{

counter() : count(0) { }

int add(int val) {

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

count += val;

return count;

}

int increment() {

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

return add(1);

}

boost::recursive_mutex mutex;

int count;

{

const int num_threads=4;

boost::thread_group threads;

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

threads.create_thread(&change_count);

threads.join_all();

return 0;