[rustyconover]

density_sketch<T, Kernel, Allocator> takes an allocator, but a sketch with anything other than std::allocator didn't work, for two reasons:

1. The kernel only accepted std::vector<T>. gaussian_kernel::operator() took const std::vector<T>&, but the sketch stores its points as std::vector<T, Allocator>. Compaction and get_estimate() call the kernel on those points, so the sketch didn't compile:

   density_sketch_impl.hpp:120:41: error: no match for call to '(const datasketches::gaussian_kernel<double>) (const std::vector<double, ...test_allocator<double>>&, ...)'
   

   The kernel now takes any two vectors of T. Existing callers that pass std::vector<T> are unaffected.

2. An unused member was always default-constructed. The class had an Allocator allocator_ member that neither constructor initialized. That fails for allocators that require an instance, such as test_allocator, whose default constructor throws for exactly this reason. Nothing reads the member, and get_allocator() already returns the levels' allocator, so this removes it.

Test

Added density sketch: custom allocator to density_sketch_test.cpp, using test_allocator. It covers:

• 1,000 updates, which puts the sketch into estimation mode
• get_estimate
• merging a second sketch
• a serialize/deserialize round trip with equal estimates
• checking that every byte is released at the end

Without these changes it doesn't compile; with them, all 17 test suites pass under CTest (GCC 14, aarch64).

We found this in the DuckDB DataSketches extension. There, aggregate states keep their sketches in DuckDB's per-query arena through a custom allocator, so they count toward the memory limit and can spill.

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