Pass-by-value vs pass-by-reference vs pass-by-value-result - pass-by-reference

Question

I've got this question, and I'm a bit confused as to what would be printed, especially for pass-by-reference. What value would be passed to x if there are two parameters? Thanks!
Consider the following program. For each of the following parameter-passing methods, what is printed?
a. Passed by value
b. Passed by reference
c. Passed by value-result
void main()
{
int x = 5;
foo (x,x);
print (x);
}
void foo (int a, int b)
{
a = 2 * b + 1;
b = a - 1;
a = 3 * a - b;
}

Answer 1

The first two should be pretty straightforward, the last one is probably throwing you because it's not really a C++ supported construct. It's something that had been seen in Fortran and Ada some time ago. See this post for more info
As for your results, I think this is what you would get:
1)
5
2)
x = 5,
a = 2 * 5 + 1 = 11
b = 11 - 1 = 10
a = 3 * 10 - 10 = 20; // remember, a and b are the same reference!
x = 20
3) Consider this (in C++ style). We will copy x into a variable, pass that by reference, and then copy the result back to x:
void main()
{
int x = 5;
int copy = x;
foo (copy,copy); // copy is passed by reference here, for sake of argument
x = copy;
print (x);
}
Since nothing in the foo function is doing anything with x directly, your result will be the same as in #2.
Now, if we had something like this for foo
void foo (int a, int b)
{
a = 2 * b + 1;
x = a - 1; // we'll assume x is globally accessible
a = 3 * a - b;
}
Then # 2 would produce the same result, but #3 would come out like so:
a = 2 * 5 + 1 = 11
x = 11 - 1 = 10 // this no longer has any effect on the final result
a = 3 * 11 - 11 = 22
x = 22