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5.5.3 Type Checking of Foreign Objects

Functions that operate on foreign objects should check that the passed SCM value indeed is of the correct type before accessing its data. They can do this with scm_assert_foreign_object_type.

For example, here is a simple function that operates on an image object, and checks the type of its argument.

clear_image (SCM image_obj)
  int area;
  struct image *image;

  scm_assert_foreign_object_type (image_type, image_obj);

  image = scm_foreign_object_ref (image_obj, 0);
  area = image->width * image->height;
  memset (image->pixels, 0, area);

  /* Invoke the image's update function.  */
  if (scm_is_true (image->update_func))
    scm_call_0 (image->update_func);