If function is false, the else works. If true, segfault.

[iQChange]

I have this function:

    bool login(const string& username, const string& password)
    {
        if(user_exists(username))
        {
            ifstream pswfile;
            pswfile.open(string(appdata + '\\' + username + '\\' + username + ".psw").c_str());
            return ispassword(password, read_userpassword(pswfile));
        }
        return false;
    }

All the functions called are these:

string decodify(const vector<int>& code)
    {
        string ret;
        for(unsigned i = 0; i < code.size(); i++)
        {
            ret[i] = ((static_cast<char>((code[i]) - 9)/7));
        }
        return ret;
    }
    vector<int> read_userpassword(ifstream& file)
    {
        vector<int> password;
        string read_string;
        for(int i = 0; getline(file, read_string); i++)
        {
            password[i] = std::stoi(read_string);
        }
        return password;
    }
    bool ispassword(const string& password_try, const vector<int>& codified_string)
    {
        return decodify(codified_string) == password_try;
    }

Then, I use like that:

...
if(user::login(usuario, senha))
{
   cout << "Welcome, " << usuario;
}
else cout << "Authentication failed";
...

When failed to authenticate, I receive the output "Authentication failed", but when I know that the password and username is correct, I receive segfault and my program stops working. Why?

[cire]

In decodify you have an empty string. You attempt to modify nonexistent elements of that empty string, resulting in undefined behavior.

In read_userpassword you have an empty vector. You attempt to modify nonexistent elements of that empty vector, resulting in undefined behavior.

Avoid undefined behavior.

[iQChange]

And how can I fix that?

[cire]

string decodify(const vector<int>& code)
    {
        string ret;
        for(unsigned i = 0; i < code.size(); i++)
        {
            ret += ((static_cast<char>((code[i]) - 9)/7));
        }
        return ret;
}

    vector<int> read_userpassword(ifstream& file)
    {
        vector<int> password;
        string read_string;
        while (getline(file, read_string))
        {
            password.push_back(std::stoi(read_string));
        }
        return password;
    }

[iQChange]

Now, if the input is correct, I get false. Any idea?

[cire]

Yes. Debug your code. Test your functions. Preferably before you start plugging them in to other code.

Presumably there is a function codify somewhere that does the opposite of decodify;

Does

#include <iostream>
#include <string>

int main()
{
    std::string s("somepwval59") ;
    std::string decodified = decodify(codify(s)) ;
    if ( decodified != s )
        std::cout << '"' << s << "\" is not equal to \"" << decodified << "\"\n" ; 
    else
        std::cout << "equal\n" ;
}

produce the expected value of equal?

You might find http://www.slideshare.net/JonJagger/larry-and-jen-do-roman-numerals-in-c
to be of interest.

[iQChange]

Haha, I got a broken gdb or anything like that. I'll test your code.