Add a custom implementation of boost::adaptors::uniqued that is compatible
with C++20 ranges library. This bridges the gap between Boost.Range and
the C++ standard library ranges until std::views::unique becomes available
in C++26. Currently, the unique view is included in
[P2214](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2760r0.html)
"A Plan for C++ Ranges Evolution", which targets C++26.
The implementation provides:
- A lazy view adaptor that presents unique consecutive elements
- No modification of source range
- Compatibility with C++20 range views and concepts
- Lighter header dependencies compared to Boost
This resolves compilation errors when piping C++20 range views to
boost::adaptors::uniqued, which fails due to concept requirements
mismatch. For example:
```c++
auto range = std::views::take(n) | boost::adaptors::uniqued; // fails
```
This change also offers us a lightweight solution in terms of smaller
header dependency.
While std::ranges::unique exists in C++23, it's an eager algorithm that
modifies the source range in-place, unlike boost::adaptors::uniqued which
is a lazy view. The proposed std::views::unique (P2214) targeting C++26
would provide this functionality, but is not yet available.
This implementation serves as an interim solution for filtering consecutive
duplicate elements using range views until std::views::unique is
standardized.
For more details on the differences between `std::ranges::unique` and
`boost::adaptors::uniqued`:
- boost::adaptors::uniqued is a view adaptor that creates a lazy view over the original range. It:
* Doesn't modify the source range
* Returns a view that presents unique consecutive elements
* Is non-destructive and lazy-evaluated
* Can be composed with other views
- std::ranges::unique is an algorithm that:
* Modifies the source range in-place
* Removes consecutive duplicates by shifting elements
* Returns an iterator to the new logical end
* Cannot be used as a view or composed with other range adaptors
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
