Copying structs with pointers

[Jonathan M Davis]

On 2011-07-20 14:49, Peter Alexander wrote:
> Just came across this issue today. I wanted to create a typical value
> type wrapper around a heap allocated object. I even created a nice
> postblit constructor like a good D programmer.
> 
> 
> struct Ptr
> {
> private int* p;
> this(int x) { p = new int; *p = x; }
> this(this) { int x = *p; p = new int; *p = x; }
> ...
> }
> 
> void main()
> {
> const Ptr a = Ptr(1);
> Ptr b = a; // error
> }
> 
> 
> Problem is, this code is illegal because you can't create a non-const
> copy of a const value type that contains pointers or references (due to
> transitive const).
> 
> I can get around it in this particular instance by creating a dup
> property (like a C++ copy constructor):
> 
> struct Ptr
> {
> @property const Ptr dup()
> {
> return Ptr(*p);
> }
> }
> 
> But it still means that my struct is non-copyable in general. For
> example, I can't fill a range with a const Ptr.
> 
> const Ptr a = Ptr(1);
> Ptr[10] arr;
> std.algorithm.fill(arr[], a); // error
> 
> 
> I understand why the shallow copy isn't allowed in general (it would
> break transitive const), but as the dup property demonstrates, I am able
> to create a deep copy of Ptr without violating const-correctness -- the
> problem is that I am not able to express this using a postblit constructor.
> 
> Am I missing something? How are you supposed to define value semantics
> on structs containing pointers?

Postblit doesn't work with const or immutable right now: 
http://d.puremagic.com/issues/show_bug.cgi?id=4867

- Jonathan M Davis