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

nlohmann::basic_json::value_t

enum class value_t : std::uint8_t {
    null,
    object,
    array,
    string,
    boolean,
    number_integer,
    number_unsigned,
    number_float,
    binary,
    discarded
};

This enumeration collects the different JSON types. It is internally used to distinguish the stored values, and the functions is_null, is_object, is_array, is_string, is_boolean, is_number (with is_number_integer, is_number_unsigned, and is_number_float), is_discarded, is_binary, is_primitive, and is_structured rely on it.

Notes

!!! note "Ordering"

The order of types is as follows:

1. `null`
2. `boolean`
3. `number_integer`, `number_unsigned`, `number_float`
4. `object`
5. `array`
6. `string`
7. `binary`

`discarded` is unordered.

!!! note "Types of numbers"

There are three enumerators for numbers (`number_integer`, `number_unsigned`, and `number_float`) to distinguish
between different types of numbers:

  - [`number_unsigned_t`](number_unsigned_t.md) for unsigned integers
  - [`number_integer_t`](number_integer_t.md) for signed integers
  - [`number_float_t`](number_float_t.md) for floating-point numbers or to approximate integers which do not fit
    into the limits of their respective type

!!! warning "Comparison operators"

`operator<` and `operator<=>` (since C++20) are overloaded and compare according to the ordering described above.
Until C++20 all other relational and equality operators yield results according to the integer value of each
enumerator. Since C++20 some compilers consider the _rewritten candidates_ generated from `operator<=>` during
overload resolution, while others do not. For predictable and portable behavior use:

  - `operator<` or `operator<=>` when wanting to compare according to the order described above
  - `operator==` or `operator!=` when wanting to compare according to each enumerators integer value

Examples

??? example

The following code how `type()` queries the `value_t` for all JSON types.

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

Output:

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

Version history

• Added in version 1.0.0.
• Added unsigned integer type in version 2.0.0.
• Added binary type in version 3.8.0.