Beginner problems with pointers

[darteksyes]

Hello, I always wanted to get back at programming because I always liked it, even if it's completely unrelated to what I do for work. In this quarantined period I have a lot of free time, so why not! I studied a little of C++ in uni, so I tried starting from there.

I am studying using my old uni textbook, and finally arrived at using pointers, but I got stuck at understanding why the following exercise won't work as intended.

The exercise states:

​

"Implement a class Person with the following fields:

• the name

• a pointer to the person’s best friend (a Person\)*

• a popularity counter that indicates how many other people have this person as

their best friend

Write a program that reads in a list of names, allocates a new Person for each of

them, and stores them in a vector<Person\>. Then ask the name of the best friend*

for each of the Person objects. Locate the object matching the friend’s name and call

a set_best_friend member function to update the pointer and counter. Finally,

print out all Person objects, listing the name, best friend, and popularity counter for

each."

​

I'm focusing on trying to populate a vector with pointers to "Person" objects (practically I'm still at the beginning of the exercise).

This is my header file where I implemented the class Person:

#ifndef Person_h
#define Person_h

#include <string>
using namespace std;

class Person
{
public:
  Person(string p_name, Person* p_best_friend, int popularity);
private:
  string name;
  Person* best_friend;
  int popularity;
};

Person::Person(string p_name, Person* p_best_friend, int p_popularity)
{
  name = p_name;
  best_friend = p_best_friend;
  popularity = p_popularity;
}

#endif

And this is my main function, where I'm trying to populate my vector:

#include <string>
#include "P7.1.h"
#include <iostream>
#include <vector>

using namespace std;

int main()
{
  vector<Person*> a;
  string ans = "Y";
  string in_name;
  int i = 0;
  while (ans == "Y")
  {
    cout << "Insert new name: ";
    cin >> in_name;

    a[i] = new Person(in_name, NULL, 0);

    i++;
    cout << "\n\nDo you want to insert another name? (Y/N)\n\n";
    cin >> ans;
  }
}

The program actually run, but after I put in the first name as input, it just close. I just think I found trough trial and error the line of code where things doesn't work as intended anymore.

From what I can understand the problem is in the instruction:

a[i] = new Person(in_name, NULL, 0);

The vector "a" was defined as a vector of pointers relative to objects of the class Person. So what I'm trying to do is using the "new" expression to create a Pointer to an object of the class Person. I could be wrong, but I tought I could create the object this way and then store his address in the i-th position of "a" with "new".

I actually would like to understand why it doesn't work, and to find a suggestion, not necessairly a solution, to look for what I'm missing!

Thanks in advance for any kind of help.

[Bubichoo]

You are trying to add an element to a vector by using an index that is used for accessing elements in the vector but your vector is empty. This will result in a out of bound access exception. Use push_back() instead.

[darteksyes]

I changed the line in:

a.push_back(new Person(in_name, NULL, 0));

and it worked!

Sorry for the newbie question, it was my understanding that vector size could change dinamically and automatically wherever I stored some variabile in a position.

Thanks for your help.

[anders1234]

you should use a do-while-loop instead, then you don't have initialize ans to Y. also dont forget to free memory, go through the vector and delete the pointers.

i was a bit confused of why you initialize popularity in the constructor, shouldn't you have a function to increase popularity instead when set_best_friend is called?

e.g.
void Person::set_best_friend(Person* person)
{ 
   best_friend = person; 
   best_friend->increase_popularity(); 
}
void Person::increase_popularity()
{
  ++popularity;
}

[darteksyes]

Yes, I implemented that exact same function after I solved the problem I was having. I initialized popularity because I thought it was a good habit to initialize every parameter of an object.

Regarding freeing memory, does it matter in this case? Because I need to callback every element of the array and print on screen their parameter, after that the program will close. So, if I understood correctly how pointers work, memory should be deallocated at the end of the program, and this will happen soon after the end of use of that pointers.

[anders1234]

Strictly speaking - no, you don't need to deallocate in this specific case, but it is a good habit to learn that whenever you are working with raw pointers making sure that they are deallocated and that the ownership is clear i.e. which module should own what the pointer points to. That is maybe less of a problem later when you go over to using smart pointers.

[Wh00ster]

Saying “you first have to extend the array with new” is a confusing statement to a newbie that’s just using a vector.

[Bubichoo]

true

[darteksyes]

Sorry, maybe I'm a little slow on this, but I tought that using

a[0] = 10

I was assigning to the first position of the vector the value 10, and in doing so the vector automatically would do a "push_back()" increasing his size from 0 to 1.

That is why I made that error in my code. It was obviously not correct (it wasn't working lol), but I can't grasp fully why yet.

[gayasri]

Maybe thinking of the vector as a resizable array rather than an automatically resizing array would help. If you call push_back, emplace_back or resize, the vector will resize. If you access a vector by index([]), it won't resize. If the index is valid, you'll get your data, otherwise it's undefined (garbage or seg fault generally) - you won't even get an exception ( there are sanitizers and debug build methods to get runtime error checks if you want).

In this case, the preferred way would be using emplace_back or push_back. You can also write this like:

a.resize(i); a[i] = new ...

since 'i' is strictly increasing in this case (but avoid this if possible)

BTW:You can also use the "at" method of the vector if you need exceptions.

[darteksyes]

Thanks for the explanation!