Separating class definition and class implementation

Class definition and implementation
- Class definition contains enough information to manage variables of that class:
  1. Reserve space in memory when programmer define class variables
  2. Identify if a given operation on a class variable is permissible (allowed; has the operation been defined)
  Class definition information is usually very compact and frequently needed to define variables of that class when you write a program that uses that class.
- Class implementation contains enough information to perform the defined operations on variables of that class:
  1. Member functions
  Class implementation information is usually very bulky (thousands of lines of code !!!) and only needed when the program is run
- Due to very different needs; it was decided (in C++) to allow the programmer to separate class definition information and class implementation information

How to separate class definition and implementation information

Class definition information consists of:
- Variable definitions in a class
- Member function (method) declations in that class
The complete class definition must be in a single file
Class implementation information consists of one or more member functions
Member function can be stored in one or more program files (it is not required to store all member functions of a class in a single file)
In fact, the situation can be pretty bad: you can use one file to store member functions from different classes !!!

Example:

class Matrix3x3 { public: float A[3][3]; void AddNum(float x) { int i, j; for (i = 1; i < 3; i = i + 1) for (j = 1; j < 3; j = j + 1) A[i][j] = A[i][j] + x; }; };

File "Matrix3x3.h"
class Matrix3x3 { public: float A[3][3]; void AddNum(float); };

File "Matrix3x3.C"
#include "Matrix3x3.h" void Matrix3x3::AddNum(float x) { int i, j; for (i = 1; i < 3; i = i + 1) for (j = 1; j < 3; j = j + 1) A[i][j] = A[i][j] + x; };

The notation Matrix3x3:: tells the C++ compiler that the function AddNum is a member function in the class Matrix3x3
Notice that we must include the class definition from file Matrix3x3.h in order to tell the C++ compiler how to manage Matrix3x3 variables.
For example, C++ compiler need to know what things are inside a Matrix3x3 variables: array A[3][3] !!!
Example Program: (Separate compilation of classes) ----
- Class definition file: click here
- Class implementation file: click here
- Main (driver) program: click here