moveFront() and friends: Request for comment

[Andrei Alexandrescu]

On 8/27/10 7:44 PDT, Pillsy wrote:
> dsimcha Wrote:
> [...]
>> I'd like any comments anyone might have on to what extent
>> arbitrarily expensive postblits should be considered in the
>> design of Phobos.
>
> I agree with you: expensive post-blits just don't seem sufficiently
> necessary in D to warp the design of the standard library around
> them, and have a distinctly anti-patternish feel to them. You have
> too many other options, like straight reference semantics, copy-on-
> write, and (with immutability and GC) safely shared structure.
>
> The last can be an incredibly useful technique for reducing the
> cost of copying, because it allows you to treat an immutable
> reference type exactly like a value type.
>
> Cheers,
> Pillsy

Clearly immutable sharing helps, and clearly reference counting and COW 
are valid techniques. However, the situation is not as cut and dried.

The problem with RC/COW is that they reduce exception safety essentially 
_everywhere_ else but the copy constructor. Once you have a type using 
RC/COW, any mutation of that object, even one that ostensibly doesn't 
cause resource allocation, might have arbitrary cost or fail. This was 
the experience with std::string in C++ - its creators did everything 
they could to enable reference counting, and the outcome was quite 
unpleasant.

Add to this the implementation annoyance of checking for aliasing in 
_every_ single method of the type. RefCounted in phobos can help with 
that, but not without a cost.

So I'm not sure it's as simple a decision as it might sound.


Andrei