C++: Bulls and Cows

This is an old game played with pencil and paper that was later implemented on computer.

The task is for the program to create a four digit random number from the digits 1 to 9, without duplication. The program should ask for guesses to this number, reject guesses that are malformed, then print the score for the guess.

The score is computed as:

The player wins if the guess is the same as the randomly chosen number, and the program ends.
A score of one bull is accumulated for each digit in the guess that equals the corresponding digit in the randomly chosen initial number.
A score of one cow is accumulated for each digit in the guess that also appears in the randomly chosen number, but in the wrong position.

#include <iostream>
#include <string>
#include <algorithm>
#include <cstdlib>

bool contains_duplicates(std::string s)
{
	std::sort(s.begin(), s.end());
	return std::adjacent_find(s.begin(), s.end()) != s.end();
}

void game()
{
	typedef std::string::size_type index;

	std::string symbols = "0123456789";
	unsigned int const selection_length = 4;

	std::random_shuffle(symbols.begin(), symbols.end());
	std::string selection = symbols.substr(0, selection_length);
	std::string guess;
	while (std::cout << "Your guess? ", std::getline(std::cin, guess))
	{
		if (guess.length() != selection_length
				|| guess.find_first_not_of(symbols) != std::string::npos
				|| contains_duplicates(guess))
		{
			std::cout << guess << " is not a valid guess!";
			continue;
		}

		unsigned int bulls = 0;
		unsigned int cows = 0;
		for (index i = 0; i != selection_length; ++i)
		{
			index pos = selection.find(guess[i]);
			if (pos == i)
			++bulls;
			else if (pos != std::string::npos)
			++cows;
		}
		std::cout << bulls << " bulls, " << cows << " cows.\n";
		if (bulls == selection_length)
		{
			std::cout << "Congratulations! You have won!\n";
			return;
		}
	}
	std::cerr << "Oops! Something went wrong with input, or you've entered end-of-file!\nExiting ...\n";
	std::exit(EXIT_FAILURE);
}

int main()
{
	std::cout << "Welcome to bulls and cows!\nDo you want to play? ";
	std::string answer;
	while (true)
	{
		while (true)
		{
			if (!std::getline(std::cin, answer))
			{
				std::cout << "I can't get an answer. Exiting.\n";
				return EXIT_FAILURE;
			}
			if (answer == "yes" || answer == "Yes" || answer == "y" || answer == "Y")
			break;
			if (answer == "no" || answer == "No" || answer == "n" || answer == "N")
			{
				std::cout << "Ok. Goodbye.\n";
				return EXIT_SUCCESS;
			}
			std::cout << "Please answer yes or no: ";
		}
		game(); 
		std::cout << "Another game? ";
	}
}