json/api/basic_json/value.md

# <small>nlohmann::basic_json::</small>value

```cpp
// (1)
template<class ValueType>
ValueType value(const typename object_t::key_type& key,
                ValueType&& default_value) const;

// (2)
template<class ValueType, class KeyType>
ValueType value(KeyType&& key,
                ValueType&& default_value) const;

// (3)
template<class ValueType>
ValueType value(const json_pointer& ptr,
                const ValueType& default_value) const;
```

1. Returns either a copy of an object's element at the specified key `key` or a given default value if no element with
   key `key` exists.
   
    The function is basically equivalent to executing
    ```cpp
    try {
       return at(key);
    } catch(out_of_range) {
       return default_value;
    }
    ```

2. See 1. This overload is only available if `KeyType` is comparable with `#!cpp typename object_t::key_type` and
   `#!cpp typename object_comparator_t::is_transparent` denotes a type.

3. Returns either a copy of an object's element at the specified JSON pointer `ptr` or a given default value if no value
   at `ptr` exists.
   
    The function is basically equivalent to executing
    ```cpp
    try {
       return at(ptr);
    } catch(out_of_range) {
       return default_value;
    }
    ```

!!! note "Differences to `at` and `operator[]`"

    - Unlike [`at`](at.md), this function does not throw if the given `key`/`ptr` was not found.
    - Unlike [`operator[]`](operator[].md), this function does not implicitly add an element to the position defined by
     `key`/`ptr` key. This function is furthermore also applicable to const objects.

## Template parameters

`KeyType`
:   A type for an object key other than [`json_pointer`](../json_pointer/index.md) that is comparable with
    [`string_t`](string_t.md) using  [`object_comparator_t`](object_comparator_t.md).
    This can also be a string view (C++17).
`ValueType` 
:   type compatible to JSON values, for instance `#!cpp int` for JSON integer numbers, `#!cpp bool` for JSON booleans,
    or `#!cpp std::vector` types for JSON arrays. Note the type of the expected value at `key`/`ptr` and the default
    value `default_value` must be compatible.

## Parameters

`key` (in)
:   key of the element to access

`default_value` (in)
:   the value to return if `key`/`ptr` found no value

`ptr` (in)
:   a JSON pointer to the element to access

## Return value

1. copy of the element at key `key` or `default_value` if `key` is not found
2. copy of the element at key `key` or `default_value` if `key` is not found
3. copy of the element at JSON Pointer `ptr` or `default_value` if no value for `ptr` is found

## Exception safety

Strong guarantee: if an exception is thrown, there are no
changes to any JSON value.

## Exceptions

1. The function can throw the following exceptions:
    - Throws [`type_error.302`](../../home/exceptions.md#jsonexceptiontype_error302) if `default_value` does not match
      the type of the value at `key`
    - Throws [`type_error.306`](../../home/exceptions.md#jsonexceptiontype_error306) if the JSON value is not an object;
      in that case, using `value()` with a key makes no sense.
2. See 1.
3. The function can throw the following exceptions:
    - Throws [`type_error.302`](../../home/exceptions.md#jsonexceptiontype_error302) if `default_value` does not match
      the type of the value at `ptr`
    - Throws [`type_error.306`](../../home/exceptions.md#jsonexceptiontype_error306) if the JSON value is not an array
      or object; in that case, using `value()` with a JSON pointer makes no sense.
    - Throws [`parse_error.106`](../../home/exceptions.md#jsonexceptionparse_error106) if an array index in the passed
      JSON pointer `ptr` begins with '0'.
    - Throws [`parse_error.109`](../../home/exceptions.md#jsonexceptionparse_error109) if an array index in the passed
      JSON pointer `ptr` is not a number.

## Complexity

1. Logarithmic in the size of the container.
2. Logarithmic in the size of the container.
3. Logarithmic in the size of the container.

## Notes

!!! 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[]`](operator[].md). However, when we call `value`
    and provide `#!c 0` as default value, then `#!c -1` is returned. This 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"
    ```

## Examples

??? example "Example: (1) access specified object element with default value"

    The example below shows how object elements can be queried with a default value.
    
    ```cpp
    --8<-- "examples/value__object_t_key_type.cpp"
    ```
    
    Output:
    
    ```json
    --8<-- "examples/value__object_t_key_type.output"
    ```

??? example "Example: (2) access specified object element using string_view with default value"

    The example below shows how object elements can be queried with a default value.
    
    ```cpp
    --8<-- "examples/value__keytype.c++17.cpp"
    ```
    
    Output:
    
    ```json
    --8<-- "examples/value__keytype.c++17.output"
    ```

??? example "Example: (3) access specified object element via JSON Pointer with default value"

    The example below shows how object elements can be queried with a default value.
    
    ```cpp
    --8<-- "examples/value__json_ptr.cpp"
    ```
    
    Output:
    
    ```json
    --8<-- "examples/value__json_ptr.output"
    ```

## See also

- see [`at`](at.md) for access by reference with range checking
- see [`operator[]`](operator%5B%5D.md) for unchecked access by reference

## Version history

1. Added in version 1.0.0. Changed parameter `default_value` type from `const ValueType&` to `ValueType&&` in version 3.11.0.
2. Added in version 3.11.0. Made `ValueType` the first template parameter in version 3.11.2.
3. Added in version 2.0.2. Extended to work with arrays in version 3.12.x.