`this` template params for struct not expressing constness.

[Simen Kjærås]

On Monday, 8 June 2020 at 09:08:40 UTC, adnan338 wrote:
> On Monday, 8 June 2020 at 08:10:19 UTC, Simen Kjærås wrote:
>> On Monday, 8 June 2020 at 07:35:12 UTC, adnan338 wrote:
>>>     Self* searchTree(this Self)(auto in ref T item) const {
>>>         if (&this is null)
>>>             return null;
>>>         if (this.item == item)
>>>             return &this;
>>>         return (this.item < item) ?
>>>             this.right.searchTree(item) :
>>>             this.right.searchTree(item);
>>>     }
>>
>> This method is const, which means 'this' is const, while Self 
>> is not. What you're looking for here is inout 
>> (https://dlang.org/spec/function.html#inout-functions):
>>
>>     auto searchTree()(auto in ref T item) inout {
>>         if (&this is null)
>>             return null;
>>         if (this.item == item)
>>             return &this;
>>         return (this.item < item) ?
>>             this.right.searchTree(item) :
>>             this.right.searchTree(item);
>>     }
>>
>> --
>>   Simen
>
> Thank you. Few followup questions, if you don't mind.
>
> 1. What does that blank template parameter mean?

Just forces the function to be a template. The only reason for 
this is it's required for auto ref to work, which you may or may 
not need on that function.


> 2. Since `inout` acts as a wildcard for 
> immutable/const/non-const qualifiers, what should I do to have 
> the compiler ensure that my method does not mutate a non-const 
> tree inside the body?

Inside inout functions, `this` is treated as const - any attempt 
to modify it should give a compile error. Since D const is 
transitive, anything reachable from `this` is also treated as 
const.  If you're able to mutate a non-const tree inside the 
body, there's a bug in the compiler.


--
   Simen