Unexpected Results When Using Pointers to Access Array Elements

Question

I have two classes. class1 and class2. class2 creates an array with elements that are of type class1. class2 has a member that points to that array. I can access the pointer with class 2's getpointer method, but I get unexpected results when I try to use the pointer to access members of the array's elements.

Here's the code:

#include <iostream>
using namespace std;

class class1{
    int a;
    int b;
public:
    class1(){}
    class1(int x, int y){a = x; b = y;}
    int geta(){return a;}
    int getb(){return b;}

};

class class2{
    int c;
    int d;
    class1 *e;
public:
    class2(int x, int y){c = x; d = y;}
    void setE(){

        class1 arr[3];

        class1 arrTemp (0,0);
        arr[0] = arrTemp;

        class1 arrTemp1 (1,1);
        arr[1] = arrTemp1;

        class1 arrTemp2 (2,2);
        arr[2] = arrTemp2;


        cout << "Element 1, A = ";
        cout << arr[0].geta() << endl;

        cout << "Element 1, B = ";
        cout << arr[0].getb() << endl;

        cout << "Element 2, A = ";
        cout << arr[1].geta() << endl;

        cout << "Element 2, B = ";
        cout << arr[1].getb() << endl;     


        cout << "Element 3, A = ";
        cout << arr[2].geta() << endl;

        cout << "Element 3, B = ";
        cout << arr[2].getb() << endl;        

        class1 *pt;
        pt = &arr[0];

        e = pt;

        }
    class1* getpointer(){return e;}
    };

int main(){

        class2 testObj (1,2);

        testObj.setE();

        class1 *objPointer = testObj.getpointer(); 

        class1 tempclass1;

        tempclass1 = *(objPointer + 0);
        cout << "Element 1, A = ";
        cout << tempclass1.geta() << endl;

        cout << "Element 1, B = ";
        cout << tempclass1.getb() << endl;

        tempclass1 = *(objPointer + 1);
        cout << "Element 2, A = ";
        cout << tempclass1.geta() << endl;

        cout << "Element 2, B = ";
        cout << tempclass1.getb() << endl;  

        tempclass1 = *(objPointer + 1);
        cout << "Element 3, A = ";
        cout << tempclass1.geta() << endl;

        cout << "Element 3, B = ";
        cout << tempclass1.getb() << endl;             



    };

This is the output:

Element 1, A = 0
Element 1, B = 0
Element 2, A = 1
Element 2, B = 1
Element 3, A = 2
Element 3, B = 2
Element 1, A = 0
Element 1, B = 0
Element 2, A = -360531408
Element 2, B = 29391
Element 3, A = -360531408
Element 3, B = 29391

With the first set of cout statements I verify that the data is going into the array correctly.

It looks like I get the correct data for the first element, but after that what I get with the pointer is completely different from the actual array. I'm very new to C++, but I thought that *(Pointer + i) was equal to arr[i].

`class1 arr[3];` - local variable - disappears at end of function. And don't write code like this - use std::vector, not arrays. — , Sep 03 '18 at 18:26

Dusteh · Accepted Answer · 2018-09-03T18:33:41.033

2

In your example there is at least one problem which is located here:

void setE(){
    class1 arr[3];
    ...
    class1 *pt;
    pt = &arr[0];

    e = pt;
}

The arr[3] is a local object which is destroyed when the method ends. Still you make the class member e point to this object.

When the method ends arr memory is released and can be freely overwritten by a different object. Hence, when using e to read the memory you observe unexpected values.

edited Sep 03 '18 at 18:33

answered Sep 03 '18 at 18:27

Dusteh

1,496
16
21

That makes sense. Do you have any recommendations of how to get around this? From my understanding I can't have methods that output arrays. – Jarom Sep 03 '18 at 18:40
@Jarom You can easily return a `std::array` or `std::vector` by value. – Jesper Juhl Sep 03 '18 at 18:51
1

Well the solution depends on what you actually need to achieve. I'd stay away from returning a pointer to a series of values since you loose the information on the data size. I'd probably go with @JesperJuhl suggestion and switch `class1 *e;` to be a `std::array e;`. Then modify the `getpointer()` to `const &std::array getData() const { return e; }`. – Dusteh Sep 03 '18 at 21:15

Unexpected Results When Using Pointers to Access Array Elements

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