'int' is enough for 'length' to migrate code from x86 to x64

[Daniel Murphy via Digitalmars-d]

"bearophile"  wrote in message news:lkcltlokangpzzdzzfjg at forum.dlang.org...

> From my experience in coding in D they are far more unlikely than 
> sign-related bugs of array lengths.

Here's a simple program to calculate the relative size of two files, that 
will not work correctly with unsigned lengths.

module sizediff

import std.file;
import std.stdio;

void main(string[] args)
{
    assert(args.length == 3, "Usage: sizediff file1 file2");
    auto l1 = args[1].read().length;
    auto l2 = args[2].read().length;
    writeln("Difference: ", l1 - l2);
}

The two ways this can fail (that I want to highlight) are:
1. If either file is too large to fit in a size_t the result will (probably) 
be wrong
2. If file2 is bigger than file1 the result will be wrong

If length was signed, problem 2 would not exist, and problem 1 would be more 
likely to occur.  I think it's clear that signed lengths would work for more 
possible realistic inputs.

While this is just an example, a similar pattern occurs in real code 
whenever array/range lengths are subtracted.