Re: [boost] [gsoc] Pointer Plus Bits Behavior and Interface

Subject: Re: [boost] [gsoc] Pointer Plus Bits Behavior and Interface
From: Stefan Strasser (strasser_at_[hidden])
Date: 2010-07-15 15:43:47

Zitat von Andrew Sutton <andrew.n.sutton_at_[hidden]>:

>>
>> > Would it be better to make the pointer_plus_bits convertible
>> > to a boolean
>> > based on the value of the pointer being stored? This would
>> > allow it to be
>> > more like to a pointer being that you could, as many people do, put it
>> > inside of an if statement to check to see if its null.
>>
>> No.  One could argue that the bits should be evaluated to produce the
>> Boolean result as easily as evaluating the pointer.  There are two aspects
>> in pointer_plus_bits and they shouldn't be obscured or conflated.
>>
>
> I strongly disagree with using views in the design of this data structure.
> It's not such a complex concept that you would need to explicitly decouple
> its concerns via views. You end up designing a pointer+bits that can't be
> used natively as a pointer or bits.
>
> The data structure is pointer_(plus_bits). First and foremost, I would
> expect its instances to model (at least some) Pointer semantics, which

I could agree with that given the name "pointer_plus_bits". but it  
seems to me that the underlying generic is something like  
"bitfield_tuple". so "pointer_plus_bits" could easily be an adapter  
(not: view) to this underlying and provide an interface to the pointer  
part by default, if you want to be able to write "if(p)".

but the problem remains that a bitfield tuple or pointer plus bits are  
2 separate logical variables (...the purpose of this library) and the  
user needs a way to differentiate between the interfaces to the two.
obvious example: pointer plus an integer of 3 bits. operator++ could  
increment the pointer/iterator, or increment the integer.

IIUC all you're saying is that you want a type that provides a view to  
the first part by default, and to the second on request.

> includes "if(p)" and not "if(pointer_view<...>(p))". Dereferencing the

probably more like "if(p.pointer())" which would return the pointer  
view. (or "if(p)" if pointer_plus_bits is an adapter as described  
above.)