Problems with sort

[Andrei Alexandrescu]

On 12/10/10 6:37 PM, Craig Black wrote:
> Thanks for the help guys. I implemented length and opIndex and now I get
> the following errors:
>
> C:\dmd2\windows\bin\..\..\src\phobos\std\algorithm.d(5341): Error:
> template std.algorithm.swap(T) if
> (!is(typeof(T.init.proxySwap(T.init)))) does not match any function
> template declaration
>
> C:\dmd2\windows\bin\..\..\src\phobos\std\algorithm.d(5341): Error:
> template std.algorithm.swap(T) if
> (!is(typeof(T.init.proxySwap(T.init)))) cannot deduce template function
> from argument types !()(_error_,Range)
>
> -Craig

This concerns a decision not yet firmed-up on whether objects should be 
cheap to copy or not.

What happens is that sort wants to swap elements at given indices. Your 
opIndex, I assume, returns by value. (If it returned by reference you'd 
have no problem sorting.) Then sort makes a number of heroic attempts at 
figuring out how to swap elements in your range (e.g. by trying 
moveAt()) before failing in frustration.

If we mandate cheap copy, then swap via two copies should be fine, which 
simplifies implementation a great deal and also your life as a user of 
it. We haven't made that decision yet. There is the concern that things 
would become too inefficient.


Andrei