1. Computing

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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