mirror of
https://github.com/nlohmann/json.git
synced 2026-07-09 20:15:12 +00:00
deploy: 7c9208bfb3
This commit is contained in:
nlohmann
@@ -0,0 +1,231 @@
|
||||
# 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)
|
||||
|
||||
// since C++20
|
||||
class basic_json {
|
||||
bool operator==(const_reference rhs) const noexcept; // (1)
|
||||
|
||||
template<typename ScalarType>
|
||||
bool operator==(ScalarType rhs) const noexcept; // (2)
|
||||
};
|
||||
```
|
||||
|
||||
1. Compares two JSON values for equality according to the following rules:
|
||||
|
||||
- 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==`.
|
||||
- Integer and floating-point numbers are automatically converted before comparison.
|
||||
|
||||
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.
|
||||
|
||||
## 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 the values `lhs`/`*this` and `rhs` are equal
|
||||
|
||||
## Exception safety
|
||||
|
||||
No-throw guarantee: this function never throws exceptions.
|
||||
|
||||
## Complexity
|
||||
|
||||
Linear.
|
||||
|
||||
## Notes
|
||||
|
||||
Comparing special values
|
||||
|
||||
- `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.
|
||||
- JSON `null` values are all equal.
|
||||
- Discarded values never compare equal to themselves.
|
||||
|
||||
Comparing floating-point numbers
|
||||
|
||||
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
|
||||
|
||||
```
|
||||
template<typename T, typename = typename std::enable_if<std::is_floating_point<T>::value, T>::type>
|
||||
inline bool is_same(T a, T b, T epsilon = std::numeric_limits<T>::epsilon()) noexcept
|
||||
{
|
||||
return std::abs(a - b) <= epsilon;
|
||||
}
|
||||
```
|
||||
|
||||
Or you can define your own equality function like this:
|
||||
|
||||
```
|
||||
bool my_equal(const_reference lhs, const_reference rhs)
|
||||
{
|
||||
const auto lhs_type = lhs.type();
|
||||
const auto rhs_type = rhs.type();
|
||||
if (lhs_type == rhs_type)
|
||||
{
|
||||
switch(lhs_type)
|
||||
// self_defined case
|
||||
case value_t::number_float:
|
||||
return std::abs(lhs - rhs) <= std::numeric_limits<float>::epsilon();
|
||||
// other cases remain the same with the original
|
||||
...
|
||||
}
|
||||
...
|
||||
}
|
||||
```
|
||||
|
||||
Comparing different `basic_json` specializations
|
||||
|
||||
Comparing different `basic_json` specializations can have surprising effects. For instance, the result of comparing the JSON objects
|
||||
|
||||
```
|
||||
{
|
||||
"version": 1,
|
||||
"type": "integer"
|
||||
}
|
||||
```
|
||||
|
||||
and
|
||||
|
||||
```
|
||||
{
|
||||
"type": "integer",
|
||||
"version": 1
|
||||
}
|
||||
```
|
||||
|
||||
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:
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <iomanip>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
int main()
|
||||
{
|
||||
nlohmann::json uj1 = {{"version", 1}, {"type", "integer"}};
|
||||
nlohmann::json uj2 = {{"type", "integer"}, {"version", 1}};
|
||||
|
||||
nlohmann::ordered_json oj1 = {{"version", 1}, {"type", "integer"}};
|
||||
nlohmann::ordered_json oj2 = {{"type", "integer"}, {"version", 1}};
|
||||
|
||||
std::cout << std::boolalpha << (uj1 == uj2) << '\n' << (oj1 == oj2) << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
true
|
||||
false
|
||||
```
|
||||
|
||||
## 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 = 17.000000000000001L;
|
||||
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] false
|
||||
{"A":"a","B":"b"} == {"A":"a","B":"b"} true
|
||||
17 == 17.0 true
|
||||
"foo" == "bar" false
|
||||
```
|
||||
|
||||
Example
|
||||
|
||||
The example demonstrates comparing several JSON types against the null pointer (JSON `null`).
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
int main()
|
||||
{
|
||||
// create several JSON values
|
||||
json array = {1, 2, 3};
|
||||
json object = {{"A", "a"}, {"B", "b"}};
|
||||
json number = 17;
|
||||
json string = "foo";
|
||||
json null;
|
||||
|
||||
// output values and comparisons
|
||||
std::cout << std::boolalpha;
|
||||
std::cout << array << " == nullptr " << (array == nullptr) << '\n';
|
||||
std::cout << object << " == nullptr " << (object == nullptr) << '\n';
|
||||
std::cout << number << " == nullptr " << (number == nullptr) << '\n';
|
||||
std::cout << string << " == nullptr " << (string == nullptr) << '\n';
|
||||
std::cout << null << " == nullptr " << (null == nullptr) << '\n';
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
[1,2,3] == nullptr false
|
||||
{"A":"a","B":"b"} == nullptr false
|
||||
17 == nullptr false
|
||||
"foo" == nullptr false
|
||||
null == nullptr true
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [operator!=](https://json.nlohmann.me/api/basic_json/operator_ne/index.md) compare for inequality
|
||||
- [operator\<=>](https://json.nlohmann.me/api/basic_json/operator_spaceship/index.md) comparison: 3-way (C++20)
|
||||
|
||||
## Version history
|
||||
|
||||
1. Added in version 1.0.0. Added C++20 member functions in version 3.11.0.
|
||||
1. Added in version 1.0.0. Added C++20 member functions in version 3.11.0.
|
||||
Reference in New Issue
Block a user