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api/basic_json/operator_spaceship/index.md

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+# nlohmann::basic_json::operator\<=>
+
+```
+// 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; // (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`](https://json.nlohmann.me/api/basic_json/value_t/index.md)).
+
+1. 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
+
+No-throw guarantee: this function never throws exceptions.
+
+## Complexity
+
+Linear.
+
+## Notes
+
+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.
+  1. Comparing a `NaN` with another `NaN`.
+  1. Comparing a `NaN` and any other number.
+
+## Examples
+
+Example: (1) comparing JSON values
+
+The example demonstrates comparing several JSON values.
+
+```
+#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:
+
+```
+[1,2,3] <=> [1,2,4] := less
+{"A":"a","B":"b"} <=> {"A":"a","B":"b"} := equivalent
+"foo" <=> 17 := greater
+"foo" <=> <discarded> := unordered
+```
+
+Example: (2) comparing JSON values and scalars
+
+The example demonstrates comparing several JSON values and scalars.
+
+```
+#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:
+
+```
+false <=> true := less
+17 <=> 17.000000 := equivalent
+17 <=> nan := unordered
+"17" <=> 17 := greater
+```
+
+## See also
+
+- [**operator==**](https://json.nlohmann.me/api/basic_json/operator_eq/index.md) - comparison: equal
+- [**operator!=**](https://json.nlohmann.me/api/basic_json/operator_ne/index.md) - comparison: not equal
+- [**operator\<**](https://json.nlohmann.me/api/basic_json/operator_lt/index.md) - comparison: less than
+- [**operator\<=**](https://json.nlohmann.me/api/basic_json/operator_le/index.md) - comparison: less than or equal
+- [**operator>**](https://json.nlohmann.me/api/basic_json/operator_gt/index.md) - comparison: greater than
+- [**operator>=**](https://json.nlohmann.me/api/basic_json/operator_ge/index.md) - comparison: greater than or equal
+
+## Version history
+
+1. Added in version 3.11.0.
+1. Added in version 3.11.0.