Editing Simula (section)

=== Classes, subclasses and virtual procedures ===
A more realistic example with use of classes,<ref name="CommonBase"/>{{rp|1.3.3, 2}} subclasses<ref name="CommonBase"/>{{rp|2.2.1}} and virtual procedures:<ref name="CommonBase"/>{{rp|2.2.3}}

 '''Begin'''
    '''Class''' Glyph;
       '''Virtual''': '''Procedure''' print '''Is''' '''Procedure''' print;;
    '''Begin'''
    '''End''';
    
    Glyph '''Class''' Char (c);
       '''Character''' c;
    '''Begin'''
       '''Procedure''' print;
         OutChar(c);
    '''End''';
    
    Glyph '''Class''' Line (elements);
       '''Ref''' (Glyph) '''Array''' elements;
    '''Begin'''
       '''Procedure''' print;
       '''Begin'''
          '''Integer''' i;
          '''For''' i:= 1 '''Step''' 1 '''Until''' UpperBound (elements, 1) '''Do'''
             elements (i).print;
          OutImage;
       '''End''';
    '''End''';
    
    '''Ref''' (Glyph) rg;
    '''Ref''' (Glyph) '''Array''' rgs (1 : 4);
    
    ''! Main program;''
    rgs (1):- '''New''' Char ('A');
    rgs (2):- '''New''' Char ('b');
    rgs (3):- '''New''' Char ('b');
    rgs (4):- '''New''' Char ('a');
    rg:- '''New''' Line (rgs);
    rg.print;
 '''End''';

The above example has one [[Superclass (computer science)|super class]] (Glyph) with two [[Subclass (computer science)|subclasses]] (<code>Char</code> and <code>Line</code>). There is one [[Virtual function|virtual procedure]] with two [[implementation]]s. The execution starts by executing the main program. Simula lacks the concept of [[abstract class]]es, since classes with pure [[Virtual function|virtual procedures]] can be [[Instance (computer science)|instantiated]]. This means that in the above example, all classes can be instantiated. Calling a pure virtual procedure will however produce a [[Runtime (program lifecycle phase)|run-time]] [[Error message|error]].