The ability to package data with functions allows you to create a new data type. This is often called encapsulation[33]. An existing data type may have several pieces of data packaged together. For example, a float has an exponent, a mantissa, and a sign bit. You can tell it to do things: add to another float or to an int, and so on. It has characteristics and behavior.

The definition of Stash creates a new data type. You can add( ), fetch( ), and inflate( ). You create one by saying Stash s, just as you create a float by saying float f. A Stash also has characteristics and behavior. Even though it acts like a real, built-in data type, we refer to it as an abstract data type, perhaps because it allows us to abstract a concept from the problem space into the solution space. In addition, the C++ compiler treats it like a new data type, and if you say a function expects a Stash, the compiler makes sure you pass a Stash to that function. So the same level of type checking happens with abstract data types (sometimes called user-defined types) as with built-in types.

You can immediately see a difference, however, in the way you perform operations on objects. You say object.memberFunction(arglist). This is “calling a member function for an object.” But in object-oriented parlance, this is also referred to as “sending a message to an object.” So for a Stash s, the statement s.add(&i) “sends a message to s” saying, “add( ) this to yourself.” In fact, object-oriented programming can be summed up in a single phrase: sending messages to objects. Really, that’s all you do – create a bunch of objects and send messages to them. The trick, of course, is figuring out what your objects and messages are, but once you accomplish this the implementation in C++ is surprisingly straightforward.