Re: Question on User-Defined Types (Embedding)

[Matthew Flatt <mflatt@cs.utah.edu>]

Quoting Brent Fulgham:
> Let's say I have a C++ class/structure that I want to pass in and out 
> of the MzScheme runtime.
> 
> For example, let's say I have a simple structure:
> 
>     struct test
>     {
>         char name[10];
>         long salary;
>     };

If you have the luxury of changing the test struct, add a
Scheme_Type element at the beginning:

     struct test
     {
        Scheme_Type tag;  /* MzScheme needs this tag */
        char name[10];
        long salary;
     };

> And I'd have previously created a type:
> 
>     Scheme_Type test_type;
> 
>     test_type = scheme_make_type("<test type>");

Use test_type as the value of `tag' in a test struct.

Because of the type tag, a test* can be safely cast to a
Scheme_Object*. And any Scheme_Object* o where *(Scheme_Type*)o ==
test_type can be safely cast to a test*.

If you can't change the test struct, you'll have to define a wrapper
struct:

     struct wrapped_test {
       Scheme_Type tag;
       struct test t;
     };

> In my real case, I wish to pass Scheme a type representing some global
> data that *should not* be collected by Scheme.

MzScheme will happily work with a Scheme_Object* value that isn't a
pointer to GC land.

See also plt/collects/mzscheme/examples/bitmatrix.c. (Probably we
should have another example with a simpler C-defined structure.)

Matthew