Re: Missing destructor call using clear and base interface ref.

[Ali Çehreli]

On 08/06/2012 06:59 AM, Roberto Delfiore wrote:
> See the following code:
>
> interface A{
> }
>
> class B : A{
> this(string name){this.name = name;}
> ~this(){
> writefln("Destructor %s", name);
> }
> string name;
> }
>
> void main(){
> B b0 = new B("b0");
> B b1 = new B("b1");
>
> A a = b0;
> clear(a);
>
> clear(b1);
> }
>
> Output:
> Destructor b1
>
> dmd 2.059
>
> Why is the B destructor not invoked in the first clear?
>
> Expected output:
> Destructor b0
> Destructor b1
>
Interesting.

I've tested the code with 2.060 after replacing 'clear' with 'destroy' 
(not required here, but because clear will be deprecated):

import std.stdio;

interface A{
}

class B : A{
    this(string name){this.name = name;}
    ~this(){
        writefln("Destructor %s", name);
    }
    string name;
}

void main(){
    B b0 = new B("b0");
    B b1 = new B("b1");

    A a = b0;
    writeln("Before clear(a)");
    destroy(a);

    writeln("Before clear(b1)");
    destroy(b1);

    writeln("Leaving main");
}

I see both of the destructor calls but the first one is executed out of 
order:

Before clear(a)
Before clear(b1)
Destructor b1
Leaving main
Destructor b0    <-- Here

Making 'a' a B produces the expected output:

    B a = b0;

Before clear(a)
Destructor b0    <-- Now at expected time
Before clear(b1)
Destructor b1
Leaving main

I guess destroy() is a no-op on an interface because your not seeing the 
destructor's effect and my seeing it can be explained by the 
non-deterministic behavior of the GC regarding destructor calls: If it 
is up to the GC, the destructor calls are not guaranteed.

Do others know? Shouldn't destroy() work on an interface?

Ali