• NotSpooky (2/2) Jan 18 2017 Is it undefined behavior to assign &this to a pointer in the
• Adam D. Ruppe (10/12) Jan 18 2017 Yes:
• pineapple (6/18) Jan 18 2017 Practically speaking I've found that if the struct was allocated
• NotSpooky (5/17) Jan 18 2017 You already answered on the IRC so thanks X2.
• Adam D. Ruppe (8/11) Jan 18 2017 Maybe... though in practice (and with C compatibility), pointers

NotSpooky <zoteman94 gmail.com> writes:

Is it undefined behavior to assign &this to a pointer in the 
constructor of a struct?

Adam D. Ruppe <destructionator gmail.com> writes:

On Wednesday, 18 January 2017 at 22:57:22 UTC, NotSpooky wrote:
 Is it undefined behavior to assign &this to a pointer in the 
 constructor of a struct?

Yes:

http://dlang.org/spec/struct.html
"A struct is defined to not have an identity; that is, the 
implementation is free to make bit copies of the struct as 
convenient."

That means it might copy and/or move it without giving you a 
chance to update the pointer. Updating in postblit can help 
sometimes but still the compiler and library are allowed to move 
structs without notice.

pineapple <meapineapple gmail.com> writes:

On Wednesday, 18 January 2017 at 23:08:07 UTC, Adam D. Ruppe 
wrote:
 On Wednesday, 18 January 2017 at 22:57:22 UTC, NotSpooky wrote:
 Is it undefined behavior to assign &this to a pointer in the 
 constructor of a struct?

 Yes:

 http://dlang.org/spec/struct.html
 "A struct is defined to not have an identity; that is, the 
 implementation is free to make bit copies of the struct as 
 convenient."

 That means it might copy and/or move it without giving you a 
 chance to update the pointer. Updating in postblit can help 
 sometimes but still the compiler and library are allowed to 
 move structs without notice.

Practically speaking I've found that if the struct was allocated 
on the heap, then &this acquires that pointer and seems not to 
break anything, e.g. how it's used in the uncopyable struct here 
https://github.com/pineapplemachine/mach.d/blob/master/mach/collect/linkedlist.d#L165

NotSpooky <zoteman94 gmail.com> writes:

On Wednesday, 18 January 2017 at 23:08:07 UTC, Adam D. Ruppe 
wrote:
 On Wednesday, 18 January 2017 at 22:57:22 UTC, NotSpooky wrote:
 Is it undefined behavior to assign &this to a pointer in the 
 constructor of a struct?

 Yes:

 http://dlang.org/spec/struct.html
 "A struct is defined to not have an identity; that is, the 
 implementation is free to make bit copies of the struct as 
 convenient."

 That means it might copy and/or move it without giving you a 
 chance to update the pointer. Updating in postblit can help 
 sometimes but still the compiler and library are allowed to 
 move structs without notice.

You already answered on the IRC so thanks X2.
So, it's problematic to have pointers to structs in all cases 
according to spec?

Adam D. Ruppe <destructionator gmail.com> writes:

On Thursday, 19 January 2017 at 00:55:42 UTC, NotSpooky wrote:
 You already answered on the IRC so thanks X2.
 So, it's problematic to have pointers to structs in all cases 
 according to spec?

Maybe... though in practice (and with C compatibility), pointers 
to ones where you know the memory management scheme is fine.

So if you declare a local, and get a pointer to it, you're OK. Or 
if you new it, or malloc it, or something like that.

The big problem with a pointer to itself in the constructor or as 
a member is that the struct itself doesn't know where it is going 
or how it was allocated, just the code outside does.