Structs should not contain pointers to internal data

[Diggory]

On Monday, 3 June 2013 at 16:00:58 UTC, Ali Çehreli wrote:
> On 06/03/2013 05:26 AM, Saurabh Das wrote:
>
> > Thank you @Ali and @Jonothan!
> >
> > So essentially since I will be storing a pointer,
> Telemetry!(T) is NOT safe
> > to use only with structs in general.
> >
> > If I have something like:
> >
> > struct UsefulStruct2
> > {
> >      this(this) @disable;
> >      this(UsefulStruct2) @disable;
> >    this(ref const(UsefulStruct2)) @disable;
> >    ref UsefulStruct2 opAssign(UsefulStruct2) @disable;
> >    ref UsefulStruct2 opAssign(ref const(UsefulStruct2))
> @disable;
> >
> >      int importantValue;
> >      auto tel1 = Telemetry!int(importantValue);
> > }
> >
> > Does that ensure that UsefulStruct2 is not relocateable and
> thus I can
> > safely store a pointer to importantValue?
>
> No. The compiler can still move a struct by blit (bit level 
> transfer). Blit is based on good old memcpy. For a "copy", 
> post-blit is for making corrections after the fact. On the 
> other hand, the programmer cannot interfere if it is a "move".
>
> For example, rvalues are moved, e.g. to an array element as in 
> the following example:
>
> import std.stdio;
> import std.array;
>
> struct UsefulStruct2
> {
>     this(this) @disable;
>     this(UsefulStruct2) @disable;
>     this(ref const(UsefulStruct2)) @disable;
>     ref UsefulStruct2 opAssign(UsefulStruct2) @disable;
>     ref UsefulStruct2 opAssign(ref const(UsefulStruct2)) 
> @disable;
>
>     int importantValue;
>     int * p;
> }
>
> UsefulStruct2 makeObject(int i)
> {
>     UsefulStruct2 u;
>     u.importantValue = i;
>     u.p = &u.importantValue;  // <-- self-referencing
>     return u;
> }
>
> void main()
> {
>     auto arr = [ makeObject(1) ];
>     assert(arr.front.p != &arr.front.importantValue);  // 
> PASSES!
> }
>
> > If not, what constraints do I need to add to my classes to
> ensure that I
> > don't run into subtle bugs when structs relocate?
>
> As you see, @disable is cripling and not a solution for this. 
> As far as I know, the only option is to observe this rule.
>
> I agree with you that a struct may become self-referencing, 
> unknowingly and indirectly through members of other types.
>
> Ali

You can get around this limitation by making a wrapper struct 
which uses special values to represent pointers which point 
within the containing struct, and does the conversion 
automatically when you dereference it.