Unintentional sharing?

[Nick Treleaven]

On Thursday, 6 June 2024 at 17:49:39 UTC, Andy Valencia wrote:
> I was using instance initialization which allocated a new 
> object.  My intention was this initialization would happen 
> per-instance, but all instances appear to share the same 
> sub-object?  That is, f1.b and f2.b appear to point to a single 
> object?  Obviously I moved the new into the initializer code, 
> but I hadn't appreciated how initial instance values were 
> calculated once.  Interestingly, this makes it similar to how 
> Python calculates default argument values for functions.
>
> class Bar {
>     int z = 3;
> }
>
> class Foo {
>     auto b = new Bar();
> }
>
> void
> main() {
>     import std.stdio : writeln;
>
>     auto f1 = new Foo(), f2 = new Foo();
>     f1.b.z = 0;
>     writeln(f2.b.z);
> }

This is a long standing issue:
https://issues.dlang.org/show_bug.cgi?id=2947

I think with the next edition we can disallow (tail) mutable 
initializers for fields (and TLS globals too).