Think twice before raising exception in constructor

Recently prolific Delphi writer Nick Hodges declared a war on the use of nil pointers. It is arguable whether nil pointers usage is bad or not, but the point is the “Guard Pattern” proposed is really an antipattern. Here is a simple demo:

program Project11;

{$APPTYPE CONSOLE}

uses
  SysUtils, Classes;

type
  TMyClass = class
  private
    FList: TStringList;
  public
    constructor Create(AList: TStringList);
    destructor Destroy; override;
  end;

constructor TMyClass.Create(AList: TStringList);
begin
  if AList = nil then
  begin
    raise Exception.Create('Don''t you dare pass me a nil reference!);
  end;
  FList:= AList;
  FList.Add('TMyClass instance created');
end;

destructor TMyClass.Destroy;
begin
  FList.Add('TMyClass instance destroyed');
  inherited Destroy;
end;

procedure Test;
begin
  TMyClass.Create(nil);
end;

begin
  try
    Test;
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
end.

If you understand how exceptions raised in a constructor work you see that the above code raises 2 exceptions: the first is the “wanted” exception in the constructor, the second is the “unwanted” access violation in the destructor.

The moral is: if you raise an exception in a constructor be very careful when writing the destructor, cause you need to treat a case of improperly initialized instance.