Yes, mongocxx::pool is thread-safe. You may access it concurrently from multiple threads. However, the individual mongocxx::client objects returned from the pool are not thread-safe, nor are the subordinate objects like collection or database obtained from the client - you must not share them between threads.

Note also that your example (which is not copied verbatim from the website but is modified from one of the examples), contains a serious programming error.

This line:

   mongocxx::client *c= pool.acquire().get();

Will obtain a pool entry, then extracts a bare pointer from it. However, the pool entry will be destroyed at the end of the statement, causing the underlying client object to be returned to the pool, allowing another thread to potentially pick it up while you continue to use it.

You should write this as:

mongocxx::instance instance{};
mongocxx::pool pool {mongocxx::uri{}};

auto threadfunc = [](mongocxx::client &client, stdx::string_view dbname) {
    client[dbname]["col"].insert({});
}
// don't even bother sharing clients. Just give each thread its own,
std::thread([]() {
    // pool.acquire() returns a mongo::pool::entry type
    auto c = pool.acquire();
    threadfunc(*c, "db1");
    threadfunc(*c, "db2");
});

std::thread([]() {
    auto c = pool.acquire();
    threadfunc(*c, "db2");
    threadfunc(*c, "db1");
});

That way, each thread retains the pool entry until it is finished using it, at which point it will be automatically returned when the unique_ptr in c is destroyed.