• Burton Radons (49/49) Feb 11 2003 My suggestion to change cast would make it expressible in my template

Burton Radons <loth users.sourceforge.net> writes:

My suggestion to change cast would make it expressible in my template 
syntax, if unimplementable:

    $to cast ($to, $from value) { ... }

But if cast were made into a normal template function for the purposes 



    int i = 4;
    object o = i; // Create an object instance representing i.
    int j = o; // Check that "o" can be casted to int and do so.

The problem is that because the boxed object results in a different 
instance of the value object, this pseudo-code:

    struct Foo { int x = 10; }
    Foo p = new Foo ();
    object box = p;
    p.x = 20;
    Console.Write(((Foo)box).x);

Behaves differently depending upon whether Foo is a struct or a class; 
it tries to bring their syntaxes too close together, IMO.  Anyway, I'm 
just justifying the fact that my boxing/unboxing takes more effort to 
use.  Here's the partial implementation:

    struct generic
    {
        TypeInfo type;
        void *data;
    }

    generic cast ($to (generic), $from value)
    {
        generic result;

        result.type = $from;
        result.data = ((ubyte *) &value) [0 .. $from.size].dup;
        return result;
    }

    $to cast ($to (int, float, double, ...), $from (generic) value)
    {
        TypeInfo type = value.type;

        if (cast (IntType) type)
            return *(int *) value.data;
        if (cast (FloatType) type)
            return *(float *) value.data;
        if (cast (DoubleType) type)
            return *(double *) value.data;
        ...
        throw new CastError (type, $to);
    }

    /* Other implementations needed for struct and class. */

And some usage:

    int i = 4;
    generic o = cast (generic, i);
    int j = cast (int, o);