I'd appreciate if somebody elaborated a bit on the purpose of the compiler flag

    -preview=dtorfields       destruct fields of partially constructed objects

On Mon, Aug 31, 2020 at 08:22:02PM +0000, Per Nordlöw via Digitalmars-d wrote:
> I'd appreciate if somebody elaborated a bit on the purpose of the compiler flag
> 
>     -preview=dtorfields       destruct fields of partially constructed
> objects

My guess is to be completely exception-safe. For example:

	class Resource(T) {
		T* handle;
		this(...) { handle = acquireResource(...); }
		~this() { releaseResource(handle); }
	}

	class ResCollection {
		Resource!A resA;
		Resource!B resB;

		this() {
			resA = new Resource!A(...);
			resB = new Resource!B(...);
		}
	}

If an exception is thrown during the call to `new Resource!B`, the current behaviour is that resA is not destructed, so there may potentially be a resource leak.  The abovementioned compiler flag is to force destruction of resA in case resB fails to be initialized (i.e., as if you wrote `scope(failure) delete(resA);` after the `resA = ...` line).


T

-- 
Кто везде - тот нигде.

On Monday, 31 August 2020 at 20:34:23 UTC, H. S. Teoh wrote:
> My guess is to be completely exception-safe. For example:

Nice. Thanks.

On Monday, 31 August 2020 at 20:22:02 UTC, Per Nordlöw wrote:
> I'd appreciate if somebody elaborated a bit on the purpose of the compiler flag
>
>     -preview=dtorfields       destruct fields of partially constructed objects

With this code:

struct S1 {
    this(int) {}
    ~this() {
        import std.stdio;
        writeln("~S1");
    }
}

struct S2 {
    S1 s1;
    S1 s2;
    this(int i) {
        s1 = S1(0);
        throw new Exception("");
        s2 = S1(0);
    }
}

void main() {
    S2 a = S2(2);
}

The current behavior is to not destruct the s1 and s2 fields, but -preview=dtorfields ensures they will be destructed. I kinda expected only s1 to be destructed above, but s2 follows it down the drain.

--
  Simen