Re: Continued looking at properties in D - interfaces and constraints

[Antonio Corbi via Digitalmars-d-learn]

On Sunday, 9 October 2016 at 11:54:50 UTC, mikey wrote:
Hi,
I'm continuing to look at properties in D and have found 
another area where I think there may be an issue - or maybe 
where I'm doing something wrong.

I have started trying to use constraints on my properties to 
constrain which values they can take I have also started trying 
to use interfaces. What I noticed was that when I combine these 
2 features the constraints get discarded.

    interface Widthy {
        @property inout(int) width() inout;
        @property void width(int width);
    }

    class Test : Widthy {
    private:
        int _w;
    public:
        @property inout(int) width() inout { return _w; }
        @property void width(int width)
        in {
            import std.exception;
            if (width < 0) {
                throw new
                Exception("width is less than zero");
            }
        }
        body {
            _w = width;
        }
    }

    void main() {
        import std.stdio;
        auto t = new Test;
        t.width = -1;
        writeln("width: ", t.width);
        // width: -1

        // hmmm... not good
    }

Hi Mikey,

I think the failure you are experimenting is a mixture of two 
problems:

1. Inheritance with contracts is evaluated in a special way, 'in 
contracts' in the base and derived method (property) are or-ed, 
so if one of them passses, the contract is believed to have 
succeeded. As you don't have a contract in the base-method: 
"@property void width(int width)", I think that the compiler 
assumes that its empty contract is always true.

So it's a matter of adding some contract to it, i.e.:

interface Widthy
{
  @property inout(int) width() inout;
  @property void width(int w) in { assert(w > 7); }
}

But then comes...

2. This approach fails with dmd version: DMD64 D Compiler v2.071.2
but it works with ldc 1.0.0 and also with ldc-git (based on DMD 
v2.071.2 and LLVM 3.8.1 ,  built with DMD64 D Compiler v2.071.2), 
so I think it's a bug in DMD, but I'm very new to D, so I hope 
that someone here with deeper knowledge can throw some light :)

Antonio