arrays - Returning Pointer from a Function in C++

Question

When I return pointer from the function, its value can be accessed individually. But when a loop is used to ouput the value of that pointer variable, wrong value is shown. Where I am making mistake, can't figure it out.

#include <iostream>
#include <conio.h>

int *cal(int *, int*);

using namespace std;

int main()
{
    int a[]={5,6,7,8,9};
    int b[]={0,3,5,2,1};
    int *c;
    c=cal(a,b);

    //Wrong outpur here
    /*for(int i=0;i<5;i++)
    {
        cout<<*(c+i);
    }*/

    //Correct output here
    cout<<*(c+0);
    cout<<*(c+1);
    cout<<*(c+2);
    cout<<*(c+3);
    cout<<*(c+4);

return 0;
}   

int *cal(int *d, int *e)
{
    int k[5];
    for(int j=0;j<5;j++)
    {
        *(k+j)=*(d+j)-*(e+j);
    }
    return k;
}

Answer 1

You are returning a pointer to a local variable.

k is created on the stack. When cal() exits the stack is unwound and that memory is free'd. Referencing that memory afterwards leads to undefined behaviour (as explained beautifully here: https://stackoverflow.com/a/6445794/78845).

Your C++ compiler should warn you about this and you should heed these warnings.

For what it's worth, here is how I'd implement this in C++:

#include <algorithm>
#include <functional>
#include <iostream>
#include <iterator>

int main()
{
    int a[] = {5, 6, 7, 8, 9};
    int b[] = {0, 3, 5, 2, 1};
    int c[5];
    std::transform (a, a + 5, b, c, std::minus<int>());
    std::copy(c, c + 5, std::ostream_iterator<int>(std::cout, ", "));
}

See it run!