How to detect overflow

[Ali Çehreli via Digitalmars-d-learn]

On 11/04/2015 12:11 AM, Namal wrote:

 >> import core.checkedint;
 >>
 >> void main() {
 >>     bool overflowed;
 >>     auto result = adds(int.max, 1, overflowed); // this overflows
 >>     adds(1, 2, overflowed);     // this does not reset the flag
 >>
 >>     assert(overflowed);
 >> }
 >>
 >> Ali
 >
 > wow, this I don't understand at all, how do those two operations
 > connected to each other? By the bool value?

The 'overflow' parameter is a 'ref' parameter, meaning that these 
functions modify the actual 'overflowed' variable above. You can imagine 
that they do not set it to 'false' ever:

Imagine that adds() is implemented like the following:

pure nothrow @nogc @safe
int adds(int x, int y, ref bool overflow) {
     // ...
     if (over_flow_detected) {
         overflow = true;
     }
}

So, the caller's variable can only go from 'false' to 'true', never the 
other way around. (All of this is just a guess. :) )

Ali