Best practices of using const

Marco de Wild mdwild at
Mon Feb 18 06:50:32 UTC 2019

On Wednesday, 13 February 2019 at 16:40:18 UTC, H. S. Teoh wrote:
> On Wed, Feb 13, 2019 at 11:32:46AM +0000, envoid via 
> Digitalmars-d-learn wrote:
> Unfortunately, that guarantee also excludes a lot of otherwise 
> useful idioms, like objects that cache data -- a const object 
> cannot cache data because that means it's being mutated, or 
> lazily-initialized objects -- because once the ctor has run, 
> the object can no longer be mutated. Most notably, D's powerful 
> range idiom is pretty much unusable with const because 
> iteration over a range requires mutating the range (though 
> having const *elements* in a range is fine).  This doesn't seem 
> as bad at first glance, but it wreaks havoc on generic code, 
> another thing that D is purportedly good at. It's very hard 
> (and often impossible) to write generic code that works with 
> both const and mutable objects.
> So ironically, the iron-clad semantics of D's const system 
> turns out to be also its own downfall.
> T

I agree that const by nature unfortunately kills lazy 
initialization. However, I don't really understand why const is a 
problem with ranges. Const elements are not a problem. Iterating 
over a range consumes it (if I understand correctly). It does not 
make sense to be able to consume a const object, so from my point 
of view it's perfectly logical to disallow iterating const 
ranges. If I'm missing something, please correct me.

I use const quite thoroughly in my project (a mahjong board game) 
and in fact I am writing a blog post explaining how it helped me 
understand what was happening in my code base. It enforces 
encapsulated mutations. In classic OOP languages, mutable objects 
propagate through the entire system, unless you actively create 
an immutable copy of it (which is a lot of work for little gain). 
If someone modifies your object on a place you don't expect (e.g. 
creating and persisting data when rendering a read-only view), it 
becomes hard to impossible to reason about the problem and debug 
Refactoring in const was a lot of work, but I think it made my 
code better in the end. I didn't run into any problems when using 
it, except when I tried to modify an object where I should not 
have (e.g. sorting a hand when rendering the view). I was able to 
untangle the spaghetti because the compiler poked me about it. As 
I didn't run into any problems and it helped clean up my code 
base, I would recommend trying it.

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