location of a datalist
the #GQuark identifying a data element
the old value to compare against
the new value to replace it with
destroy notify for the new value
destroy notify for the existing value
Return: %TRUE if the existing value for @key_id was replaced by @newval, %FALSE otherwise.
2.34
Compares the member that is associated with @key_id in @datalist to @oldval, and if they are the same, replace @oldval with @newval.
This is like a typical atomic compare-and-exchange operation, for a member of @datalist.
If the previous value was replaced then ownership of the old value (@oldval) is passed to the caller, including the registred destroy notify for it (passed out in @old_destroy). Its up to the caller to free this as he wishes, which may or may not include using @old_destroy as sometimes replacement should not destroy the object in the normal way.