How do I make my adress show the values in my array?

Question

Thank you for your help in advance!!

I've created a class to which I pass the name of an array as an argument.

Element(string, string, short*);  //Constructor for my class Element

Next, I pass an array to instance H of my class.

short neutrons_H[] = {0, 1, 2};
Element H("Hydrogen", "H", neutrons_H);

Now, I can access the correct values from the array if I ask for them in the constructor. However, if I request the values in a method from the same class, I only get -13108 as a response. Changing the index doesn't remedy the situation. I want to retrieve the correct values from the address stored under 'neutrons'.

I've added all three files below for clarity.

Main program:

#include <iostream>
#include "Element.h"
using namespace std;

Element PeriodicTable[118];

int FillPeriodicTable() {
    short neutrons_H[] = {0, 1, 2};
    Element H("Hydrogen", "H", neutrons_H);
    PeriodicTable[Element::getElementNumber()-1] = H;

    short neutrons_He[] = {2};
    Element He("Helium", "He", neutrons_He);
    PeriodicTable[Element::getElementNumber()-1] = He;

    cout << endl;
    return Element::getElementNumber();
}

int main()
{
    cout << FillPeriodicTable() << " elements out of the total 118 have been created and added!" << endl;

    PeriodicTable[0].showProperties();
}

Header file for class Element:

#pragma once
using namespace std;

class Element
{
    private:
        static int elementNumber;

        string name = "-";
        string abbreviation = "";
        short protons = 0;
        short* neutrons = 0;
        short electrons = 0;

    public:
        Element();
        Element(string, string, short*);

        void showProperties();

        static int getElementNumber() { return elementNumber; }
};

Source code for class Element:

#include <iostream>
#include "Element.h"
using namespace std;

int Element::elementNumber = 0;

Element::Element() {

}

Element::Element(string name, string abbreviation, short* neutron) {
    cout << ++elementNumber << " :  " << abbreviation << " - " << name << endl;

    this->name = name;
    this->abbreviation = abbreviation;
    this->protons = elementNumber;
    this->neutrons = neutron;
    this->electrons = elementNumber;
}

void Element::showProperties() {
    cout << abbreviation << " - " << name << ":" << endl << endl;
    cout << "Protons:" << endl << "   " << protons << endl;
    cout << endl << "Neutrons:" << endl;
    for (int isotope = 0; isotope < 3; isotope++) {
        cout << "   " << neutrons[isotope] << endl;             // Returns only -13108
    }
    cout << endl << "Electrons:" << endl << "   " << electrons << endl;
}

Answer 1

You're storing a dangling pointer.

Not an array; a pointer.

By the time you try to use it, the array (which was a local variable in FillPeriodicTable() ) is dead. Gone. Buried. Turned to dust long ago.

If you want the class objects to contain arrays, then make them do that thing, preferably by storing a std::array .

Answer 2

If you run under valgrind you will have:

bruno@bruno-XPS-8300:/tmp$ valgrind ./a.out
==4288== Memcheck, a memory error detector
==4288== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==4288== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
==4288== Command: ./a.out
==4288== 
1 :  H - Hydrogen
2 :  He - Helium

2 elements out of the total 118 have been created and added!
H - Hydrogen:

Protons:
   1

Neutrons:
==4288== Invalid read of size 2
==4288==    at 0x401640: Element::showProperties() (in /tmp/a.out)
==4288==    by 0x40109A: main (in /tmp/a.out)
==4288==  Address 0xffefffc60 is on thread 1's stack
==4288==  240 bytes below stack pointer
==4288== 
   30240
   1401
   0

Electrons:
   1
==4288== 
==4288== HEAP SUMMARY:
==4288==     in use at exit: 72,704 bytes in 1 blocks
==4288==   total heap usage: 2 allocs, 1 frees, 73,728 bytes allocated
==4288== 
==4288== LEAK SUMMARY:
==4288==    definitely lost: 0 bytes in 0 blocks
==4288==    indirectly lost: 0 bytes in 0 blocks
==4288==      possibly lost: 0 bytes in 0 blocks
==4288==    still reachable: 72,704 bytes in 1 blocks
==4288==         suppressed: 0 bytes in 0 blocks
==4288== Rerun with --leak-check=full to see details of leaked memory
==4288== 
==4288== For counts of detected and suppressed errors, rerun with: -v
==4288== ERROR SUMMARY: 3 errors from 1 contexts (suppressed: 0 from 0)

The problems comes because Element saves pointer to short and that pointer is the address of a local variable in FillPeriodicTable

To store your array you can allocate it in the heap or to use a std::vector simplifying all

So the constructor of Element has the signature

Element::Element(string name, string abbreviation, const vector<short> & neutron)

To write the list of isotopes in showProperties the loop can be replaced by:

for (auto isotope : neutrons)
  cout << "   " << isotope << endl; 

notice this is compatible with any number of isotope, xenon and cesium have 26 isotopes. Your code supposed the array has (at least) 3 elements but this was false for the helium having only 1

In the class Element the field neutron becomes

vector<short> neutrons;

and of course in main :

vector<short> neutrons_H = {0, 1, 2};
...
vector<short> neutrons_He = {2};

Now the execution under valgrind is:

bruno@bruno-XPS-8300:/tmp$ valgrind ./a.out
==4406== Memcheck, a memory error detector
==4406== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==4406== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
==4406== Command: ./a.out
==4406== 
1 :  H - Hydrogen
2 :  He - Helium

2 elements out of the total 118 have been created and added!
H - Hydrogen:

Protons:
   1

Neutrons:
   0
   1
   2

Electrons:
   1
==4406== 
==4406== HEAP SUMMARY:
==4406==     in use at exit: 72,704 bytes in 1 blocks
==4406==   total heap usage: 8 allocs, 7 frees, 73,752 bytes allocated
==4406== 
==4406== LEAK SUMMARY:
==4406==    definitely lost: 0 bytes in 0 blocks
==4406==    indirectly lost: 0 bytes in 0 blocks
==4406==      possibly lost: 0 bytes in 0 blocks
==4406==    still reachable: 72,704 bytes in 1 blocks
==4406==         suppressed: 0 bytes in 0 blocks
==4406== Rerun with --leak-check=full to see details of leaked memory
==4406== 
==4406== For counts of detected and suppressed errors, rerun with: -v
==4406== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
bruno@bruno-XPS-8300:/tmp$