C++ Hangman

#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <ctime>
#include <cctype>
using namespace std;

char getGuess(string &used); //Prototype Function Declaration
char isCorrect(); //Prototype Function Declaration

using namespace std;

int main()
{
// set-up
const int MAX_WRONG = 10; // maximum number of incorrect guesses allowed

vector<string> words; // collection of possible words to guess
words.push_back("GUESS");
words.push_back("HANGMAN");
words.push_back("DIFFICULT");

srand(static_cast<unsigned int>(time(0)));
random_shuffle(words.begin(), words.end());
const string THE_WORD = words[0]; // word to guess
int wrong = 0; // number of incorrect guesses
string soFar(THE_WORD.size(), '-'); // word guessed so far
string used = ""; // letters already guessed

cout << "Welcome to Hangman. Good luck!\n";

// main loop
while ((wrong < MAX_WRONG) && (soFar != THE_WORD))
{
cout << "\n\nYou have " << (MAX_WRONG - wrong);
cout << " incorrect guesses left.\n";
cout << "\nYou've used the following letters:\n" << used << endl;
cout << "\nSo far, the word is:\n" << soFar << endl;


char guess = getGuess(used);

if (THE_WORD.find(guess) != string::npos)
{
cout << "That's right! " << guess << " is in the word.\n";

// update soFar to include newly guessed letter
for (unsigned int i = 0; i < THE_WORD.length(); ++i)
{
if (THE_WORD[i] == guess)
{
soFar[i] = guess;
}
}
}
else
{
cout << "Sorry, " << guess << " isn't in the word.\n";
++wrong;
}
}

// shut down
if (wrong == MAX_WRONG)
cout << "\nYou've been hanged!";
else
cout << "\nYou guessed it!";

cout << "\nThe word was " << THE_WORD << endl;

return 0;
}

//----getGuess Function Return starts here---

char getGuess(string &used)
{
char guess;
cout << "\n\nEnter your guess: ";
cin >> guess;
guess = toupper(guess); //make uppercase since secret word in uppercase
while (used.find(guess) != string::npos); //if guess IS in the string, ask for guess again in while loop
{
cout << "\nYou've already guessed " << guess << endl;
cout << "Enter your guess: ";
cin >> guess;
guess = toupper(guess);
}
used += guess;
return guess;
}