Florian Albrechtskirchinger 6b97599a27

Fix C++20/gcc-12 issues (Part 2) (#3446 )

* Add C++20 3-way comparison operator and fix broken comparisons

Fixes #3207.
Fixes #3409.

* Fix iterators to meet (more) std::ranges requirements

Fixes #3130.
Related discussion: #3408

* Add note about CMake standard version selection to unit tests

Document how CMake chooses which C++ standard version to use when
building tests.

* Update documentation

* CI: add legacy discarded value comparison

* Fix internal linkage errors when building a module

2022-05-29 13:08:06 +02:00

2.3 KiB

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nlohmann::basic_json::operator<=>

// since C++20
class basic_json {
    std::partial_ordering operator<=>(const_reference rhs) const noexcept;  // (1)

    template<typename ScalarType>
    std::partial_ordering operator<=>(const ScalarType rhs) const noexcept; // (2)
};

3-way compares two JSON values producing a result of type std::partial_ordering according to the following rules:
- Two JSON values compare with a result of std::partial_ordering::unordered if either value is discarded.
- If both JSON values are of the same type, the result is produced by 3-way comparing their stored values using their respective operator<=>.
- Integer and floating-point numbers are converted to their common type and then 3-way compared using their respective operator<=>. For instance, comparing an integer and a floating-point value will 3-way compare the first value convertered to floating-point with the second value.
- Otherwise, yields a result by comparing the type (see value_t).
3-way compares a JSON value and a scalar or a scalar and a JSON value by converting the scalar to a JSON value and 3-way comparing both JSON values (see 1).

Template parameters

ScalarType: a scalar type according to std::is_scalar<ScalarType>::value

Parameters

rhs (in): second value to consider

Return value

the std::partial_ordering of the 3-way comparison of *this and rhs

Exception safety

No-throw guarantee: this function never throws exceptions.

Complexity

Linear.

Notes

!!! note "Comparing NaN"

- `NaN` values are unordered within the domain of numbers.
  The following comparisons all yield `std::partial_ordering::unordered`:
    1. Comparing a `NaN` with itself.
    2. Comparing a `NaN` with another `NaN`.
    3. Comparing a `NaN` and any other number.

Version history

Added in version 3.11.0.
Added in version 3.11.0.

2.3 KiB Raw Blame History