#include <vector>
#include <string>
#include <iostream>
#include <algorithm>
 
struct User {
    std::string name, game;
    User(const char* nam, const char* gam) : name(nam), game(gam) {}
    const std::string& getUsername() const {
        return name;
    }
};
 
int main(){
    std::vector<User> users;
    users.push_back({"foo","bar"});
    std::string username;
    bool first = true;
    do {
        if (!first) std::cout << username << ": no good!\n";
        else first = false;
        std::cout << "Enter Username: ";
        std::cin >> username;
    } while(std::any_of(users.begin(), users.end(),
            [username](const auto& u){ return u.getUsername() == username; }));
 
    return 0;
}

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Zm9vCmZvbwpmb28KR3JlZw==

foo
foo
foo
Greg