Method memory_pool

cpp/src/arrow/memory_pool_test.cc:43–47

#ifdef ARROW_JEMALLOC
struct JemallocMemoryPoolFactory {
  static MemoryPool* memory_pool() {
    MemoryPool* pool;
    ABORT_NOT_OK(jemalloc_memory_pool(&pool));
    return pool;
  }
};
#endif