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107 lines
3.7 KiB
Markdown
107 lines
3.7 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 two JSON values for inequality. Returns `#!cpp !(lhs == rhs)`.
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- This means the comparison is simply the logical negation of `operator==`, including for special values like `NaN` and `discarded`.
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2. Compares a JSON value and a scalar or a scalar and a JSON value for inequality by converting the scalar to a JSON
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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 the values `lhs`/`*this` and `rhs` are not equal
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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 "C++20"
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Since C++20, `basic_json` declares no `operator!=`. The compiler rewrites `#!cpp a != b` as `#!cpp !(a == b)`
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using [`operator==`](operator_eq.md), so the result is the same as described above.
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!!! note "Comparing `NaN` and `discarded`"
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Since `operator!=` is defined as `!(a == b)`, the behavior for special values follows that of `operator==`:
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- For `NaN` values: `NaN == NaN` yields `#!cpp false`, so `NaN != NaN` yields `#!cpp true`.
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- For `discarded` values: `discarded == x` yields `#!cpp false` for any `x`, so `discarded != x` yields `#!cpp true`.
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## Examples
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??? example "Example: (1) compare JSON values"
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The example demonstrates comparing several JSON types.
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```cpp
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--8<-- "examples/operator__notequal.cpp"
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```
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Output:
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```json
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--8<-- "examples/operator__notequal.output"
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```
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??? example "Example: (2) compare JSON values with `#!cpp nullptr`"
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The example demonstrates comparing several JSON types against the null pointer (JSON `#!json null`).
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```cpp
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--8<-- "examples/operator__notequal__nullptr_t.cpp"
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```
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Output:
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```json
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--8<-- "examples/operator__notequal__nullptr_t.output"
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```
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## See also
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- [operator==](operator_eq.md) comparison: equal
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- [operator<=>](operator_spaceship.md) comparison: 3-way (C++20)
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## Version history
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1. Added in version 1.0.0. Added a C++20 member function in version 3.11.0. Changed in version 3.13.0 to remove
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special-casing for `NaN` and `discarded` values; `operator!=` now consistently means `!(a == b)`. Removed the C++20
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member function in version 3.13.0; since C++20, the compiler rewrites `a != b` using `operator==`.
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2. Added in version 1.0.0. Changed in version 3.13.0 to remove special-casing for `NaN` and `discarded` values;
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`operator!=` now consistently means `!(a == b)`. Since C++20, the compiler rewrites `a != b` using `operator==`.
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Made conditionally `#!cpp noexcept` in version 3.13.0; before, a throwing conversion called `std::terminate`.
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