json/docs/mkdocs/docs/api/basic_json/value.md

nlohmann::basic_json::value

// (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;

This is equivalent to Python's dict.get(key, default).

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

   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

   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.

!!! note "Integer keys"

Calling this function with an integer `key` argument (for example, `#!cpp value(0, 1)`) does not compile in
C++11, where `object_comparator_t` is not transparent: such an argument would otherwise implicitly convert to a
null `#!cpp const char*` and, from there, cause undefined behavior when constructing a `#!cpp std::string` for the
object key. To access an array element with a default value, use [`at`](at.md) together with a `#!cpp try`/`#!cpp
catch` block, or compare against [`size`](size.md) instead.

Template parameters

KeyType

A type for an object key other than json_pointer that is comparable with string_t using object_comparator_t. 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 if default_value does not match the type of the value at key
   • Throws type_error.306 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 if default_value does not match the type of the value at ptr
   • Throws type_error.306 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 if an array index in the passed JSON pointer ptr begins with '0'.
   • Throws parse_error.109 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"
```

!!! warning "Deprecation"

Overload (3) also accepts a [`json_pointer`](../json_pointer/index.md) whose template argument is a `basic_json`
specialization (e.g., `nlohmann::json_pointer<nlohmann::json>`) instead of a string type. This is deprecated since
version 3.11.0 and will be removed in a future major version; use `basic_json::json_pointer` (for `json`,
`nlohmann::json_pointer<std::string>`) instead.

You should be warned by your compiler with a `-Wdeprecated-declarations` warning if you are using a deprecated
function.

See the [migration guide](../../integration/migration_guide.md#json-pointers) for how to update existing code.

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"
```

??? example "Example: (1) type_error.302 and type_error.306 exceptions"

The example below shows how `value()` throws `type_error.302` when the default value's type does not match the type
of the stored value, and `type_error.306` when `value()` is called on a JSON value that is not an object.

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

Output:

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

See also

• see at for access by reference with range checking
• see operator[] 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. Deleted overload for integral key types added in version 3.13.0 to reject such calls at compile time instead of causing undefined behavior at runtime.
2. Added in version 3.11.0. Made ValueType the first template parameter in version 3.11.2. Fixed in version 3.13.0 to consistently accept std::string_view-convertible keys, as already supported by operator[], at, find, and other lookup functions.
3. Added in version 2.0.2. Extended to work with arrays in version 3.13.0, including fixing an issue where resolving ptr through an array unexpectedly threw out_of_range instead of returning the resolved element (or default_value, as documented).