json/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.

```mermaid
flowchart LR
    A[null] --> B[boolean]
    B --> C["number_integer / number_unsigned / number_float"]
    C --> D[object]
    D --> E[array]
    E --> F[string]
    F --> G[binary]
```

!!! 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
#include <iostream>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    // create JSON values
    json j_null;
    json j_boolean = true;
    json j_number_integer = -17;
    json j_number_unsigned = 42u;
    json j_number_float = 23.42;
    json j_object = {{"one", 1}, {"two", 2}};
    json j_array = {1, 2, 4, 8, 16};
    json j_string = "Hello, world";

    // call type()
    std::cout << std::boolalpha;
    std::cout << (j_null.type() == json::value_t::null) << '\n';
    std::cout << (j_boolean.type() == json::value_t::boolean) << '\n';
    std::cout << (j_number_integer.type() == json::value_t::number_integer) << '\n';
    std::cout << (j_number_unsigned.type() == json::value_t::number_unsigned) << '\n';
    std::cout << (j_number_float.type() == json::value_t::number_float) << '\n';
    std::cout << (j_object.type() == json::value_t::object) << '\n';
    std::cout << (j_array.type() == json::value_t::array) << '\n';
    std::cout << (j_string.type() == json::value_t::string) << '\n';
}

```

Output:

```json
true
true
true
true
true
true
true
true

```

See also

• type return the type of the JSON value
• type_name return the type as string
• operator value_t return the type of the JSON value
• is_primitive return whether the type is primitive
• is_structured return whether the type is structured

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.