Difference between revisions of "How to use procedural variables"

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{{How_to_use_procedural_variables}}
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Copy the text below and it will demonstrate the use of procedural variables, this is a fully working program. You don't even need to understand how it works. The syntax is pretty simple.
 
Copy the text below and it will demonstrate the use of procedural variables, this is a fully working program. You don't even need to understand how it works. The syntax is pretty simple.
  

Latest revision as of 08:30, 17 February 2020

English (en)

Copy the text below and it will demonstrate the use of procedural variables, this is a fully working program. You don't even need to understand how it works. The syntax is pretty simple.

program Test;

{$mode objfpc}{$H+}
uses
  {$IFDEF UNIX}{$IFDEF UseCThreads}
  cthreads,
  {$ENDIF}{$ENDIF}
  Classes;

// Make the Types the type corresponds to a function signature
type
  TFuncNoArgsString = function(): String;
  TFuncOneArgsString = function(x: string): string;

// Example functions
function Hello: String;
begin
  Result := 'Hello There';
end;

function Woah(G: String): String;
begin
  Result := 'Woah ' + G;
end;

// Overloaded function takes the two types of function
// pointers created above
procedure Take(f: TFuncNoArgsString); overload;
begin
  WriteLn(f());
end;

procedure Take(f: TFuncOneArgsString); overload;
begin
  WriteLn(f('there!!!'));
end;

var
  ptr: Pointer;
  List: TList;
begin
  // the "@" symbol turns the variable into a pointer.
  // This must be done in order pass a function as a
  // parameter.  This also demonstrates that pascal
  // keeps track of the pointer type so the overloading works!

  Take(@Hello);
  Take(@Woah);

  // Now put a function in an untyped pointer
  ptr := @Hello;
  // Type the pointer and call it all at the same time
  WriteLn(TFuncNoArgsString(ptr));
  // A TList Example
  List := TList.Create;
  List.Add(@Hello);
  WriteLn(TFuncNoArgsString(List[0]));
  ReadLn;
end.

With {$modeswitch classicprocvars+} the @-address-operator is not necessary to refer to methods. Also, if you are using the @-address-operator usage of {$typedaddress on} is advised in order to prevent programming mistakes.