Why double not? (!!)

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

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) {

}