How to Use a Function or a Procedure as a Parameter in another Function

In Delphi, procedural types (method pointers) allow you to treat procedures and functions as values that can be assigned to variables or passed to other procedures and functions.

Here's how to call a function (or procedure) as a parameter of another function (or procedure) :

1. Declare the function (or procedure) that will be used as a parameter. In the example below, this is "TFunctionParameter".
2. Define a function that will accept another function as a parameter. In the example below this is "DynamicFunction"

type
   TFunctionParameter = function(const value : integer) : string;

...

function One(const value : integer) : string;
begin
   result := IntToStr(value) ;
end;

function Two(const value : integer) : string;
begin
   result := IntToStr(2 * value) ;
end;

function DynamicFunction(f : TFunctionParameter) : string;
begin
   result := f(2006) ;
end;

...

//Example usage:

var
   s : string;
begin
   s := DynamicFunction(One) ;
   ShowMessage(s) ; //will display "2006"

   s := DynamicFunction(Two) ;
   ShowMessage(s) ; // will display "4012"
end;

• Of course, you decide on the signature of the "TFunctionParameter": whether it is a procedure or a function, how many parameters does it take, etc.

• If "TFunctionParameter" is a method (of an instance object) you need to add the words of object to the procedural type name, as in:

  TFunctionParameter = function(const value : integer) : string of object;

• If you expect "nil" to be specified as the "f" parameter, you should test for this using the Assigned function.

• Fixing the "Incompatible type: 'method pointer and regular procedure'"

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