Article: Why Const Sucks

[Steven Schveighoffer]

On 3/8/18 9:58 AM, joe wrote:
> On Monday, 5 March 2018 at 10:57:35 UTC, Jonathan M Davis wrote:
>> Here's something I wrote up on const:
>> /snip
> 
> May be not entirely related, but a little gotcha.
> 
> given:
> 
> interface XY {}
> 
> class Foo: XY {}
> class Bar: XY {}
> 
> void doSomething(in XY sth)
> {
>    auto foo = cast(Foo)sth; // error in @safe code
> }
> 
> But the compiler doesn't emit a warning that const got cast away in 
> @system code.
> Since @system is the default setting and casting const away is by 
> definition undefined behavior, there should be some feedback.
> 
> If you cast away const in C++ you need to be explicit about it by using 
> const_cast, documents the intention rather than D's swiss-army-cast.

This is a problem with the cast system, not const. D has one cast 
mechanism, and it's used to:

a) dynamic type cast
b) hook user-supplied opCast
c) enter undefined behavior by casting away const/immutable.

I really think we should have a separate mechanism for a and b, as they 
are far less dangerous than c.

std.conv.to does this as well, but the language should have some 
safeguards against using "safe casting" incorrectly.

-Steve