• Salih Dincer (20/20) Nov 26 2021 Hi All;
• Stanislav Blinov (5/8) Nov 26 2021 `empty` is not required to be `const`, but it is required to
• Steven Schveighoffer (9/30) Nov 26 2021 No, there is no specification of whether any range methods have to be

Salih Dincer <salihdb hotmail.com> writes:

Hi All;

I have two questions that make each other redundant. Please 
answer one of them. I'm implementing ```bool empty() const``` for 
ranges as below:

```d
   bool empty() // const
   {
     bool result;

     if(!head)
     {
       result = true;
       fastRewind();
     }
     return result; // head ? false : true;
   }
```
* Is the const essential for ranges?
* Is it possible to rewind the pointer (```Node * head;```) when 
my head is empty by the const?

Thanks...

Stanislav Blinov <stanislav.blinov gmail.com> writes:

On Friday, 26 November 2021 at 10:44:10 UTC, Salih Dincer wrote:

 * Is the const essential for ranges?
 * Is it possible to rewind the pointer (```Node * head;```) 
 when my head is empty by the const?

`empty` is not required to be `const`, but it is required to 
yield the same result if called multiple times without mutating 
the range (see 
https://dlang.org/phobos/std_range_primitives.html#.isInputRange). In other
words, you really ought not to mutate the range in implementation of `empty`,
or at least not to the extent you seem to want to.

Steven Schveighoffer <schveiguy gmail.com> writes:

On 11/26/21 5:44 AM, Salih Dincer wrote:
 Hi All;
 
 I have two questions that make each other redundant. Please answer one 
 of them. I'm implementing ```bool empty() const``` for ranges as below:
 
 ```d
    bool empty() // const
    {
      bool result;
 
      if(!head)
      {
        result = true;
        fastRewind();
      }
      return result; // head ? false : true;
    }
 ```
 * Is the const essential for ranges?

No, there is no specification of whether any range methods have to be 
const. As Stanislav says, it is only a requirement that subsequent calls 
return the same value as long as popFront has not been called.

 * Is it possible to rewind the pointer (```Node * head;```) when my head 
 is empty by the const?

If const is set, then any members are treated as const, and anything 
they point to are treated as const. So no.

That being said, if `fastRewind` changes `empty` from true to false, the 
method is invalid.

-Steve