DIP53 and DIP49 (ver2) - New definitions for qualified constructors and postblits

[Michel Fortin]

On 2013-12-20 01:31:03 +0000, Kenji Hara <k.hara.pg at gmail.com> said:

> 
> 2013/12/20 Michel Fortin <michel.fortin at michelf.ca>
> 
>> On 2013-12-18 03:42:38 +0000, "Kenji Hara" <k.hara.pg at gmail.com> said:
>> 
>> http://wiki.dlang.org/DIP49
>>> Improved points from version 1:
>>> - Swap meanings of 'this(this) inout' and 'this(this) const'
>>> - Inout postblit now covers all cheap (== not rebind indirections) copies
>>> between same qualifiers
>>> 
>> 
>> I can't help but notice that if you want class-typed members to be
>> assignable in a "this(this) immutable" context you'll need to add
>> tail-constness for classes in the language.
> 
> 
> Sorry I can't understand what you saying. DIP49 does not touch class
> reference semantics. It's not a purpose of DIP49.

But what if your struct has a class-typed member:

	struct A {
		Object o;
		int* a;

		this(this) {
			a = new int;
			o = new Object;
		}

		this(this) immutable {
			a = new immutable(int);
			o = new immutable(Object); // will that work?
		}
	}

On the second postblit, the type of "a" has to be "immutable(int)*" to 
allow you to assign something else to the pointer while not being able 
to affect what's at the other end of the indirection.

So then, what is the type of "o"? Again, you need to be able to assign 
the variable while not affecting what is at the other end of the 
indirection. You need a tail-immutable object reference, which doesn't 
exist.


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