On 04/18/2013 06:22 AM, Joseph Rushton Wakeling wrote:> On 04/17/2013
10:29 PM, Ali Çehreli wrote:
>> Yes but as I said I am not sure how useful or needed this whole
thing is.
>>
>> The language handles copy, move, and assignment for structs.
takeOver() seems to
>> be a primitive operation for classes.
>
> Could you expand a bit on that? Just that I'm not sure I understand your
> meaning completely, and I'm curious. :-)
Being value types, structs enjoy automatic handling of copy, assignment,
and in the case of rvalues, move.
struct S
{
int i;
}
S makeS()
{
return S();
}
void main()
{
auto a = S();
auto b = a; // a is copied to b
b = a; // a is assigned to b
b = makeS(); // the returned rvalue is 'moved' to b
}
Of course that works only for structs with simple value members. When
the automatic handling is not sufficient or does the wrong thing, then
the programmer must provide this(this), opAssign(), or ~this(). Things
get more interesting when the members are mutable or immutable references.
With the above definition of S, all of the following compile:
immutable(S) imm0;
S mut = imm0; // immutable to mutable
immutable(S) imm1 = mut; // mutable to immutable
immutable(S) imm2 = imm0; // immutable to immutable
Now naively add another member to S:
struct S
{
int i;
int[] arr; // <-- added
}
Only the latter of the three assignments compile:
immutable(S) imm0;
S mut = imm0; // compilation ERROR
immutable(S) imm1 = mut; // compilation ERROR
immutable(S) imm2 = imm0; // compiles
Anyway... I am trying to wrap up what I have learned so far before going
any further. :)
For classes, none of these operations make sense: There is no automatic
copying, no automatic assignment, nor there are rvalues to move.
> I'd assumed this problem would be some fairly straightforward aspect
of the
> language that I just wasn't familiar with, so I'm quite struck by the
fact that
> it actually seems a non-trivial problem.
My experience has been with C++ so I don't know how this is solved in
Java or C#. Maybe this is not a problem with the common idioms in those
languages, which would also mean that this is not a problem with D
classes. Rather, if one has decided to go with a class, then value
semantics were not a consideration to begin with.
Ali
