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digitalmars.D.learn - Can passing an address of this to a non-copyable object be made

reply aliak <something something.com> writes:
So if we had this:

struct A(T) {
   auto proxy()  trusted {
     return B!T(&this);
   }
}

struct B(T) {
   private A!T* source;
   private this(A!T* s) { source = s; }
    disable this();
    disable this(this) {}
    disable void opAssign(B!T);
}

In order for f to be "safe" I need to ensure that B!T(&this) does 
not escape the scope of A!T. I figured disable construction and 
copying may work, but it seems you can still get it moved:

void main()  safe {
   auto f() {
     auto a = A!int();
     return a.proxy;
   }
   auto escaped = f; // escaped.source is gone...
}

Anyway around this?

Cheers,
- Ali
Aug 26 2018
parent reply Nicholas Wilson <iamthewilsonator hotmail.com> writes:
On Sunday, 26 August 2018 at 20:17:30 UTC, aliak wrote:

 So if we had this:

 struct A(T) {
   auto proxy()  trusted {
     return B!T(&this);
   }
 }

 struct B(T) {
   private A!T* source;
   private this(A!T* s) { source = s; }
    disable this();
    disable this(this) {}
    disable void opAssign(B!T);
 }

 In order for f to be "safe" I need to ensure that B!T(&this) 
 does not escape the scope of A!T. I figured disable 
 construction and copying may work, but it seems you can still 
 get it moved:

 void main()  safe {
   auto f() {
     auto a = A!int();
     return a.proxy;
   }
   auto escaped = f; // escaped.source is gone...
 }

 Anyway around this?

 Cheers,
 - Ali


Not sure abut the current language but DIP1014 
https://github.com/dlang/DIPs/blob/master/DIPs/DIP1014.md#final-review

"The point was made that allowing opPostMove to be overidden 
raises the question of what to do when it is annotated with 
 disable. The concensus was that, in such a case, an actual 
attempt to move the object would result in a compilation error."

So, soon™?
Aug 26 2018
parent reply Jonathan M Davis <newsgroup.d jmdavisprog.com> writes:
On Sunday, August 26, 2018 5:10:29 PM MDT Nicholas Wilson via Digitalmars-d-
learn wrote:

 On Sunday, 26 August 2018 at 20:17:30 UTC, aliak wrote:

 So if we had this:

 struct A(T) {

   auto proxy()  trusted {

     return B!T(&this);

   }

 }

 struct B(T) {

   private A!T* source;
   private this(A!T* s) { source = s; }
    disable this();
    disable this(this) {}
    disable void opAssign(B!T);

 }

 In order for f to be "safe" I need to ensure that B!T(&this)
 does not escape the scope of A!T. I figured disable
 construction and copying may work, but it seems you can still
 get it moved:

 void main()  safe {

   auto f() {

     auto a = A!int();
     return a.proxy;

   }
   auto escaped = f; // escaped.source is gone...

 }

 Anyway around this?

 Cheers,
 - Ali


 Not sure abut the current language but DIP1014
 https://github.com/dlang/DIPs/blob/master/DIPs/DIP1014.md#final-review

 "The point was made that allowing opPostMove to be overidden
 raises the question of what to do when it is annotated with
  disable. The concensus was that, in such a case, an actual
 attempt to move the object would result in a compilation error."

 So, soon™?


Yeah. Hopefully, we're able to disable moving at some point in the near
future. However, right now, it's definitely not possible. So, if you have a
type where it won't work properly if it's ever moved, then either you need
to rethink what you're doing, or you must be _very_ careful with how you use
any object of that type so that you don't ever use it in a way that even
might result in it being moved.

- Jonathan M Davis
Aug 26 2018
parent aliak <something something.com> writes:
On Monday, 27 August 2018 at 00:15:18 UTC, Jonathan M Davis wrote:

 On Sunday, August 26, 2018 5:10:29 PM MDT Nicholas Wilson via

 So, soon™?




Bah humbug. Was afraid of this :p


 Yeah. Hopefully, we're able to disable moving at some point in 
 the near future. However, right now, it's definitely not 
 possible. So, if you have a type where it won't work properly 
 if it's ever moved, then either you need to rethink what you're 
 doing, or you must be _very_ careful with how you use any 
 object of that type so that you don't ever use it in a way that 
 even might result in it being moved.

 - Jonathan M Davis


I guess I'll just stay  system for now then. Maybe I'll be able 
to figure something out some time.
Aug 27 2018