structs, ~this(), this(this) and reference counting

[Michel Fortin]

On 2012-02-18 13:25:54 +0000, Benjamin Thaut <code at benjamin-thaut.de> said:

> Am 18.02.2012 11:32, schrieb Sönke Ludwig:
>> After implementing automatic reference counting for resource management
>> and spending the last two days with trying to figure out where the
>> crashes and leaks come from and write workarounds for the corresponding
>> compiler bugs, I came to the conclusion that this approach is futile.
>> The code base is too large, as is the number of patterns that lead to
>> compiler bugs.
>> 
>> Since the number of issues unusually high, instead of posting one bug
>> report for each (posted two), I compiled a small test suite with a
>> number of bug patterns that I discovered (and some working patterns).
>> The list is probably not complete and I know that some cases that are OK
>> in the test suite now failed for me in real code.
>> 
>> Anyway, do you think it makes sense to post this test suite as a meta
>> bug for reference counting (excluding phobos RefCounted), so that
>> somebody can look into it as a whole?
>> 
>> I get the feeling that otherwise it might take a very long time until
>> everything is stable in this area - and IMO this is an extremely
>> important area for anyone who needs to (partially or fully) avoid the
>> GC. And, last but not least, just like linker crashes and ICEs, it may
>> cause a very fragile impression for anyone who encounters this.
>> 
>> Test suite: http://pastebin.com/niZwRKpc
>> (Run with "dmd -g -run" or "dmd -g -version=BAILOUT -run")
>> 
> 
> You did not implement the assignment operator for your refcounted 
> struct. If you do that it should heavily reduce the number of issues 
> you have. I actually only run into 1 severe issue with refcounting and 
> that is that array slice copying does not call any copy constructor / 
> assignment operator

It'd be nice though if the default assignment operator for structs 
would be generated like this:

	if (&other != &this)
	{
		~this(); // destruct
		memcpy(&this, &other, other.sizeof); // blit
		this(this); // postblit
	}

because this is the correct thing to do on assignment 99% of the time. 
So you'd have less boilerplate code to write.


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