{

counter() : count(0) { }

int increment() {

boost::unique_lock<boost::mutex> scoped_lock(mutex);

return ++count;

}

boost::mutex mutex;

int count;

{

int i = c.increment();

boost::unique_lock<boost::mutex> scoped_lock(io_mutex);

std::cout << "count == " << i << std::endl;

{

const int num_threads = 4;

boost::thread_group thrds;

for (int i=0; i < num_threads; ++i)

thrds.create_thread(&change_count);

thrds.join_all();

return 0;