Smart pointers instead of GC?

[Adam D. Ruppe]

On Sunday, 2 February 2014 at 07:35:21 UTC, Andrei Alexandrescu 
wrote:
> What I'm driving at is that right now the type is A, as is in a 
> variety of other situations. It would be a whole different 
> language that would be wholly incompatible with D.

A == Borrowed!A in my view. Consider this:

class A { A foo() { return this; } }

A a;

void crashy() {
    ubyte[__traits(classInstanceSize, A)] Abuffer;
    auto localA = emplace!A(Abuffer);

    a = localA.foo();
}


That's obviously wrong, when crashy returns, a is dead... but it 
has no way of knowing that.

Let's consider we wrote a nice little Owned!T. Internally, it 
keeps that buffer, disables postblit, frees in the destructor and 
might have a release method to transfer ownership. All stuff we 
can do in the library today.


Owned!A a;
void crashy() {
    Owned!A localA = Owned!A();
    a = localA; // prohibited, this(this) is annotated with 
@disable
    a = localA.foo(); // should ALSO be prohibited, cannot convert 
A to Owned!A
}


The class object itself doesn't know how it is stored. If it 
returns this, it MUST assume it is borrowed unless it explicitly 
allocates a new thing.

For all it knows, it was emplaced into a static array or 
malloced. A == Borrowed!A. typeof(this) == A.




So, what about the global a up there? I think there's two 
solutions:

1) ideally, Owned!A has something like this:

scope A borrow() { return payload; }
alias borrow this;

A a = localA; // legal, A is a local variable in the same or 
inner scope
globalA = localA; // illegal, since borrow returns scope, it 
isn't allowed to escape


2) a worse but perhaps doable option would be

A globalA; // illegal, non-local variable must not be borrowed

struct foo {
   A a; // now foo is itself considered a borrowed reference type 
so the rules apply to it
}

GC!A globalA; // ok cool



What's interesting about this is we could, in theory (and I've 
done a limited proof of concept), implement it in the library 
using a static assert in RTInfo!

anyway i g2g might talk more later