COW vs. in-place.

[Reiner Pope]

> Why not:
> 
>     str = toupper(str);     // in-place
>     str = toupper(str.dup); // COW

This is not copy on write. That is simply 'always copy', and this 
performs worse than COW (which in turn performs worse than in-place, if 
in-place is possible). Walter has also said earlier that, with COW, it 
should be the responsibility of the writer to ensure the copy, not the 
caller.