Question

I’m tidying up some code I’ve been given, for some reason, if the variables are defined under ‘Private’ (where they need to be to constrain access within this unit). I get EAccessViolation error when writing to them. If I loosely define them under Var just before implementaion they can be accessed ok. I’m comparing my structure with other similar units, where the Private units work fine & cant spot any significant differences.. Any suggestions?

Error MSG: Project — raised exception class EAccessViolation Error with message ‘access vilation error in module—-. Read of address 0000050F

interface
uses
  dialogs, math, dateutils, SysUtils, classes;
type
     //double = extended;   
     TDoubleDoubleArray = array of array of double;
     TSunPositionAlgorithm = class (TObject)
         private
           FLocationChanged: boolean;
         public
           Constructor Create;
           Destructor Destroy;
           procedure SetDefaults;
         end;
  Var
    SunPositionAlgorithm : TSunPositionAlgorithm;
           F_L0: Double;
           F_L1: TDoubleDoubleArray;

implementation
  {TSunPositionAlgorithm }

constructor TSunPositionAlgorithm.Create;
begin
  SetDefaults;
end;

procedure TSunPositionAlgorithm.SetDefaults;
Begin
F_L0:= 1;                   // works ok
  FLocationChanged:=true;  // throws eaccess violation error
End;

Calling function (added to Post after David H’s 1st question):

  procedure TSun.NRELPositionOfSun(const DateTime: TDateTime; var Azimuth, Elevation, Declination: double);
  Var

    LSunPositionAlgorithm : TSunPositionAlgorithm;
  Begin
    LSunPositionAlgorithm := TSunPositionAlgorithm.Create;
    Try
      LSunPositionAlgorithm.SetDefaults;

  blah..


    Finally
      LSunPositionAlgorithm.Destroy;
    End;
  End;

Answer 1

You haven’t shown the code that calls this code. However, it’s pretty obvious that you don’t have a valid TSunPositionAlgorithm object.

Get one of those like this:

procedure Test;
var
  spa: TSunPositionAlgorithm;
begin
  spa := TSunPositionAlgorithm.Create;
  try
    spa.SetDefaults;
  finally
    spa.Free;
  end;
end;

You probably have code like this:

procedure Test;
var
  spa: TSunPositionAlgorithm;
begin
  spa.SetDefaults;//oops, spa has not been initialised yet  
end;

or possibly like this:

procedure Test;
var
  spa: TSunPositionAlgorithm;
begin
  spa.Create;//oops, spa has not been initialised yet  
end;

Update You’ve now shown the calling code but you clearly haven’t shown it all, because the code in the question does not exhibit the behaviour you describe. The point I make remains, you must have an invalid object reference somewhere. I’ve shown a couple of the most common ways for that to come about. But there are other ways to get an invalid object reference.

---

The other thing wrong with the code in the question is your destructor. They must always be marked with the override directive.

destructor Destroy; override;

You have to do this in order for your destructor to be called when an object is freed.