New to C++, program not responding when dereferencing part of a vector? - c++

Question

I learned C++ yesterday an I'm trying to solve USACO training problems.
http://train.usaco.org/usacoprob2?a=iKSzALidh4Q&S=gift1
For this one, I have created a vector of People pointers. However, after some troubleshooting, I discovered that when I try to do something like
Person bob = *(people.at(i));
or
people.at(i) -> setbalance(giveself); // giveself is an int
The program is not responding and:
Process terminated with status -1073741819 (0 minute(s), 3 second(s).
I'm also new to this forum.
Here is my code:
include statments
using namespace std;
class Person
{
private:
int balance;
int origbalance;
string name;
public:
int getbalance() {return balance;}
string getname() {return name;}
void setbalance(int b){balance +=b;}
void setorigbalance(int o) {origbalance = o;}
int getorigbalance() {return origbalance;}
void giveTo(int num, Person* y) {y->setbalance(num);}
~Person();
Person(string n);
};
Person::Person(string n)
{
name = n;
}
Person::~Person()
{
}
int main()
{
ofstream fout ("gift1.out");
ifstream fin ("gift1.in");
int NP;
fin>>NP;
cout<<NP<<endl;
vector<Person*> people(NP);
cout<<"Created vector\n"<<endl;
for(int i = 0; i<NP; i++)
{
string nam;
fin>>nam;
Person* p = new Person(nam);
people.push_back(p);
cout<<nam<<endl;
}
cout<<"\nFilled vector, size = "<<people.size()<<endl;
for(int i = 0; i<NP; i++)
{
string temp;
fin>>temp;
cout<<"\nNow receiving "<<temp<<endl;
int togive, numgiving;
fin>>togive>>numgiving;
cout<<"\n"<<temp<<" is dividing "<<togive<<" among "<<numgiving<<" people"<<endl;
Person bob = *(people.at(i));
cout<<"hi bob"<<endl;
//(*people.at(i)).setorigbalance(togive);
cout<<"Original balance set"<<endl;
int giveeach = togive/numgiving;
cout<<"or "<<giveeach<<" to each person"<<endl;
int giveself = togive%numgiving;
cout<<"and "<<giveself<<" to himself :/"<<endl;
people.at(i) -> setbalance(giveself);
for(int j=0; j<numgiving; j++)
{
string nametogiveto;
fin>> nametogiveto;
cout<<nametogiveto<<endl;
for(int k=0; k<NP; k++)
{
string namy = people.at(k)->getname();
if(namy==nametogiveto)
{
cout<<"\nHere you go "<<namy<<" have "<<giveeach<<endl;
people.at(k)->setbalance(giveeach);
people.at(i)->setbalance(-giveeach);
break;
}
}
}
}
for(int i=0; i<NP; i++)
{
cout<<people.at(i)->getname()<<endl;;
cout<<people.at(i)->getorigbalance() - people.at(i)->getbalance()<<endl;
cout<<endl;
fout<<people.at(i)->getname();
fout<<people.at(i)->getorigbalance() - people.at(i)->getbalance()<<endl;
}
return 0;
}

Answer 1

You're running into undefined behavior due to dereferencing null pointers.
vector<Person*> people(NP);
This line creates the vector with NP null pointers. You later add on your actual pointers but only ever access the first NP elements which are the null pointers.
That said you don't even need pointers here and I'd recommend getting rid of them. In fact you have memory leaks due to allocating pointers with new but never calling delete on them. In my experience it's typical for people just starting out with C++ to overuse pointers, so think about ways to avoid them first.
Change
vector<Person*> people(NP);
to
vector<Person> people;
and fill it using:
for(int i = 0; i<NP; i++)
{
string nam;
fin>>nam;
Person p(nam); // no more need for pointer or new here
people.push_back(p);
cout<<nam<<endl;
}
later when accessing it you don't need any dereferencing anymore either. That means you can get rid of all the * for example:
Person bob = *(people.at(i));
turns into:
Person bob = people.at(i);
and you can access member functions with . instead of -> everywhere, for example:
people.at(k)->setbalance(giveeach);
would turn into:
people.at(k).setbalance(giveeach);
This means getting rid of a lot of unneeded dereferencing of pointers and also of the memory leak you previously would have had.

Answer 2

For me, it seems that you are getting that error since opening input file fin is not successful. To check it, add the following line after you define fin variable:
ifstream fin ("gift1.in");
if(!fin) {
cout << "Error opening input file.\n";
}
In any case, it's always a good practice to check if the file is opened successfully.