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

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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; 

    Note:

    • 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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