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In C++ duck typing there is the concept of a callable, e.g. a symbol that can be called like a function. I'd like to emulate this with Delphi generics.

I have a problem where I need a procedure that can accept either a normal procedure, a class method or a lambda as argument. Right now I have 3 almost identical overloaded implementations. Is there a way to use Delphi generics to avoid this duplication?

jpo234
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1 Answers1

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You don't need generics for this.

The reference to X is compatible with X, X of object, and reference to X (where X is a procedural type such as procedure(X: Integer)).

For example,

program Project1;

{$APPTYPE CONSOLE}

uses
  System.SysUtils;

type
  TTestProc = procedure(X: Integer);
  TTestMeth = procedure(X: Integer) of object;
  TTestRef = reference to procedure(X: Integer);

// Ordinary procedure
procedure TestProc(X: Integer);
begin
end;

// Method
type
  TMyClass = class
    class procedure TestMeth(X: Integer);
  end;

class procedure TMyClass.TestMeth(X: Integer);
begin
end;

begin
  var A: TTestRef;
  A := TestProc;
  A := TMyClass.TestMeth;
  A := procedure(X: Integer) begin end;
end.
Andreas Rejbrand
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    Generic [TProc](https://docwiki.embarcadero.com/Libraries/en/System.SysUtils.TProc) and [TFunc](https://docwiki.embarcadero.com/Libraries/en/System.SysUtils.TFunc) from RTL are worth mentioning here. Note that they don't have `const` modifier on their parameters, which can be an issue with auto ref. counted types like strings. – Peter Wolf Jan 13 '23 at 10:22