Inherited const when you need to mutate

[Jonathan M Davis]

On Monday, July 09, 2012 18:34:14 David Piepgrass wrote:
> I guess D does not have 'mutable' (like C++) to override const on
> methods?

No, const is actually const. You can't modify anything which is const through 
that reference (or pointer) to that data. Casting away const and mutating 
something is undefined behavior. This provides much stronger guarantees and 
better enables compiler optimizations. More importantly, it's required for 
immutabel, since a const variable could actually be immutable underneath, in 
which case, depending on how it was constructed (e.g. it was put in ROM), it 
could result in a segfault if you tried to mutate it after casting away const.

http://stackoverflow.com/questions/4219600/logical-const-in-d

So, const gives you much greater benefits in D than in C++, but it also comes 
at a much higher cost. And it comes to a bit of a head in Object, because 
certain function _must_ be const in Object in order to work with const (namely 
opEquals, opCmp, toHash, and toString), but they _can't_ be const for certain 
schemes (e.g. caching) to work. The open question is how to fix this (or if it 
even can be fixed). But the current plan is to make all 4 of those functions be 
@safe pure const nothrow, and that _will_ make certain schemes effectively 
infeasible.

- Jonathan M Davis