Pointers as Functions Returns in C/C++

Notes

Let’s look at this code right here, that uses

#include <iostream>
using std::cout;
using std::endl;

int Add(int a, int b)
{
    // Called Function
    cout << "Address a in Add = " << &a <<  endl;
    int c = a + b;
    return c;

}
int main()
{
    // Calling Function

    int a = 2, b = 4;
    cout << "Address a in main = " << &a <<  endl;

    // Call by Value
    int c = Add(a,b); // Value of a of main is copied to a of Add
                      // Value in b of main is copied to b of Add

    cout << "Sum = " << c << endl; 
    return 0;
}

Output

Address a in main = 0x7ffee1a35608
Address a in Add = 0x7ffee1a355dc
Sum = 6

We can see that the a in Add and the a in main are not the same as we print the addresses of the a’s. Which means these variables are not the same, they’re at different memory addresses.

The names of variables are local or specific to a particular function

Passing the Addresses

Lets pass the addresses of a and b to the Add function

#include <iostream>
using std::cout;
using std::endl;

int Add(int* a, int* b)
{
    // Called Function
    // a and b are pointers to integers
    cout << "Address a in Add = " << &a <<  endl;
    cout << "Value in a of Add (address of a of main) = " << a <<  endl; // Should give address of a in main
    cout << "Value at address stored in a of Add = " << *a <<  endl;

    int c = (*a) + (*b);
    return c;

}
int main()
{   // Calling Function
    int a = 2, b = 4;
    cout << "Address a in main = " << &a <<  endl;

    // Call by Value
    int c = Add(&a,&b); // a and b are integers local to main

    cout << "Sum = " << c << endl; 
}

Output:

Address a in main = 0x7ffeeb23e60c
Address a in Add = 0x7ffeeb23e5d8
Value in a of Add (address of a of main) = 0x7ffeeb23e60c
Value at address stored in a of Add = 2
Sum = 6