Expression invalid null pointer - c++

Question

I am having some issues with my program, what I want to do is generate a md5 password which then save it to a text file and this part is not working for me, ("Expression invalid null pointer") any help would be greatly appreciated.
C++ Visual Studio 2015
main.cpp
#include <iostream>
#include <istream>
#include <string>
#include <sstream>
#include <fstream>
#include <iterator>
#include "s_encrypt.h"
#include "encrypt_copy.h"
using namespace std;
int main(int argc, char *argv[])
{
string password = "";
cout << "Please enter a password to be encrypted\n";
getline(cin, password);
cout << "MD5 Encryption of " << password << " " << "is this" << " " << md5(password);
cout << "Saving MD5 generated password to text file";
std::string p = md5(password);
CopyEncryptedPw(p);
return 0;
}
encrypt_copy.cpp
#include <istream>
#include <iostream>
#include <fstream>
#include <string>
#include "encrypt_copy.h"
using namespace std;
std::string CopyEncryptedPw(std::string pass)
{
fstream outfile;
outfile.open("C:\encrypted_pass.txt", ios::out);
outfile << pass;
return 0;
}
encrypt_copy.h
#pragma once
#ifndef ENCRYPT_H
#define ENCRYPT_H
std::string CopyEncryptedPw(std::string pass);
#endif

Answer 1

There are two issues with your code:
Issue 1:
outfile.open("C:\encrypted_pass.txt", ios::out);
If we assume that your OS is Windows, this should be:
outfile.open("C:\\encrypted_pass.txt", ios::out);
Also, the forward slash can be used for the standard stream functions:
outfile.open("C:/encrypted_pass.txt", ios::out);
Issue 2:
You're returning 0 for a function that is supposed to return a std::string.
std::string CopyEncryptedPw(std::string pass)
{
//...
return 0; // <-- This is bad
}
This code exhibits undefined behavior on return, since what will happen is that a 0 is assigned to the std::string return value, and assigning 0 to a std::string is undefined behavior.
Either return a string type (or a type that is convertible to a std::string), or return int:
int CopyEncryptedPw(std::string pass)
{
fstream outfile;
outfile.open("C:\\encrypted_pass.txt", ios::out);
outfile << pass;
return 0;
}
You could also have a void function that doesn't return anything, but you probably want an int return value for example, to return an error code (or OK indicator).