Concept of Proper Type

• Type should be responsible for its own data
• keep data protected from client tampering
• keep data protected from other programs
• Type should be responsible for its own behavior
• document behavior
• make behavior available to clients
• implement behavior separately out of public view
• Type should be responsible for its own existence
• when a client needs a variable of proper type, asking for it should be enough
• when a client is finished with a variable, it should go away peacefully

C strings fail on all counts