Question

What difference does it make when I use a const parameter in a procedure?

Take the following procedure for example:

procedure DoSomething(Sender: TObject; const Text: String; var Reply: String);
begin
  //Text is read-only and Reply will be passed back wherever DoSomething() was called
  Reply:= Text;
end;

The parameter Text: String is prefixed with const so that (as far as I know), a copy of the value is made and used – and is read-only. What I was wondering is how is does this affect the application any differently than if I didn’t put const there? Perhaps a performance trick?

Answer 1

Looking at the documentation states:

“Using const allows the compiler to optimize code for structured – and string-type parameters. It also provides a safeguard against unintentionally passing a parameter by reference to another routine.”

In case of a string for example the optimization means there is no additional refcounting when passing as const. Also passing as const does not mean it’s a copy. Often it internally passes as reference because the compiler ensures no write access to it.

Some very interesting articles to completly understand what’s going on under the hood:

http://delphitools.info/2010/07/28/all-hail-the-const-parameters

http://vcldeveloper.com/articles/different-function-parameter-modifiers-in-delphi

Edit:

A simple example to show that const may result in pass by reference internally:

program Project1;

{$APPTYPE CONSOLE}

type
  PMyRecord = ^TMyRecord;
  TMyRecord = record
    Value1: Cardinal;
    Value2: Cardinal;
  end;

procedure PassAsConst(const r: TMyRecord);
begin
  PMyRecord(@r).Value1 := 3333;
  PMyRecord(@r).Value2 := 4444;
end;

procedure PassByVal(r: TMyRecord);
begin
  PMyRecord(@r).Value1 := 3333;
  PMyRecord(@r).Value2 := 4444;
end;

var
  r: TMyRecord;
begin
  r.Value1 := 1111;
  r.Value2 := 2222;
  PassByVal(r);
  Writeln(r.Value1);
  Writeln(r.Value2);

  PassAsConst(r);
  Writeln(r.Value1);
  Writeln(r.Value2);

  Readln;
end.