Copying structs via function (not bitwise)

[Bill Baxter]

Matthias Walter wrote:
> 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

You can't make natural syntax work for everything but you _can_ make 
unnatural syntax work for everything.  I.e. you could make it so that:

T i,j = 0;
assign(i,j);
j++;
assert(i==0);

works for all the T's you want it to.
Not quite as sweet as having it all appear to be built-in but better 
than a spoon in the eye.

--bb