Classes

Separate Compilation

Special code for the compiler must be put in a header file to prevent the file from being compiled twice.
```
#ifndef PRODUCT_H
#define PRODUCT_H
. . .
#endif
```
The source file includes its own header file.
```
#include "Product.h"
```
The source file does not contain a main function because many programs may use this class.
Shared constants are placed in the header file.
To share global variables among source files, declare them in a header file as extern.
```
extern GraphicWindow cwin;
```
The source file contains the definition.
```
GraphicWindow cwin;
```