opAssign(int) necessitates this(this) for automatic opAssign to work

[Simen kjaeraas]

On Sun, 27 Dec 2009 02:23:47 +0100, Ali Çehreli <acehreli at yahoo.com> wrote:

> Simen kjaeraas wrote:
>  > On Sun, 27 Dec 2009 01:42:07 +0100, Ali Çehreli <acehreli at yahoo.com>  
> wrote:
>  >
>  >> I've tested the following with dmd 2.037.
>  >>
>  >> The compiler generated opAssign is disabled by the definition of
>  >> opAssign(int). The compiler rejects the following assignment
>  >> operation. (The error message is in the comment below.)
>  >>
>  >> Is this by design?
>  >>
>  >> When I also define post-blit, the compiler generated opAssign is
>  >> available again and seems to work correctly. (My struct doesn't have
>  >> any members for brevity.)
>  >>
>  >> The program below compiles when this(this) is provided.
>  >>
>  >> void main()
>  >> {
>  >>      S s0;
>  >>      s0 = s0;    // ERROR
>  >> }
>  >
>  > This piece of code does post-blit, not opAssign(int).
>
> post-blit is when an object is constructed from another one. The above  
> has two already constructed objects on both sides. So I think it is  
> assignment.
>
>  > Try defining opAssign(S). That should work here.
>
> I know; but the problem is, defining opAssign(int) disables the  
> compiler-generated opAssign(S); which is then interestingly available  
> again, once this(this) is defined.
>
> I just wanted to know whether there is a compiler bug in this behavior.
>
> Ali

Structs being what they are (Plain Old Data), opAssign(typeof(this)) is
basically the same as this(this). In both cases, you create a copy of an
existing struct, and thus need to .dup referenced data that should be
unique, and whatever else your copying function needs.

If you define an opAssign, you've basically informed the compiler that
this struct should only be assignable from whatever parameters opAssign
takes. If you define postblit, you've said 'this struct can be
constructed from another instance of the same struct', meaning it's
assignable to itself. If you can't, then when would you ever do a
postblit?

-- 
Simen