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

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

```cpp
// (1)
template<typename ValueType>
ValueType get() const noexcept(
    noexcept(JSONSerializer<ValueType>::from_json(
        std::declval<const basic_json_t&>(), std::declval<ValueType&>())));

// (2)
template<typename BasicJsonType>
BasicJsonType get() const;

// (3)
template<typename PointerType>
PointerType get() noexcept;

template<typename PointerType>
const PointerType get() const noexcept;  // constexpr since C++14
```

1. Explicit type conversion between the JSON value and a compatible value which is
   [CopyConstructible](https://en.cppreference.com/w/cpp/named_req/CopyConstructible) and
   [DefaultConstructible](https://en.cppreference.com/w/cpp/named_req/DefaultConstructible). The value is converted by
   calling the [`json_serializer<ValueType>`](json_serializer.md) `from_json()` method.
   
    The function is equivalent to executing
    ```cpp
    ValueType ret;
    JSONSerializer<ValueType>::from_json(*this, ret);
    return ret;
    ```

    This overload is chosen if:
    
    - `ValueType` is not `basic_json`,
    - `json_serializer<ValueType>` has a `from_json()` method of the form
      `void from_json(const basic_json&, ValueType&)`, and
    - `json_serializer<ValueType>` does not have a `from_json()` method of the form
      `ValueType from_json(const basic_json&)`

    If the type is **not** [CopyConstructible](https://en.cppreference.com/w/cpp/named_req/CopyConstructible) and
    **not** [DefaultConstructible](https://en.cppreference.com/w/cpp/named_req/DefaultConstructible), the value is
    converted by calling the `json_serializer<ValueType>` `from_json()` method.
   
    The function is then equivalent to executing
    ```cpp
    return JSONSerializer<ValueTypeCV>::from_json(*this);
    ``` 
   
    This overload is chosen if:
    
    - `ValueType` is not `basic_json` and
    - `json_serializer<ValueType>` has a `from_json()` method of the form
     `ValueType from_json(const basic_json&)`

    If `json_serializer<ValueType>` has both overloads of `from_json()`, the latter one is chosen.

2. Overload for `basic_json` specializations. The function is equivalent to executing
    ```cpp
    return *this;
    ```

3. Explicit pointer access to the internally stored JSON value. No copies are made.

## Template parameters

`ValueType`
:   the value type to return

`BasicJsonType`
:   a specialization of `basic_json`

`PointerType`
:   pointer type; must be a pointer to [`array_t`](array_t.md), [`object_t`](object_t.md), [`string_t`](string_t.md),
    [`boolean_t`](boolean_t.md), [`number_integer_t`](number_integer_t.md), or
    [`number_unsigned_t`](number_unsigned_t.md), [`number_float_t`](number_float_t.md), or [`binary_t`](binary_t.md).
    Other types will not compile.

## Return value

1. copy of the JSON value, converted to `ValueType`
2. a copy of `#!cpp *this`, converted into `BasicJsonType`
3. pointer to the internally stored JSON value if the requested pointer type fits to the JSON value; `#!cpp nullptr`
   otherwise

## Exception safety

Depends on what `json_serializer<ValueType>` `from_json()` method throws for overloads (1) and (2); the JSON value
itself is never modified, since `get()` is a `#!cpp const` member function. No-throw guarantee for overload (3): this
function never throws exceptions.

## Exceptions

Depends on what `json_serializer<ValueType>` `from_json()` method throws

## Complexity

Depends on the `json_serializer<ValueType>::from_json()` implementation for overloads (1) and (2); constant for
overload (3).

## Notes

!!! danger "Undefined behavior for pointers"

    Writing data to the pointee (overload 3) of the result yields an undefined state.

!!! danger "Undefined behavior for numeric conversions"

    Conversions between numeric types are performed by the corresponding
    `from_json()` implementation using the target C++ type. When converting
    between numeric types, the library does not check whether the source
    value is representable by the target type.
    
    If the source value is outside the range of the target type, the behavior
    is the same as the corresponding C++ conversion. In particular, converting
    a floating-point value to an integer type that cannot represent the value
    results in undefined behavior.

    See [Number conversion](../../features/types/number_handling.md#number-conversion)
    for more information.

!!! note "`std::optional` conversions"

    Prior to version 3.13.0, `#!cpp get<std::optional<T>>()` (and other conversions to `std::optional<T>`) failed to
    compile in every configuration, due to an internal implementation bug that made the `from_json` overload for
    `std::optional` unreachable regardless of the [`JSON_USE_IMPLICIT_CONVERSIONS`](../macros/json_use_implicit_conversions.md)
    setting. This has been fixed.

## Examples

??? example "Example: (1) explicit conversion to compatible types"

    The example below shows several conversions from JSON values
    to other types. There a few things to note: (1) Floating-point numbers can
    be converted to integers, (2) A JSON array can be converted to a standard
    `std::vector<short>`, (3) A JSON object can be converted to C++
    associative containers such as `std::map<std::string, json>`.
        
    ```cpp
    --8<-- "examples/get__ValueType_const.cpp"
    ```
    
    Output:
    
    ```json
    --8<-- "examples/get__ValueType_const.output"
    ```

??? example "Example: (2) explicit conversion to another `basic_json` specialization"

    The example below shows how a `json` value is converted to an `ordered_json` value using `get<BasicJsonType>()`.

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

    Output:

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

??? example "Example: (3) explicit pointer access to the stored value"

    The example below shows how pointers to internal values of a JSON value can be requested. Note that no type
    conversions are made and a `#cpp nullptr` is returned if the value and the requested pointer type does not match.
        
    ```cpp
    --8<-- "examples/get__PointerType.cpp"
    ```
    
    Output:
    
    ```json
    --8<-- "examples/get__PointerType.output"
    ```

## See also

- [get_to](get_to.md) convert and write into a passed value
- [get_ptr](get_ptr.md) get a pointer to the stored value
- [get_ref](get_ref.md) get a reference to the stored value
- [operator ValueType](operator_ValueType.md) get a value via implicit conversion
- [Converting values](../../features/conversions.md) - the type conversions article

## Version history

1. Since version 2.1.0.
2. Since version 2.1.0. Extended to work with other specializations of `basic_json` in version 3.2.0.
3. Since version 1.0.0.