Delphi pitfalls: Enumerated types and for loops

Not all for loops are created equal.

Consider
for x in [value1, value2, value3]

You would expect to see x vary in the order of the values in the list.

However – if x and the values are of an enumerated type, looping the “list”
does NOT loop in the apparent order of the constant, but in the order of the enumerated type declaration, such as it would for any set.

An example:

      
        program EnumArrayvsEnumSet;
      
        {$APPTYPE CONSOLE}
      
        {$R *.res}
      
        uses
      
          System.SysUtils;
      
        type
      
          TEnum = (plough, foo, bar, wtf);
      
        procedure Output(const aEnum: TEnum);
      
        begin
      
          case aEnum of
      
            wtf: Writeln('wtf');
      
            plough: Writeln('plough');
      
            bar: Writeln('bar');
      
            foo: Writeln('foo');
      
            else Writeln('Missed one!');
      
          end;
      
        end;
      
        procedure TestEnumSet;
      
        var
      
          enum: TEnum;
      
        begin
      
          for enum in [wtf, plough, bar, foo]
      
          do Output(enum);
      
        end;
      
        procedure TestEnumArray;
      
        var
      
          enum: TEnum;
      
          enumArray: TArray<TEnum>;
      
        begin
      
          enumArray := [wtf, plough, bar, foo];
      
          for enum in enumArray
      
          do Output(enum);
      
        end;
      
        begin
      
          try
      
            try
      
              Writeln('Declared: TEnum = (plough, foo, bar, wtf)');
      
              Writeln('Test order: [wtf, plough, bar, foo]');
      
              Writeln;
      
              Writeln('Looping an Enum Set');
      
              TestEnumSet;
      
              Writeln;
      
              Writeln('Looping an Enum Array');
      
              TestEnumArray;
      
            except
      
              on E: Exception do
      
                Writeln(E.ClassName, ': ', E.Message);
      
            end;
      
          finally
      
            Writeln;
      
            Write('Press Enter:');
      
            Readln;
      
          end;
      
        end.

view raw EnumArrayvsEnumSet.dpr hosted with ❤ by GitHub

The above code yields

Declared: TEnum = (plough, foo, bar, wtf)
Test order: [wtf, plough, bar, foo]

Looping an Enum Set
plough
foo
bar
wtf

Looping an Enum Array
wtf
plough
bar
foo

Press Enter:

This can be easy to miss, so if loop order is important – make sure you code your enums into an actual array and do not leave them as a set.

Delphi pitfalls: Enumerated types and for loops

Published by Lars Fosdal

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