How To Pass Parameters To A Function – By Value, By Reference

Function parameters are passed by value. This means that parameters are variables which are assigned the values of the arguments used when a function is called. For example, given the function definition: 
int multiplyByTwo(int a)
{
    a =  a * 2;
    return a;
}
if the function is called for example the following way: 
int x = 6;
multiplyByTwo(x);
The value of x is assigned to a, then a is assigned a * 2 and finally a is returned. But the original value of x will not change (in this case it will still be 6). In case you want the original argument passed to a function to be modifiable by the function's statements, you can pre-pone the parameter's name with a &: 
int multipyByTwoCallByReference(int &b)
{
    b = b *2;
     return b;
}
in this case the original argument would get doubled. Another way to pass an argument by reference is to pass a pointer: 
int multiplyByTwoPointerReference(int *pint)
{
     *pint = *pint * 2;
     return *pint;
}
In this case the variable located at the address pint is modified. Finally, a function can also return a pointer. The pointer, though, mustn't contain the address of a local variable or formal parameter, because they get destroyed at the function’s exit. 
int *multiplyByTwoPointerReference2(int *pint)
{
     *pint = *pint * 2;
     return pint;
}


Example


#include <iostream>
using namespace std;

int multiplyByTwo(int a)
{
    a =  a * 2;
    return a;
}

int multipyByTwoCallByReference(int &b)
{
    b = b *2;
    return b;
}

int multiplyByTwoPointerReference(int *pint)
{
     *pint = *pint * 2;
     return *pint;
}

int *multiplyByTwoPointerReference2(int *pint)
{
     *pint = *pint * 2;
     return pint;
}

int main()
{
     int x = 3;
     int *pinteger;
     cout << "x is: " << x << endl;
     cout << "multiplyByTwo(x); returns " << multiplyByTwo(x) << endl;
     cout << "x is: " << x << endl;

     cout << "x is: " << x << endl;
     cout << "multipyByTwoCallByReference(x); returns " << multipyByTwoCallByReference(x) << endl;
     cout << "x is: " << x << endl;

     cout << "x is: " << x << endl;
     cout << "multiplyByTwoPointerReference(&x); returns " << multiplyByTwoPointerReference(&x) << endl;
     cout << "x is: " << x << endl;

     cout << "x is: " << x << endl;
     cout << "multiplyByTwoPointerReference2(&x); returns " << (pinteger = multiplyByTwoPointerReference2(&x)) << endl;
     cout << "x is: " << x << endl;
     cout << "*pinteger is: " << *pinteger << endl;
}

Output

x is: 3
multiplyByTwo(x); returns 6
x is: 3
x is: 3
multipyByTwoCallByReference(x); returns 6
x is: 6
x is: 6
multiplyByTwoPointerReference(&x); returns 12
x is: 12
x is: 12
multiplyByTwoPointerReference2(&x); returns 0012F4CC
x is: 24
*pinteger is: 24