json/api/basic_json/operator_spaceship.md

# <small>nlohmann::basic_json::</small>operator<=>

```cpp
// since C++20
class basic_json {
    std::partial_ordering operator<=>(const_reference rhs) const noexcept;  // (1)

    template<typename ScalarType>
    std::partial_ordering operator<=>(const ScalarType rhs) const noexcept(/* see below */); // (2)
};
```

1. 3-way compares two JSON values producing a result of type `std::partial_ordering` according to the following rules:
    - Two JSON values compare with a result of `std::partial_ordering::unordered` if either value is discarded.
    - If both JSON values are of the same type, the result is produced by 3-way comparing their stored values using
      their respective `operator<=>`.
    - Integer and floating-point numbers are converted to their common type and then 3-way compared using their
      respective `operator<=>`.
      For instance, comparing an integer and a floating-point value will 3-way compare the first value converted to
      floating-point with the second value.
    - Otherwise, yields a result by comparing the type (see [`value_t`](value_t.md)).
  
2. 3-way compares a JSON value and a scalar or a scalar and a JSON value by converting the scalar to a JSON value and
   3-way comparing both JSON values (see 1).

## Template parameters

`ScalarType`
:   a scalar type according to `std::is_scalar<ScalarType>::value`

## Parameters

`rhs` (in)
:   second value to consider 

## Return value

the `std::partial_ordering` of the 3-way comparison of `*this` and `rhs`

## Exception safety

1. No-throw guarantee: this function never throws exceptions.
2. No-throw guarantee if converting the scalar to a JSON value cannot throw, as for numbers, Booleans, and
   `#!cpp nullptr`; the function is `#!cpp noexcept` exactly in that case. Otherwise, it throws what the conversion
   throws, for example `std::bad_alloc` when converting a string, or
   [`out_of_range.410`](../../home/exceptions.md#jsonexceptionout_of_range410) for an enum value not mapped by
   [`NLOHMANN_JSON_SERIALIZE_ENUM_STRICT`](../macros/nlohmann_json_serialize_enum_strict.md).

## Complexity

Linear.

## Notes

!!! note "Comparing `NaN`"

    - `NaN` values are unordered within the domain of numbers.
      The following comparisons all yield `std::partial_ordering::unordered`:
        1. Comparing a `NaN` with itself.
        2. Comparing a `NaN` with another `NaN`.
        3. Comparing a `NaN` and any other number.

## Examples

??? example "Example: (1) comparing JSON values"

    The example demonstrates comparing several JSON values.
    
    ```cpp
    #include <compare>
    #include <iostream>
    #include <nlohmann/json.hpp>
    
    using json = nlohmann::json;
    
    const char* to_string(const std::partial_ordering& po)
    {
        if (std::is_lt(po))
        {
            return "less";
        }
        else if (std::is_gt(po))
        {
            return "greater";
        }
        else if (std::is_eq(po))
        {
            return "equivalent";
        }
        return "unordered";
    }
    
    int main()
    {
        // create several JSON values
        json array_1 = {1, 2, 3};
        json array_2 = {1, 2, 4};
        json object_1 = {{"A", "a"}, {"B", "b"}};
        json object_2 = {{"B", "b"}, {"A", "a"}};
        json number = 17;
        json string = "foo";
        json discarded = json(json::value_t::discarded);
    
        // output values and comparisons
        std::cout << array_1 << " <=> " << array_2 << " := " << to_string(array_1 <=> array_2) << '\n'; // *NOPAD*
        std::cout << object_1 << " <=> " << object_2 << " := " << to_string(object_1 <=> object_2) << '\n'; // *NOPAD*
        std::cout << string << " <=> " << number << " := " << to_string(string <=> number) << '\n'; // *NOPAD*
        std::cout << string << " <=> " << discarded << " := " << to_string(string <=> discarded) << '\n'; // *NOPAD*
    }
    
    ```
    
    Output:
    
    ```json
    [1,2,3] <=> [1,2,4] := less
    {"A":"a","B":"b"} <=> {"A":"a","B":"b"} := equivalent
    "foo" <=> 17 := greater
    "foo" <=> <discarded> := unordered
    
    ```

??? example "Example: (2) comparing JSON values and scalars"

    The example demonstrates comparing several JSON values and scalars.
    
    ```cpp
    #include <compare>
    #include <iostream>
    #include <nlohmann/json.hpp>
    
    using json = nlohmann::json;
    
    const char* to_string(const std::partial_ordering& po)
    {
        if (std::is_lt(po))
        {
            return "less";
        }
        else if (std::is_gt(po))
        {
            return "greater";
        }
        else if (std::is_eq(po))
        {
            return "equivalent";
        }
        return "unordered";
    }
    
    int main()
    {
        using float_limits = std::numeric_limits<json::number_float_t>;
        constexpr auto nan = float_limits::quiet_NaN();
    
        // create several JSON values
        json boolean = false;
        json number = 17;
        json string = "17";
    
        // output values and comparisons
        std::cout << std::boolalpha << std::fixed;
        std::cout << boolean << " <=> " << true << " := " << to_string(boolean <=> true) << '\n'; // *NOPAD*
        std::cout << number << " <=> " << 17.0 << " := " << to_string(number <=> 17.0) << '\n'; // *NOPAD*
        std::cout << number << " <=> " << nan << " := " << to_string(number <=> nan) << '\n'; // *NOPAD*
        std::cout << string << " <=> " << 17 << " := " << to_string(string <=> 17) << '\n'; // *NOPAD*
    }
    
    ```
    
    Output:
    
    ```json
    false <=> true := less
    17 <=> 17.000000 := equivalent
    17 <=> nan := unordered
    "17" <=> 17 := greater
    
    ```

## See also

- [**operator==**](operator_eq.md) - comparison: equal
- [**operator!=**](operator_ne.md) - comparison: not equal
- [**operator<**](operator_lt.md) - comparison: less than
- [**operator<=**](operator_le.md) - comparison: less than or equal
- [**operator>**](operator_gt.md) - comparison: greater than
- [**operator>=**](operator_ge.md) - comparison: greater than or equal

## Version history

1. Added in version 3.11.0.
2. Added in version 3.11.0.
   Made conditionally `#!cpp noexcept` in version 3.13.0; before, a throwing conversion called `std::terminate`.