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nlohmann
116 lines
3.3 KiB
Markdown
116 lines
3.3 KiB
Markdown
# nlohmann::basic_json::operator\<
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```
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// until C++20
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bool operator<(const_reference lhs, const_reference rhs) noexcept; // (1)
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template<typename ScalarType>
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bool operator<(const_reference lhs, const ScalarType rhs) noexcept; // (2)
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template<typename ScalarType>
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bool operator<(ScalarType lhs, const const_reference rhs) noexcept; // (2)
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```
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1. Compares whether one JSON value `lhs` is less than another JSON value `rhs` according to the following rules:
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- If either operand is discarded, the comparison yields `false`.
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- If both operands have the same type, the values are compared using their respective `operator<`.
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- Integer and floating-point numbers are automatically converted before comparison.
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- In case `lhs` and `rhs` have different types, the values are ignored and the order of the types is considered, which is:
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1. null
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1. boolean
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1. number (all types)
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1. object
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1. array
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1. string
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1. binary For instance, any boolean value is considered less than any string.
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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.
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## Template parameters
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`ScalarType` : a scalar type according to `std::is_scalar<ScalarType>::value`
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## Parameters
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`lhs` (in) : first value to consider
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`rhs` (in) : second value to consider
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## Return value
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whether `lhs` is less than `rhs`
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## Exception safety
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No-throw guarantee: this function never throws exceptions.
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## Complexity
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Linear.
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## Notes
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Comparing `NaN`
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`NaN` values are unordered within the domain of numbers. The following comparisons all yield `false`:
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1. Comparing a `NaN` with itself.
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1. Comparing a `NaN` with another `NaN`.
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1. Comparing a `NaN` and any other number.
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Operator overload resolution
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Since C++20 overload resolution will consider the *rewritten candidate* generated from [`operator<=>`](https://json.nlohmann.me/api/basic_json/operator_spaceship/index.md).
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## Examples
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Example
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The example demonstrates comparing several JSON types.
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```
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#include <iostream>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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// create several JSON values
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json array_1 = {1, 2, 3};
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json array_2 = {1, 2, 4};
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json object_1 = {{"A", "a"}, {"B", "b"}};
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json object_2 = {{"B", "b"}, {"A", "a"}};
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json number_1 = 17;
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json number_2 = 17.0000000000001L;
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json string_1 = "foo";
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json string_2 = "bar";
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// output values and comparisons
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std::cout << std::boolalpha;
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std::cout << array_1 << " == " << array_2 << " " << (array_1 < array_2) << '\n';
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std::cout << object_1 << " == " << object_2 << " " << (object_1 < object_2) << '\n';
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std::cout << number_1 << " == " << number_2 << " " << (number_1 < number_2) << '\n';
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std::cout << string_1 << " == " << string_2 << " " << (string_1 < string_2) << '\n';
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}
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```
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Output:
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```
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[1,2,3] == [1,2,4] true
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{"A":"a","B":"b"} == {"A":"a","B":"b"} false
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17 == 17.0000000000001 true
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"foo" == "bar" false
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```
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## See also
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- [**operator\<=>**](https://json.nlohmann.me/api/basic_json/operator_spaceship/index.md) comparison: 3-way
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## Version history
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1. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0.
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1. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0.
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