On 16.09.2014 17:38, Andrei Alexandrescu wrote:
On 9/15/14, 4:49 PM, Rainer Schuetze wrote:
On 15.09.2014 10:24, Andrei Alexandrescu wrote:
Hmm, seems fine when I try it. It feels like a bug in the type system,
though: when you make a copy of const(RCXString) to some RCXString, it
removes the const from the referenced RCBuffer struct mbuf!?
The conversion relies on pure constructors. As I noted in the opening
post, I also think there's something too lax in there. If you have a
reduced example that shows a type system breakage without cast, please
submit.
Here's an example:
module module2;
struct S
{
union
{
immutable(char)* iptr;
char* ptr;
}
}
void main()
{
auto s = immutable(S)("hi".ptr);
S t = s;
t.ptr[0] = 'A';
}
It seems the union is hiding the fact that there are mutable references.
Only the first field is verified when copying the struct. Is this by
design? (typeof(s.ptr) is "immutable(char*)")
Not sure whether that's a bug or feature :o). In fact I'm not even
kidding. The "it's a bug" view is obvious. The "it's a feature" view
goes by the reasoning: if you're using a union, it means you plan to do
gnarly things with the type system anyway, so the compiler may as well
tread carefully around you.
Through a rather interesting coincidence, I was talking to Walter during
the weekend about the idiom:
union
{
immutable T data;
T mdata;
}
which I found useful for things like incrementing the reference counter
for non-mutable data. I was discussing how it would be cool if the
compiler recognized the construct and did something interesting about
it. It seems it already does.
Andrei
There is already bug report for this:
https://issues.dlang.org/show_bug.cgi?id=12885
It also references the issue why this has been changed pretty recently:
https://issues.dlang.org/show_bug.cgi?id=11257
I'm on the fence whether this is convenient or makes it too easy to
break const "guarantees". It seems strange that you can modify a
const-reference only after you make a copy of the "pointer". ATM I'd
prefer seeing an explicite cast for that.
