mirror of
https://github.com/nlohmann/json.git
synced 2026-07-09 03:55:09 +00:00
nlohmann
174 lines
4.6 KiB
Markdown
174 lines
4.6 KiB
Markdown
# <small>nlohmann::basic_json::</small>operator==
|
|
|
|
```cpp
|
|
// 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.
|
|
|
|
2. 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
|
|
|
|
!!! note "Comparing special values"
|
|
|
|
- `NaN` values are unordered within the domain of numbers.
|
|
The following comparisons all yield `#!cpp false`:
|
|
1. Comparing a `NaN` with itself.
|
|
2. Comparing a `NaN` with another `NaN`.
|
|
3. Comparing a `NaN` and any other number.
|
|
- JSON `#!cpp null` values are all equal.
|
|
- Discarded values never compare equal to themselves.
|
|
|
|
!!! note "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
|
|
|
|
```cpp
|
|
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:
|
|
|
|
```cpp
|
|
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
|
|
...
|
|
}
|
|
...
|
|
}
|
|
```
|
|
|
|
!!! note "Comparing different `basic_json` specializations"
|
|
|
|
Comparing different `basic_json` specializations can have surprising effects. For instance, the result of comparing
|
|
the JSON objects
|
|
|
|
```json
|
|
{
|
|
"version": 1,
|
|
"type": "integer"
|
|
}
|
|
```
|
|
|
|
and
|
|
|
|
```json
|
|
{
|
|
"type": "integer",
|
|
"version": 1
|
|
}
|
|
```
|
|
|
|
depends on whether [`nlohmann::json`](../json.md) or [`nlohmann::ordered_json`](../ordered_json.md) is used:
|
|
|
|
```cpp
|
|
--8<-- "examples/operator__equal__specializations.cpp"
|
|
```
|
|
|
|
Output:
|
|
|
|
```json
|
|
--8<-- "examples/operator__equal__specializations.output"
|
|
```
|
|
|
|
## Examples
|
|
|
|
??? example
|
|
|
|
The example demonstrates comparing several JSON types.
|
|
|
|
```cpp
|
|
--8<-- "examples/operator__equal.cpp"
|
|
```
|
|
|
|
Output:
|
|
|
|
```json
|
|
--8<-- "examples/operator__equal.output"
|
|
```
|
|
|
|
??? example
|
|
|
|
The example demonstrates comparing several JSON types against the null pointer (JSON `#!json null`).
|
|
|
|
```cpp
|
|
--8<-- "examples/operator__equal__nullptr_t.cpp"
|
|
```
|
|
|
|
Output:
|
|
|
|
```json
|
|
--8<-- "examples/operator__equal__nullptr_t.output"
|
|
```
|
|
|
|
## See also
|
|
|
|
- [operator!=](operator_ne.md) compare for inequality
|
|
- [operator<=>](operator_spaceship.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.
|
|
2. Added in version 1.0.0. Added C++20 member functions in version 3.11.0.
|