 42,
    "hobbies": ["running", "reading", null, "cooking"]
}
```

Notes

!!! info "Design rationale"

The library behaves differently to `#!cpp std::vector` and `#!cpp std::map`:

- `#!cpp std::vector::operator[]` never inserts a new element.
- `#!cpp std::map::operator[]` is not available for const values.

The type `#!cpp json` wraps all JSON value types. It would be impossible to remove
[`operator[]`](../../api/basic_json/operator%5B%5D.md) for const objects. At the same time, inserting elements for
non-const objects is really convenient as it avoids awkward `insert` calls. To this end, we decided to have an
inserting non-const behavior for both arrays and objects.

!!! info

The access is unchecked. In case the passed object key does not exist or the passed array index is invalid, no
exception is thrown.

!!! danger

- It is **undefined behavior** to access a const object with a non-existing key.
- It is **undefined behavior** to access a const array with an invalid index.
- In debug mode, an **assertion** will fire in both cases. You can disable assertions by defining the preprocessor
  symbol `#!cpp NDEBUG` or redefine the macro [`JSON_ASSERT(x)`](../../api/macros/json_assert.md). See the
  documentation on [runtime assertions](../assertions.md) for more information.

!!! failure "Exceptions"

`operator[]` 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_error305) is thrown.

Performance: reserving array capacity

There is no public reserve(count) member on basic_json for pre-allocating array capacity. If you are building a large array incrementally (e.g., via repeated push_back()) and know its final size ahead of time, you can reserve capacity via get_ref() to access the underlying array_t directly:

json j = json::array();
j.get_ref<json::array_t&>().reserve(1000);
for (int i = 0; i < 1000; ++i) {
    j.push_back(i);
}

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	reference to newly inserted #!json null value is returned	undefined behavior; runtime assertion in debug mode
access to invalid array index	reference to newly inserted #!json null value is returned; any index between previous maximal index and passed index are filled with #!json null	undefined behavior; runtime assertion in debug mode