TITLE: Terminology for ADT, abstract class, etc

PROBLEM: impellus@cs.ucsd.edu (Tom Impelluso)

Could someone explain the differences between the terms Class vs.
Object vs. Abstract Data Type.  


RESPONSE: rmartin@rcmcon.com (Robert Martin), 9 Apr 94

There are two definitions of ADT.  One has it as a class with nothing
but pure virtual functions.  The other has it as a data structure with
an entorage of associated functions.  I prefer the latter.  In C++ it
corresponds to a class with member functions, none of which are
virtual.

[ The first is an ABC (abstract base class); the second is an ADT. -adc ]

A class is the definition of a data structure and its associated
functions.  Similar to a 'struct' in C, but with functions associated
with it.  

	class complex
	{
	  public:
  		double Magnitude() {return sqrt(itsReal*itsReal + 
                                                itsImaginary+itsImaginary);}

	  private:
		double itsReal;
		double itsImaginary;
	};
An object is an instance of a class:

	complex c;  // the class is 'complex', the object is 'c'.


RESPONSE: lincmad@netcom.com (Linc Madison)

An abstract data type is an encapsulated representation of the data
needed to describe some category of objects with the operations that can
be performed on those objects.

The term for a class which has at least one (not "nothing but" but only
"at least one") pure virtual function is an *abstract class*, not an
abstract data type.  The representation of an ADT in C++ may or may not
have virtual member functions; that question is orthogonal.