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name: cpp-header-api description: "Operate hnswlib's C++11 header-only API for HNSW and brute-force indexes, filters, persistence, deletion/replacement, stop conditions, multivector search, and bounded concurrency." metadata: disco-role: operating disable-model-invocation: true license: Apache 2.0

C++ header API

Use this sub-skill when the task is about the C++ or header-only interface: including hnswlib/hnswlib.h, constructing HierarchicalNSW, searchKnn, searchKnnCloserFirst, BaseFilterFunctor, markDelete, addPoint, saveIndex, CMake, epsilon search, multivector search, or C++ concurrency. This skill covers the 0.9.0 header API and the installed CPU-native extension facts; it does not expand the Python binding surface.

Operating contract

• Require an include root containing the public hnswlib/ headers. Do not assume a particular checkout layout, build directory, virtual environment, or dataset.
• Treat the library as header-only. A C++11 compiler compiles the implementation into the client; there is no hnswlib binary or CUDA API to link against.
• Keep the SpaceInterface object alive until every index using it has been destroyed. The index retains its distance function and parameter pointer.
• Use float with L2Space or InnerProductSpace for the normal vector API. Read api-reference.md before selecting an optional stop-condition or multivector signature.

Standard workflow

1. Normalize the request. Identify dimension, metric, capacity, whether labels can be deleted/replaced, query ordering, filter requirements, and whether the request is optional epsilon or multivector search.
2. Check the build boundary. Include <hnswlib/hnswlib.h>, compile as C++11, add the include root, and use -pthread (or the platform equivalent) for code using the library's mutexes/threads. Use workflows.md for direct compiler and CMake patterns.
3. Create the space before the index. Construct L2Space(dim) for squared Euclidean distance or InnerProductSpace(dim) for 1 - inner product. Construct HierarchicalNSW<float> with capacity, M, ef_construction, seed, and replacement policy. Prefer stack ownership or an RAII owner.
4. Populate deliberately. Pass a contiguous vector pointer and a stable, unique external label to addPoint. A repeated label updates its existing item. Increase capacity with resizeIndex before inserting, or use the persistence-based capacity extension described in the references.
5. Query with the intended ordering. searchKnn returns a priority queue whose top() is the farthest member of the retained result set. Drain and reverse it, or call searchKnnCloserFirst for a vector ordered closest first. A filter is a live pointer to a functor that accepts external labels.
6. Apply mutation preconditions. markDelete only marks; it does not remove graph connections. Replacement requires construction/load with allow_replace_deleted=true and addPoint(..., true). Do not call unmarkDelete as a recovery mechanism while replacement is enabled.
7. Persist and reload safely. Call saveIndex(file), destroy the index if desired, then construct the load form with the same compatible space. Set ef again after loading: it is not saved and loading initializes it to 10.
8. Verify a small invariant. Assert result count/order, filter acceptance, and a post-load query. Run the bundled smoke wrapper with an explicit include root; do not use a large stress or benchmark as the basic API gate.
9. Escalate optional APIs. For epsilon or multivector work, use the exact declarations and data layout in api-reference.md and the recipes in workflows.md. Do not infer a Python equivalent or invent a convenience overload.
10. Bound concurrency. Concurrent read-only searches are the normal safe pattern; concurrent insertion/update calls are demonstrated and internally label-serialized. Coordinate structural operations, persistence, resizing, destruction, and mixed read/write phases externally. See concurrency.md.

API routing map

• Metric and memory layout: api-reference.md.
• Construction, query, filter, persistence, and mutation recipes: workflows.md.
• Threads, compiler flags, OpenMP, native SIMD flags, and ownership: concurrency.md.
• Failure diagnosis, including low ef, capacity, paths, deletion policy, and benchmark exclusions: troubleshooting.md.
• Executable smoke check: scripts/run_cpp_smoke.sh --include-dir <include-root>.

Hard guardrails

• Do not claim CUDA support, GPU kernels, a compiled library ABI, or a required OpenMP dependency. The exposed C++ API is CPU-native and header-only.
• Do not treat a priority-queue pop sequence as closest-first. The queue's further-first behavior is intentional; the vector helper reverses it.
• Do not pass replace_deleted=true unless the index was created or loaded with replacement enabled. Do not use a filter to make an unavailable k appear available: filtering can legitimately return fewer than k items.
• Do not let a temporary query buffer, metric space, filter, stop condition, or data backing an in-flight operation go out of scope.
• Do not use the smoke result to make recall, latency, SIMD portability, or multi-thread stress claims. Those require a separately chosen workload and compatible hardware.

Completion checklist

Before handing off a C++ solution, confirm:

• the include root and compiler standard are explicit;
• the space outlives the index and matches the data dimension/metric;
• M, ef_construction, capacity, and query ef have an intentional value;
• queue versus closer-first semantics are documented at the call site;
• filters inspect external labels and tolerate fewer than k matches;
• deletion/replacement flags match the requested lifecycle;
• persistence reloads with a compatible space and resets ef explicitly;
• optional stop-condition signatures were copied from the bundled reference;
• no operation is concurrently resizing, saving, loading, or destroying the index;
• the smoke program compiles and passes with assertions.