What's wrong with just a runtime-checked const?

[Andrei Khropov]

Reiner Pope wrote:

> Disclaimer: my background is a small amount of Java and C#, so perhaps I   am
> completely confusing the issue because of practically no C++ experience.
> Please tell me if that's the case.
> 
> Walter seems to have two main objections to a C++-style const:
> 1. it can be subverted with a const_cast, removing any guarantees about it.

Different C++ style casts are not supported in D so it's not an issue, I think.

> 2. it is hard to implement, but this doesn't seem to concern him as much as
> the former.
> 
> People have expressed a desire for const especially to ensure the
> Copy-on-Write idiom, and to make libraries more robust.
> 
> Wouldn't a runtime const check be much more flexible than a compile-time
> check? Const-safeness is fundamentally a correctness-checking feature, just
> like unit tests, so why not make it operate exactly like unit tests?

I think things that can be checked at compile-time should be checked at
compile-time and don't slow down runtime code.

Your proposed type of checks also has a kind of dynamic flavor
(Smalltalk/Python like) which is not very D way :-)

Unit tests are different. They are rather "post-compile" or "pre-runtime" in
fact.

I suppose you actually mean DbC, right?

> The even better thing about this is that most code doesn't need to have
> const-correctness in mind when writing it

I think that const-correct code is important and expresses good design and also
aids compiler in optimization.

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Anyway, I think const discussion should be postponed until we get all the
things with imports right which is the highest priority for now, IMHO.