Question

There are three variables with the following types

 uint64_t old_addr, new_addr;
 int delta;

and I want to do this assignment

 new_addr = old_addr + delta;

However the problem is, when old_addr=915256 and delta=-6472064, the new_addr becoms 18446744069414584325

to fix that I have to check some things:

 if ( delta < 0 ) {
if ( old_addr < abs(delta) )
   new_addr = 0;
    else   
       new_addr = old_addr + delta;
 }

Is there a better and efficient way?

Answer 1

The question is what values old_addr and new_addr can take. And why
they are uint64_t, rather than simply int. The simplest expression
would be:

new_addr = old_addr + std::min( delta, -static_cast<int>( old_addr ) );

, but if old_addr can be greater than INT_MAX, this won’t work.
Otherwise, the rules of mixed signed/unsigned arithmetic in C/C++ are
such that you’re probably safest using explicit ifs, and not risking
any mixed arithmetic before being sure of the values.

And note that on most machines, abs( delta ) will still be negative if
delta is equal to INT_MIN. To correctly handle all of the cases,
you’ld need something like:

if ( delta > 0 ) {
    new_addr = std::numeric_limits<uin64_t>::max() - delta > old_addr
            ?   old_addr + delta
            :   std::numeric_limits<uint64_t>::max();
} else if ( delta < 0 ) {
    new_addr = old_addr != 0 && -(delta + 1) < old_addr - 1
            ?   old_addr + delta
            :   0;
} else {
    new_addr = old_addr;
}

(Just off the top of my head. There could easily be an off by one error
in there.)