[Dlang-study] [lifetime] Initial thoughts on lifetime management

[Andrei Alexandrescu]

On 10/28/2015 07:24 AM, Michel Fortin wrote:
> True. Note that "each reference" must also counts temporaries. So if you have this:
>
> 	foo(getObject());
>
> what you really have is this:
>
> 	foo(getObject(){-1});
>
> where {-1} is a notation I made up to denote that the counter of the object returned by getObject() is decremented at the end of the statement. (The returned value is already {+1} by the caller.)

Nice notation. There is an amendment I'd need to add (I think I 
mentioned this already): at least in v1.0 we're considering having the 
callER do the increment and the callEE do the decrement. This is how D 
currently handles copy construction and destruction, and it turns out it 
has a number of advantages over C++ (where the caller does all the work).

So, the protocol in foo(getObject()) is, foo's caller gets an rvalue 
with the already-incremented refcount and bitblits it into the call to 
foo, no additional operation. The code generated by foo includes the 
refcount decrement.

This will make for a simple implementation for v1.0 (uses the same 
backend as the ctors/dtors), but we need to figure whether that is the 
optimal approach for manipulating refcounts.

> And if you have this:
>
> 	c = getObject();
>
> it becomes this:
>
> 	c{-1}{+1} = getObject(){-1};

Applying the already-discussed manipulation, the temporary returned by 
getObject() comes with the already-bumped refcount so all we need to do 
is decref on c prior to the assignment. Then bitblt.


Andrei