Better demo for http://stackoverflow.com/q/18993194/827263

demo.cpp

#include <iostream>
using namespace std;

int agenext (int age);

int main ()
{
    int age = 42;
    cout << "age = " << age << "\n";
    agenext(age);
    cout << "After calling agenext(age), age = " << age << "\n";
}

int agenext (int x)
{
    cout << "entering agenext, x = " << x << "\n";
    x++;
    cout << "agenext returning " << x << "\n";
    return x;
}