int nreaders = 0;
mutex mreaders;
mutex mvalue;

void do_write(…)
{
	lock_guard<mutex> lock(mvalue);
	perform write
}

foo_t do_read(…)
{
	{
		lock_guard<mutex> rlock(mreaders);
		nreaders++;
		if (nreaders==1)
			mvalue.lock();
	}

	perform read and save in rv

	{
		lock_guard<mutex> rlock(mreaders);
		nreaders--;
		if (nreaders==0)
			mvalue.unlock();
	}

	return rv;
}