Using 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; 

Important Notes

  • 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'"