(glib:define-gboxed-opaque text-iter "GtkTextIter"
  :export t
  :type-initializer "gtk_text_iter_get_type"
  :alloc (%text-iter-alloc))  

Most text manipulation is accomplished with iterators, represented by a gtk:text-iter instance. An iterator represents a position between two characters in the text buffer.

Iterators are not valid indefinitely. Whenever the text buffer is modified in a way that affects the number of characters in the text buffer, all outstanding iterators become invalid. Note that deleting 5 characters and then reinserting 5 characters still invalidates iterators, though you end up with the same number of characters you pass through a state with a different number.