Why double not? (!!)

[Is it possible to store different generic types? via Digitalmars-d-learn]

On Saturday, 19 November 2016 at 07:51:36 UTC, Is it possible to 
store different generic types? wrote:
> On Saturday, 19 November 2016 at 06:58:38 UTC, Era Scarecrow 
> wrote:
>> On Saturday, 19 November 2016 at 04:54:22 UTC, Xinok wrote:
>>> On Saturday, 19 November 2016 at 03:52:02 UTC, Ryan wrote:
>>>> Why do I see double `not` operators sometimes in D code? An 
>>>> example it the last post of this thread.
>>>>
>>>> http://forum.dlang.org/thread/ktlpnikvdwgbvfaamrsk@forum.dlang.org
>>>>
>>>>> import core.sys.windows.windows : GetConsoleCP;
>>>>> bool hasConsole = !!GetConsoleCP();
>>>>
>>>> Thanks.
>>>
>>> It's a more concise way of writing:
>>>     GetConsoleCP() != 0
>>>
>>> You can do this in C/C++ as well (and presumably some other 
>>> languages).
>>
>>  Hmmm... thinking about it, it does make perfect sense. The 
>> first ! converts it to bool, the other inverts it back to it's 
>> positive/negative state.
>>
>>  Although it's a combination of logic I wouldn't have through 
>> of unless I saw it. But testing the result on any number 
>> (float, double or real) won't be precise and would take far 
>> longer (and more complicated) using another method.
>
> It's a very common practice in any language that uses 
> truthy/falsey, especially seen a lot in Javascript.
>
> Generally it's not necessary unless you want to be explicit 
> about checking upon a bool.
>
> Ex.
>
> auto hasModel = !!view.model;
>
> if (hasModel) {
>     ...
> }
>
> Could very well just be
>
> auto model = view.model;
>
> if (model) {
>
> }

It's especially difficult with numbers like you did point out and 
it completely depends on languages.

Most languages have false when it's 0, null, undefined etc. and 
everything else is true.

Which means -1 would be true, 0 would be false, 1 would be true, 
0.000001 would be true, -0.000000001 would be true.