Can you find this Delphi coding error (part 6)?

July 11, 2017

Can you spot the error in the following (somewhat artificial) Delphi code?

program ResultMaker;

{$APPTYPE CONSOLE}

uses
  System.SysUtils;

type  
  IResultMaker = interface
    function CalcResult : Integer;
  end;
  
  TResultMaker = class(TInterfacedObject, IResultMaker)
  public
    function CalcResult : Integer;
  end;

  function TResultMaker.CalcResult : Integer;
  begin
    // ..
	// ..
  end;
  
  procedure DisplayResult;
  var
    ResultMaker : IResultMaker;
  begin
    ResultMaker := TResultMaker.Create;
	
	try
	  Writeln('Result: '+IntToStr(ResultMaker.CalcResult));
	finally
	  FreeAndNil(ResultMaker);
	end;
  end;
  
begin
  DisplayResult;
end.

This was quite easy, right?

Solution: This code will most probably result in a runtime error (access violation). The reason is that FreeAndNil is passed an interface. However, the compiler will not complain. Also, the code does not make sense at all, because the ResultMaker variable is reference-counted and will be freed automatically. So no need to call FreeAndNil.

Not so hard to spot! But imagine finding this problem among thousands of lines of code spread over hundreds of units! And consider also a code base that is evolving all the time, so the code must constantly be rechecked.

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