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

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* 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
This commit is contained in:
Florian Albrechtskirchinger
2022-05-29 13:08:06 +02:00
committed by GitHub
parent ede6667858
commit 6b97599a27
35 changed files with 1963 additions and 690 deletions
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61
docs/mkdocs/docs/api/basic_json/operator_lt.md
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@@ -1,31 +1,34 @@
# <small>nlohmann::basic_json::</small>operator<
```cpp
bool operator<(const_reference lhs, const_reference rhs) noexcept;
// 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;
bool operator<(const_reference lhs, const ScalarType rhs) noexcept; // (2)
template<typename ScalarType>
bool operator<(ScalarType lhs, const const_reference rhs) noexcept;
bool operator<(ScalarType lhs, const const_reference rhs) noexcept; // (2)
```
Compares whether one JSON value `lhs` is less than another JSON value `rhs` according to the following rules:
1. Compares whether one JSON value `lhs` is less than another JSON value `rhs` according to the
following rules:
- If either operand is discarded, the comparison yields `#!cpp false`.
- If both operands have the same type, the values are compared using their respective `operator<`.
- Integer and floating-point numbers are automatically converted before comparison.
- In case `lhs` and `rhs` have different types, the values are ignored and the order of the types
is considered, which is:
1. null
2. boolean
3. number (all types)
4. object
5. array
6. string
7. binary
For instance, any boolean value is considered less than any string.
- If `lhs` and `rhs` have the same type, the values are compared using the default `<` operator.
- Integer and floating-point numbers are automatically converted before comparison
- Discarded values a
- In case `lhs` and `rhs` have different types, the values are ignored and the order of the types is considered, which
is:
1. null
2. boolean
3. number (all types)
4. object
5. array
6. string
7. binary
For instance, any boolean value is considered less than any string.
2. Compares wether a JSON value is less than a scalar or a scalar is less than a JSON value by converting
the scalar to a JSON value and comparing both JSON values according to 1.
## Template parameters
@@ -52,6 +55,21 @@ No-throw guarantee: this function never throws exceptions.
Linear.
## Notes
!!! note "Comparing `NaN`"
`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.
!!! note "Operator overload resolution"
Since C++20 overload resolution will consider the _rewritten candidate_ generated from
[`operator<=>`](operator_spaceship.md).
## Examples
??? example
@@ -68,6 +86,11 @@ Linear.
--8<-- "examples/operator__less.output"
```
## See also
- [**operator<=>**](operator_spaceship.md) comparison: 3-way
## Version history
- Added in version 1.0.0.
1. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0.
2. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0.