tail const

[Michel Fortin]

On 2010-12-04 08:55:19 -0500, Andrei Alexandrescu 
<SeeWebsiteForEmail at erdani.org> said:

> If conversion is allowed only for values (i.e. perform a memberwise 
> copy of one struct to another), it looks like things could work. 
> Almost. The problem is that that surreptitious copy completely bypasses 
> the constructor:
> 
> struct A(T) {
>      private T obj;
>      private bool isObject;
>      this(T obj_) {
>          obj = obj_;
>          static if (is(T == Object)) isObject = true;
>      }
> }
> 
> auto a = A!Widget(new Widget);
> A!Object b = a; // works, new automatic conversion rule
> assert(!b.isObject); // passes, invariant is messed up

Copying a struct always bypasses the constructor, whether it's a 
template or not. If you want to maintain invariants, add the 
"this(this)" postblit constructor. Your invariant in the case above is 
a little silly since it stores a value deduced from the template 
parameter as a member variable. But if it's that important, fixing it 
is trivial: add a postblit constructor.

	this(this) {
		static if (is(T == Object)) isObject = true;
		else isObject = false;
	}

I'm not sure why you imply it won't work for references. That's the 
whole point of the proposal. It can work for references as long as the 
memory layout is the same and each member can also be converted as a 
reference.


-- 
Michel Fortin
michel.fortin at michelf.com
http://michelf.com/