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Kryvich

Warning W1010: Method hides virtual method of base type

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I like to write code without warnings and hints issued by the compiler. Today I encountered the following warning:

program TestCaseW1010;
{$APPTYPE CONSOLE}
type
  TPerson = class
    Name: string;
    function ToString(Quote: boolean): string; overload;
  end;

function TPerson.ToString(Quote: boolean): string;
begin
  if Quote then
    Result := '"' + Name + '"'
  else
    Result := Name
end;

var
  Person: TPerson;

begin
  Person := TPerson.Create;
  Person.Name := 'John';
  Writeln(Person.ToString);         // --> TPerson
  Writeln(Person.ToString(False));  // --> John
  Writeln(Person.ToString(True));   // --> "John"
  Readln;
  Person.Free;
end.

The compiler shows W1010 Method 'ToString' hides virtual method of base type 'TObject'. In fact, he does not. Is it a false positive warning, or is it something wrong with my code?

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The correct declaration would be:

type
  TPerson = class
    Name: string;
    function ToString(Quote: boolean): string; reintroduce; overload;
  end;

 

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Ouch! Thank you. :) 

P.S. I encountered this situation in GOLD Parser Engine 5 that I've converted from Lazarus. Now this project compiles without any warnings, I'm happy!

Edited by Kryvich
  • Like 1

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That fact that you have to mark overloads to a virtual method as reintroduce always bothers me but I never cba to actually report it.

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I think the requirement to explicitly state reintroduce is a good thing. It avoids the issue of forgetting to add the override directive - if you really intended to do so. 

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From a pure linguistic point of view: Where does the need for reintroducing a purely virtual aka non existent thing come from? There seems to be a need for this that is deep in the compiler. Would be a nice research project, or something the experts can explain.

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43 minutes ago, David Heffernan said:

Why don't you override the method from TObject? 

That's exactly the problem and as well the reason for the warning. The OP does not  actually override TObject.ToString but introduces a new method (because of the different signature). The override just does nothing. 

Edited by Zacherl

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22 hours ago, Zacherl said:

That's exactly the problem and as well the reason for the warning. The OP does not  actually override TObject.ToString but introduces a new method (because of the different signature). The override just does nothing. 

It's not that the override does nothing. There is no override. It makes no sense to me to reintroduce a method of that name. 

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