(guile.info.gz) Type checking

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 18.3.3 Type checking
 --------------------
 
 Functions that operate on smobs should aggressively check the types of
 their arguments, to avoid misinterpreting some other datatype as a smob,
 and perhaps causing a segmentation fault.  Fortunately, this is pretty
 simple to do.  The function need only verify that its argument is a
 non-immediate, whose first word is the type tag returned by
 `scm_make_smob_type'.
 
    For example, here is a simple function that operates on an image
 smob, and checks the type of its argument.  We also present an expanded
 version of the `init_image_type' function, to make `clear_image' and
 the image constructor function `make_image' visible to Scheme code.
 
      SCM
      clear_image (SCM image_smob)
      {
        int area;
        struct image *image;
 
        SCM_ASSERT (SCM_SMOB_PREDICATE (image_tag, image_smob),
                    image_smob, SCM_ARG1, "clear-image");
 
        image = (struct image *) SCM_SMOB_DATA (image_smob);
        area = image->width * image->height;
        memset (image->pixels, 0, area);
 
        /* Invoke the image's update function.  */
        if (image->update_func != SCM_BOOL_F)
          scm_apply (image->update_func, SCM_EOL, SCM_EOL);
 
        return SCM_UNSPECIFIED;
      }
 
 
      void
      init_image_type (void)
      {
        image_tag = scm_make_smob_type ("image", sizeof (struct image));
        scm_set_smob_mark (image_tag, mark_image);
        scm_set_smob_free (image_tag, free_image);
        scm_set_smob_print (image_tag, print_image);
 
        scm_c_define_gsubr ("clear-image", 1, 0, 0, clear_image);
        scm_c_define_gsubr ("make-image", 3, 0, 0, make_image);
      }
 

Info Catalog

(guile.info.gz) Creating Instances

(guile.info.gz) Defining New Types (Smobs)

(guile.info.gz) Garbage Collecting Smobs