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nlohmann
232 lines
6.2 KiB
Markdown
232 lines
6.2 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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// since C++20
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class basic_json {
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bool operator==(const_reference rhs) const noexcept; // (1)
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template<typename ScalarType>
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bool operator==(ScalarType rhs) const noexcept; // (2)
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};
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```
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1. Compares two JSON values for equality according to the following rules:
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- Two JSON values are equal if (1) neither value is discarded, and (2) they are of the same type and their stored values are the same according to their respective `operator==`.
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- Integer and floating-point numbers are automatically converted before comparison.
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1. Compares a JSON value and a scalar or a scalar and a JSON value for equality 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 the values `lhs`/`*this` and `rhs` are equal
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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 special values
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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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- JSON `null` values are all equal.
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- Discarded values never compare equal to themselves.
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Comparing floating-point numbers
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Floating-point numbers inside JSON values numbers are compared with `json::number_float_t::operator==` which is `double::operator==` by default. To compare floating-point while respecting an epsilon, an alternative [comparison function](https://github.com/mariokonrad/marnav/blob/master/include/marnav/math/floatingpoint.hpp#L34-#L39) could be used, for instance
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```
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template<typename T, typename = typename std::enable_if<std::is_floating_point<T>::value, T>::type>
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inline bool is_same(T a, T b, T epsilon = std::numeric_limits<T>::epsilon()) noexcept
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{
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return std::abs(a - b) <= epsilon;
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}
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```
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Or you can define your own equality function like this:
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```
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bool my_equal(const_reference lhs, const_reference rhs)
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{
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const auto lhs_type = lhs.type();
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const auto rhs_type = rhs.type();
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if (lhs_type == rhs_type)
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{
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switch(lhs_type)
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// self_defined case
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case value_t::number_float:
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return std::abs(lhs - rhs) <= std::numeric_limits<float>::epsilon();
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// other cases remain the same with the original
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...
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}
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...
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}
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```
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Comparing different `basic_json` specializations
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Comparing different `basic_json` specializations can have surprising effects. For instance, the result of comparing the JSON objects
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```
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{
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"version": 1,
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"type": "integer"
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}
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```
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and
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```
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{
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"type": "integer",
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"version": 1
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}
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```
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depends on whether [`nlohmann::json`](https://json.nlohmann.me/api/json/index.md) or [`nlohmann::ordered_json`](https://json.nlohmann.me/api/ordered_json/index.md) is used:
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```
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#include <iostream>
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#include <iomanip>
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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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nlohmann::json uj1 = {{"version", 1}, {"type", "integer"}};
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nlohmann::json uj2 = {{"type", "integer"}, {"version", 1}};
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nlohmann::ordered_json oj1 = {{"version", 1}, {"type", "integer"}};
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nlohmann::ordered_json oj2 = {{"type", "integer"}, {"version", 1}};
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std::cout << std::boolalpha << (uj1 == uj2) << '\n' << (oj1 == oj2) << std::endl;
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}
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```
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Output:
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```
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true
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false
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```
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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 = 17.000000000000001L;
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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] false
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{"A":"a","B":"b"} == {"A":"a","B":"b"} true
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17 == 17.0 true
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"foo" == "bar" false
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```
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Example
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The example demonstrates comparing several JSON types against the null pointer (JSON `null`).
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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, 2, 3};
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json object = {{"A", "a"}, {"B", "b"}};
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json number = 17;
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json string = "foo";
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json null;
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// output values and comparisons
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std::cout << std::boolalpha;
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std::cout << array << " == nullptr " << (array == nullptr) << '\n';
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std::cout << object << " == nullptr " << (object == nullptr) << '\n';
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std::cout << number << " == nullptr " << (number == nullptr) << '\n';
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std::cout << string << " == nullptr " << (string == nullptr) << '\n';
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std::cout << null << " == nullptr " << (null == nullptr) << '\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] == nullptr false
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{"A":"a","B":"b"} == nullptr false
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17 == nullptr false
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"foo" == nullptr false
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null == nullptr true
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```
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## See also
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- [operator!=](https://json.nlohmann.me/api/basic_json/operator_ne/index.md) compare for inequality
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- [operator\<=>](https://json.nlohmann.me/api/basic_json/operator_spaceship/index.md) comparison: 3-way (C++20)
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## Version history
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1. Added in version 1.0.0. Added C++20 member functions in version 3.11.0.
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1. Added in version 1.0.0. Added C++20 member functions in version 3.11.0.
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