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131 lines
4.1 KiB
Markdown
131 lines
4.1 KiB
Markdown
# <small>nlohmann::basic_json::</small>operator<
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```cpp
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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(/* see below */); // (2)
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template<typename ScalarType>
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bool operator<(ScalarType lhs, const const_reference rhs) noexcept(/* see below */); // (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
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following rules:
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- If either operand is discarded, the comparison yields `#!cpp 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
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is considered, which is:
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1. null
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2. boolean
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3. number (all types)
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4. object
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5. array
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6. string
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7. binary
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For instance, any boolean value is considered less than any string.
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2. Compares whether a JSON value is less than a scalar or a scalar is less than a JSON value by converting
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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`
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: a scalar type according to `std::is_scalar<ScalarType>::value`
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## Parameters
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`lhs` (in)
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: first value to consider
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`rhs` (in)
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: 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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1. No-throw guarantee: this function never throws exceptions.
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2. No-throw guarantee if converting the scalar to a JSON value cannot throw, as for numbers, Booleans, and
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`#!cpp nullptr`; the function is `#!cpp noexcept` exactly in that case. Otherwise, it throws what the conversion
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throws, for example `std::bad_alloc` when converting a string, or
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[`out_of_range.410`](../../home/exceptions.md#jsonexceptionout_of_range410) for an enum value not mapped by
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[`NLOHMANN_JSON_SERIALIZE_ENUM_STRICT`](../macros/nlohmann_json_serialize_enum_strict.md).
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## Complexity
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Linear.
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## Notes
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!!! note "Comparing `NaN`"
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`NaN` values are unordered within the domain of numbers.
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The following comparisons all yield `#!cpp false`:
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1. Comparing a `NaN` with itself.
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2. Comparing a `NaN` with another `NaN`.
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3. Comparing a `NaN` and any other number.
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!!! note "Operator overload resolution"
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Since C++20 overload resolution will consider the _rewritten candidate_ generated from
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[`operator<=>`](operator_spaceship.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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```cpp
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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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```json
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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<=>**](operator_spaceship.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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2. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0.
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Made conditionally `#!cpp noexcept` in version 3.13.0; before, a throwing conversion called `std::terminate`.
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