Passing by pointer

• Caller has to take the address -> not transparent
• A 0 value can be provided to mean nothing. This can be used to provide optional arguments.

Pass by reference

• Caller just passes the object -> transparent. Has to be used for operator overloading, since overloading for pointer types is not possible (pointers are builtin types). So you can't do string s = &str1 + &str2; using pointers.
• No 0 values possible -> Called function doesn't have to check for them
• Reference to const also accepts temporaries: void f(const T& t); ... f(T(a, b, c));, pointers cannot be used like that since you cannot take the address of a temporary.
• Last but not least, references are easier to use -> less chance for bugs.

When you pass a parameter by reference, the parameter inside the function is an alias to the variable you passed from the outside. When you pass a variable by a pointer, you take the address of the variable and pass the address into the function. The main difference is that you can pass values without an address (like a number) into a function which takes a const reference, while you can't pass address-less values into a function which takes const pointers.

Typically a C++ compiler implement a reference as a hidden pointer.

You can change your function into the pointer variant this way:

void flip(int *i) // change the parameter to a pointer type
{
    cout << "          flip start "<<"i="<< *i<<"\n"; // replace i by *i
    *i = 2*(*i); // I'm not sure it the parenthesis is really needed here,
                 // but IMHO this is better readable
    cout << "          flip exit  "<<"i="<< *i<<"\n";
}

int main()
{
    int j =1;
    cout <<"main j="<<j<<endl;
    flip(&j); // take the address of j and pass this value
    // adjust all other references ...
}