遍历类向量导致问题

我的程序正在尝试创建一个新帐户，并将用户名与数据库中的用户名进行比较。1来自用户输入，另一个来自矢量。我必须遍历向量并比较这两个字符串。但由于某种未知的原因，它只比较FIRST值，而不是比较矢量内的所有值。

这是我的代码：
我的登录Ac.cpp

class AccountInfo {
public:
string username;
string password;
string type;
}; 
bool LoginAcc::checkAccountNameValid(string username) {
vector <AccountInfo> accInfo;
AccountInfo user;
ifstream UserDatabase("UserDatabase.txt");
string line = "";
while (getline(UserDatabase,line)) {
    stringstream linestream(line);
    getline(linestream,user.username,':');
    accInfo.push_back(user);
}
UserDatabase.close();
for(vector<AccountInfo>::iterator itr = accInfo.begin(); itr != accInfo.end(); ++itr) {
    if (username.compare((*itr).username) != 0)
        return true;
    else 
        return false;
}

我的主.cpp

        case 'n':
        while (!flag) {
            cout << "Please enter a username with 8 characters. " << endl;
            cout << "Username : ";
            cin >> username;
            if (username.length() != 8) {
                cout << "Username does not meet the requirements" << endl;
                cout << "Username : ";
                cin >> username;
            }
            else {
                valid = login.checkAccountNameValid(username);
                if (valid == true) {
                    cout << "Please enter a password with 8 characters." << endl;
                    cout << "Password : " << endl;
                    cin >> password;
                    cout << "1. Student" << endl;
                    cout << "2. Lecturer" << endl;
                    cout << "3. Admin" << endl;
                    cout << "Please choose the category you belong to : ";
                    cin >> category;
                    login.createNewAcc(username,password,category);
                }
                else {
                    cout << "Username has been taken. Please choose another. " << endl;
                    cout << "Username : ";
                    cin >> username;
                }
            }
        }
        break;

我觉得我的"checkAccountNameValid"方法中的逻辑有问题。有人能提出建议吗？谢谢