Delphi: Call a function whose name is stored in a string

Question

Is it possible to call a function whose name is stored in a string in Delphi?

Answer 1

Please give more details on what are you trying to achieve.

As far as I know:

• It is not possible to call a random function like that.
• For class and object functions (MyObject.Function) this can be done with RTTI, but it's a lot of work.
• If you just need to call one particular type of functions (say, function(integer, integer): string), it's a lot easier.

For the last one, declare a function type, then get a function pointer and cast it like this:

type
  TMyFuncType = function(a: integer; b: integer): string of object;

  TMyClass = class
  published
    function Func1(a: integer; b: integer): string;
    function Func2(a: integer; b: integer): string;
    function Func3(a: integer; b: integer): string;
  public
    function Call(MethodName: string; a, b: integer): string;
  end;

function TMyClass.Call(MethodName: string; a, b: integer): string;
var m: TMethod;
begin
  m.Code := Self.MethodAddress(MethodName); //find method code
  m.Data := pointer(Self); //store pointer to object instance
  Result := TMyFuncType(m)(a, b);
end;

{...}

//use it like this
var MyClass: TMyClass;
begin
  MyClass := TMyClass.Create;
  MyClass.Call('Func1', 3, 5);
  MyClass.Call('Func2', 6, 4);
  MyClass.Destroy;
end.

Answer 2

You didn't specify your Delphi version, However if you have Delphi 2010(+) you can do it using the enhanced RTTI, I'm not expert on them, but I tried this sample for you:

  TProcClass = class
    public
      procedure SayHi;
      function GetSum(X,Y:Integer): Integer;
  end;

uses
  Rtti;

{ TProcClass }

procedure TProcClass.SayHi;
begin
  ShowMessage('Hi');
end;

function TProcClass.GetSum(X, Y: Integer): Integer;
begin
  ShowMessage(IntToStr(X + Y));
end;

procedure ExecMethod(MethodName:string; const Args: array of TValue);
var
 R : TRttiContext;
 T : TRttiType;
 M : TRttiMethod;
begin
  T := R.GetType(TProcClass);
  for M in t.GetMethods do
    if (m.Parent = t) and (m.Name = MethodName)then
      M.Invoke(TProcClass.Create,Args)
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  ExecMethod('SayHi',[]);
  ExecMethod('GetSum',[10,20]);
end;

The good things, if you have procedure or function with parameters it will work without more work.

Answer 3

I'm surprised no one has suggested a dispatch table. This is exactly what it's for.

program RPS;

uses
  SysUtils,
  Generics.Collections;

type
  TDispatchTable = class(TDictionary<string, TProc>);

procedure Rock;
begin
end;

procedure Paper;
begin
end;

procedure Scissors;
begin
end;

var
  DispatchTable: TDispatchTable;

begin
  DispatchTable := TDispatchTable.Create;
  try
    DispatchTable.Add('Rock', Rock);
    DispatchTable.Add('Paper', Paper);
    DispatchTable.Add('Scissors', Scissors);

    DispatchTable['Rock'].Invoke; // or DispatchTable['Rock']();
  finally
    DispatchTable.Free;
  end;
end.

The implementation I wrote uses generics so it would only work with Delphi 2009+. For older versions it would probably be easiest to implement using TStringList and the command pattern