Copying structs via function (not bitwise)

[Matthias Walter]

Bill Baxter Wrote:

> Matthias Walter wrote:
> > Bill Baxter Wrote:
> > 
> >> Matthias Walter wrote:
> >>> Is there any other way of copying structs with '=' operator, other than the default way of a bitwise copy? I quote the FAQ:
> >>>
> >>> "Structs, being value objects, do get copied about. A copy is defined in D to be a bit copy. I've never been comfortable with any object in C++ that does something other than a bit copy when copied. Most of this other behavior stems from that old problem of trying to manage memory. Absent that, there doesn't seem to be a compelling rationale for having anything other than a bit copy."
> >>>
> >>> The reason, I'm asking for this, is that I'm working on a GMP port. Because numbers _must_ be value types, I must use structs. But as different instances of the same number must reference another memory region of the GMP internals, I must call special copy functions, if I do "a = b;" on two gmp-structs, a refers to the same gmp-internal memory as b.
> >>>
> >>> Is there any way to do this?
> >>>
> >>> best regards
> >>> Matthias Walter
> >> opAssign can be used to override the meaning of A=B where A and B are 
> >> different types.  But by design you can't change the meaning A=B where A 
> >> and B are the same type.
> >>
> >> But you could make it work using any of the following:
> >>
> >> A ~= B;
> >> A.foo = B;
> >> A = B.foo;
> >> assign(A,B);
> >> A.assign(B);
> >> etc..
> >>
> >> And I'm sure downs could suggest a few more workable syntaxes. :-)
> >>
> >> --bb
> > 
> > Problem is that GMP-structs should behave like normal numbers and one also does not write:
> > int i;
> > i.value = 10;
> > 
> > Does this mean, that in the current situation, it is impossible to port the GMP library with structs so that 
> > 
> > T i,j = 0;
> > i = j;
> > j++;
> > assert (i == 0);
> > 
> > works for T = int, T = long, T = float, T = gmp_struct ?
> > 
> > best regards
> > Matthias Walter
> 
> Yep.  That's what it means.  I don't know what's in gmp_struct but if it 
> includes a pointer to something that must be allocated then D ain't 
> gonna be able to do it.
> 
> But D2 is supposedly going to be getting features to make ref-counting 
> smart pointer structs possible, and I'm pretty sure that means there has 
> to be some way to trigger side effects upon assignments.
> 
> --bb

Okay, then I know on it for now. The problem is indeed, that the C-library for GMP must allocate some memory for the number, meaning you always only maintain some pointers to that memory. bitwise-copy would then lead to a double reference to the same number...

best regards
Matthias Walter