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api/basic_json/operator_lt/index.md

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+# nlohmann::basic_json::operator\<
+
+```
+// until C++20
+bool operator<(const_reference lhs, const_reference rhs) noexcept;   // (1)
+
+template<typename ScalarType>
+bool operator<(const_reference lhs, const ScalarType rhs) noexcept;  // (2)
+
+template<typename ScalarType>
+bool operator<(ScalarType lhs, const const_reference rhs) noexcept;  // (2)
+```
+
+1. Compares whether one JSON value `lhs` is less than another JSON value `rhs` according to the following rules:
+
+   - If either operand is discarded, the comparison yields `false`.
+   - If both operands have the same type, the values are compared using their respective `operator<`.
+   - Integer and floating-point numbers are automatically converted before comparison.
+   - In case `lhs` and `rhs` have different types, the values are ignored and the order of the types is considered, which is:
+     1. null
+     1. boolean
+     1. number (all types)
+     1. object
+     1. array
+     1. string
+     1. binary For instance, any boolean value is considered less than any string.
+
+1. Compares whether a JSON value is less than a scalar or a scalar is less than a JSON value by converting the scalar to a JSON value and comparing both JSON values according to 1.
+
+## Template parameters
+
+`ScalarType` : a scalar type according to `std::is_scalar<ScalarType>::value`
+
+## Parameters
+
+`lhs` (in) : first value to consider
+
+`rhs` (in) : second value to consider
+
+## Return value
+
+whether `lhs` is less than `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 `false`:
+
+1. Comparing a `NaN` with itself.
+1. Comparing a `NaN` with another `NaN`.
+1. Comparing a `NaN` and any other number.
+
+Operator overload resolution
+
+Since C++20 overload resolution will consider the *rewritten candidate* generated from [`operator<=>`](https://json.nlohmann.me/api/basic_json/operator_spaceship/index.md).
+
+## Examples
+
+Example
+
+The example demonstrates comparing several JSON types.
+
+```
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+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_1 = 17;
+    json number_2 = 17.0000000000001L;
+    json string_1 = "foo";
+    json string_2 = "bar";
+
+    // output values and comparisons
+    std::cout << std::boolalpha;
+    std::cout << array_1 << " == " << array_2 << " " << (array_1 < array_2) << '\n';
+    std::cout << object_1 << " == " << object_2 << " " << (object_1 < object_2) << '\n';
+    std::cout << number_1 << " == " << number_2 << " " << (number_1 < number_2) << '\n';
+    std::cout << string_1 << " == " << string_2 << " " << (string_1 < string_2) << '\n';
+}
+```
+
+Output:
+
+```
+[1,2,3] == [1,2,4] true
+{"A":"a","B":"b"} == {"A":"a","B":"b"} false
+17 == 17.0000000000001 true
+"foo" == "bar" false
+```
+
+## See also
+
+- [**operator\<=>**](https://json.nlohmann.me/api/basic_json/operator_spaceship/index.md) comparison: 3-way
+
+## Version history
+
+1. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0.
+1. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0.