On 2010-12-04 08:55:19 -0500, Andrei Alexandrescu
<[email protected]> 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
[email protected]
http://michelf.com/
