String to boolean inconsistency

[Andrej Mitrovic]

Coolio. I should pay a visit to druntime.. sometime. :)

On 12/12/10, Jonathan M Davis <jmdavisProg at gmx.com> wrote:
> On Saturday 11 December 2010 19:00:55 Jonathan M Davis wrote:
>> On Saturday 11 December 2010 18:18:54 Tomek Sowiński wrote:
>> > string s = "";
>> > assert(s);  // ok
>> > assert(s != null);  // fails
>> >
>> > I guess that's a bug. But which one is right?
>>
>> A null array and an empty array are essentially the same thing as far as D
>> is concerned. I believe that the only difference is whether "is null"
>> returns true or not. So, this program
>>
>> import std.range;
>> import std.stdio;
>>
>> void main()
>> {
>>     string s = "";
>>
>>     writefln("[%s]", s);
>>     writeln(s is null);
>>     writeln(s == null);
>>     writeln(s.empty);
>>     writeln();
>>
>>     string t = null;
>>
>>     writefln("[%s]", t);
>>     writeln(t is null);
>>     writeln(t == null);
>>     writeln(t.empty);
>> }
>>
>>
>> prints
>>
>> []
>> false
>> true
>> true
>>
>> []
>> true
>> true
>> true
>>
>>
>> "==" is _not_  the correct operator to use to check for null anyway. "is"
>> is the correct operator for that.
>>
>> This behavior is intended. Arrays are actually something like this under
>> the hood:
>>
>> struct array(T)
>> {
>>     T* arr;
>>     size_t length;
>> }
>>
>>
>> By declaring an array to be null, all you're doing is setting arr to null.
>> std.range.empty() will check length and ignore arr. "==" will presumably
>> only check arr if the length of the two arrays is the same. If they are
>> the same, then each element is checked fore equality. But if their length
>> is zero, then you don't need to check any elements. By doing something
>> like
>>
>> assert("" == null)
>>
>> you're likely creating an array which has whose arr value is non-null (but
>> it's not actually part of the array; the memory there is extra capacity
>> which will be used if/when you append to the array; it avoids extra memory
>> re-allocations that way) and an array whose arr value is null. Both will
>> have their lengths be 0.
>>
>> When "is null" is used, it checks that arr is null. If "==" is used, it
>> checks whether the elements of the two arrays are the same. Since they
>> have the same length, that would mean iterating through all of their
>> elements to verify that each element is the same. But since there are no
>> elements in either, no elements get checked. The fact that the "" array
>> has extra capacity is irrelevant. It's not part of the array, just arr.
>> So, the fact that
>>
>> assert("" != null);
>>
>> fails is intended. It's a result of how arrays are designed.
>
> On reflection, I should have used the name ptr instead of array, since that
> _is_
> its actual name. It slipped my mind I guess...
>
> - Jonathan M Davis
>