The C++ API uses Boost.Asio for asynchronous operations. There is a :cppspead2::thread_pool class (essentially the same as the Python :pyspead2.ThreadPool class). However, it is not required to use this, and you may for example run everything in one thread to avoid multi-threading issues.
spead2::thread_pool
Classes that perform asynchronous operations take a parameter of type :cppspead2::io_service_ref. This can be (implicitly) initialised from either a :cppboost::asio::io_service reference, a :cppspead2::thread_pool reference, or a :cppstd::shared_ptr\<spead2::thread_pool\>. In the last case, the receiving class retains a copy of the shared pointer, providing convenient lifetime management of a thread pool.
spead2::io_service_ref
A number of the APIs use callbacks. These follow the usual Boost.Asio guarantee that they will always be called from threads running :cppboost::asio::io_service::run. If using a :cpp~spead2::thread_pool, this will be one of the threads managed by the pool. Additionally, callbacks for a specific stream are serialised, but there may be concurrent callbacks associated with different streams.