What is causing this Segmentation Fault: 11? - c++

Question

Okay, so this is effectively the first C++ I have ever written. Consequently, my attempts to research this Segmentation Fault error have failed to yield a useful response that I can understand. Most suggestions say that I'm exceeding the heap memory, but I don't see how I could be in this code. Maybe I'm trying to reference something that I can't reference?
Can someone please take a look at this code and possibly tell me what is causing this error? The code compiles fine, it just throws this error when it runs. I have tried commenting out all the code involving vectors, and that didn't solve the problem. I'm guessing it's something stupid that I'm missing or just a concept about C++ that I don't understand. Either way, some insight would be much appreciated! Thanks in advance.
#include <iostream>
#include <string>
#include <sstream>
#include <vector>
using namespace std;
const int DEFAULT_SIZE = 15;
//---------------------------------User----------------------------------------------------------------------------------
//User Interface Definition
class User
{
public:
User(string nameValue);
string getName();
string getWall();
string getHome();
private:
string name;
};
//User Implementation Definition
User::User(string nameValue) : name(nameValue)
{
}
string User::getName()
{
return name;
}
//---------------------------------Group----------------------------------------------------------------------------------
//Group Interface Definition
class Group
{
public:
Group(string nameValue);
string getName();
void addMember(User newMember);
private:
string name;
vector<User> members;
};
//Group Implementation
Group::Group(string nameValue) : name(nameValue)
{
}
string Group::getName()
{
return name;
}
void Group::addMember(User newMember)
{
members.push_back(newMember);
}
//---------------------------------MessageBuffer----------------------------------------------------------------------------------
//MessgeBuffer Interface Declaration
class MessageBuffer
{
public:
MessageBuffer();
void insert(string user, string recipient, string message);
string readByGroup(string groupName);
string readByUser(string userName);
string readPublic();
private:
string buffer;
};
//MessageBuffer Implementation
MessageBuffer::MessageBuffer() : buffer("")
{
}
void MessageBuffer::insert(string user, string recipient, string message)
{
buffer = "|<" + user + "::" + recipient + ">|" + message + buffer;
}
string MessageBuffer::readByGroup(string groupName)
{
return "Something";
}
string MessageBuffer::readByUser(string userName)
{
return "Something";
}
string MessageBuffer::readPublic()
{
return "Something";
}
//---------------------------------System-------------------------------------------------------------------------------
//System Interface Definition
class System
{
public:
System();
void start();
private:
bool running;
User currentUser;
MessageBuffer messageBuffer;
vector<User> users;
vector<Group> groups;
};
//System Implementation Definition
System::System() : running(false), currentUser(0)
{
}
void System::start()
{
running = true;
string userSelection;
cout << "===========================================================" << endl;
cout << "|\t Welcome to the Auburn Messaging System! \t |" << endl;
cout << "===========================================================" << endl;
while (running) {
cout << "1) Create a new user\n";
cout << "2) Broadcast a message\n";
cout << "3) Multicast a message\n";
cout << "4) Unicast a message\n";
cout << "5) Display Wall page\n";
cout << "6) Display Home page\n";
cout << "7) Create a new group\n";
cout << "8) Join a group\n";
cout << "9) Switch to a different user\n";
cout << "10) Quit Auburn Messaging System\n";
cin >> userSelection;
if (userSelection == "1")
{
running = false;
}
}
}
//----------------------------------------------------------------------------------------------------------------------
int main() {
cout << "Part 1";
System system1;
cout << "Part 2";
system1.start();
cout << "Part 3";
return 0;
}

Answer 1

In the System constructor, currentUser(0) is attempting to initialise a the first parameter of the User constructor (a string) with a literal zero which will be interpreted as a null char pointer.
The std::string constructor (look for basic_string<char> in the debugger) which constructs a string from a char* will be called with a null pointer, which it will dereference and invoke undefined behaviour.