how to add a string to a class object in iphone

Question

i have a search view.in that i am searching for the topics starting with some character.when i press 1 character it will show the results in table view.the search results in textDidChange method are storing in string type. how can i convert this string type into class "Topic" type. "Topic" is a class.i have created aTopic object for "Topic class"

Thanks in advance