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

nlohmann::basic_json::get

// (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 and DefaultConstructible. The value is converted by calling the json_serializer<ValueType> from_json() method.

   The function is equivalent to executing

   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 and not DefaultConstructible, the value is converted by calling the json_serializer<ValueType> from_json() method.

   The function is then equivalent to executing

   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

   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, object_t, string_t, boolean_t, number_integer_t, or number_unsigned_t, number_float_t, or binary_t. 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 "Booleans as numbers"

A JSON boolean converts to `#!cpp 0` or `#!cpp 1` for every arithmetic type **except** [`number_integer_t`](number_integer_t.md),
[`number_unsigned_t`](number_unsigned_t.md) and [`number_float_t`](number_float_t.md) (by default `#!cpp std::int64_t`,
`#!cpp std::uint64_t`, and `#!cpp double`), for which `#!cpp get<T>()` throws
[`type_error.302`](../../home/exceptions.md#jsonexceptiontype_error302) ("type must be number, but is boolean"). So
`#!cpp json(true).get<int>()` is `#!cpp 1`, while `#!cpp json(true).get<std::int64_t>()` and
`#!cpp json(true).get<double>()` throw. This is the behavior of all 3.x versions;
[`basic_json_view::get<T>()`](../basic_json_view/get.md) behaves the same.

!!! note "std::u8string conversions"

Since version 3.13.0, `#!cpp get<std::u8string>()` (C++20) reads a JSON string; it also accepts an array of numbers,
the form in which version 3.12.0 stored a `#!cpp std::u8string`. Any other JSON type throws
[`type_error.302`](../../home/exceptions.md#jsonexceptiontype_error302) ("type must be string or array").

!!! 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 - convert and write into a passed value
• get_ptr - get a pointer to the stored value
• get_ref - get a reference to the stored value
• operator ValueType - get a value via implicit conversion
• Converting values - the type conversions article
• basic_json_view::get - the same conversion on a zero-copy view (many types are converted without ever building a basic_json value)
• Arbitrary Type Conversions - make your own types usable with get

Version history

1. Added in version 2.1.0.
2. Added in version 2.1.0. Extended to work with other specializations of basic_json in version 3.1.2.
3. Added in version 1.0.0.