Implementing pure virtual functions

Dan Saks

January 9, 2013

Dan Saks

Unfortunately, compilers may fail to catch some pure virtual function calls. Such failures are rare, but nevertheless possible. As a second line of defense, if a pure virtual function has no definition, most compilers generate a definition for that function that makes a stink if executed. The C code to do this for the shape class might look like:

double shape_area(shape const *s) {
   pure_virtual_alert();
   return 0;
}

double shape_perimeter(shape const *s) {
   pure_virtual_alert();
   return 0;
}

~~~

shape_vtbl the_shape_vtbl = {
   shape_area,
   shape_perimeter
};

On a desktop system, the pure_virtual_alert function might simply write a message to the system's stderr stream or display a message box, and then terminate the program. For an embedded system, you would have to tailor the response to be something more appropriate.

Dan Saks is president of Saks & Associates, a C/C++ training and consulting company. For more information about Dan Saks, visit his website at www.dansaks.com. Dan also welcomes your feedback: e-mail him at dan@dansaks.com..