When pass a variable to a function, why the function only gets a duplicate of the variable?

Question

When pass a variable to a function, why the function only gets a copy/duplicate of the variable?

int n=1;

void foo(int i)
{
    i++;
}

As

Answer 1

If you really want a function to change its actual parameter, you can pass it by reference in C++

void foo(int& i)
{
    i++;
}