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

Access with default value: value

Overview

In many situations such as configuration files, missing values are not exceptional, but may be treated as if a default value was present. For this case, use value(key, default_value) which takes the key you want to access and a default value in case there is no value stored with that key.

Example

??? example

Consider the following JSON value:

```json
{
    "logOutput": "result.log",
    "append": true
}
```

Assume the value is parsed to a `json` variable `j`.

| expression                                  | value                                                |
|---------------------------------------------|------------------------------------------------------|
| `#!cpp j`                                   | `#!json {"logOutput": "result.log", "append": true}` |
| `#!cpp j.value("logOutput", "logfile.log")` | `#!json "result.log"`                                |
| `#!cpp j.value("append", true)`             | `#!json true`                                        |
| `#!cpp j.value("append", false)`            | `#!json true`                                        |
| `#!cpp j.value("logLevel", "verbose")`      | `#!json "verbose"`                                   |

Notes

!!! failure "Exceptions"

- `value` can only be used with objects. For other types, a [`basic_json::type_error`](../../home/exceptions.md#jsonexceptiontype_error306) is thrown.

!!! warning "Return type"

The value function is a template, and the return type of the function is determined by the type of the provided
default value unless otherwise specified. This can have unexpected effects. In the example below, we store a 64-bit
unsigned integer. We get exactly that value when using [`operator[]`](../../api/basic_json/operator[].md). However,
when we call `value` and provide `#!c 0` as default value, then `#!c -1` is returned. The occurs, because `#!c 0`
has type `#!c int` which overflows when handling the value `#!c 18446744073709551615`.

To address this issue, either provide a correctly typed default value or use the template parameter to specify the
desired return type. Note that this issue occurs even when a value is stored at the provided key, and the default
value is not used as the return value.

```cpp
--8<-- "examples/value__return_type.cpp"
```

Output:

```json
--8<-- "examples/value__return_type.output"
```

See also

• value for access with default value
• documentation on checked access