Difference between revisions of "How to use procedural variables"
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| − | < | + | <delphi>program Test; |
| − | program | ||
{$mode objfpc}{$H+} | {$mode objfpc}{$H+} | ||
| Line 10: | Line 9: | ||
cthreads, | cthreads, | ||
{$ENDIF}{$ENDIF} | {$ENDIF}{$ENDIF} | ||
| − | Classes | + | Classes; |
| − | + | ||
| − | //Make the Types the type corresponds to a function signature | + | // Make the Types the type corresponds to a function signature |
type | type | ||
| − | + | TFuncNoArgsString = function(): String; | |
| − | + | TFuncOneArgsString = function(x: string): string; | |
| − | // | + | // Example functions |
| − | function Hello | + | function Hello: String; |
begin | begin | ||
| − | + | Result := 'Hello There'; | |
end; | end; | ||
| − | function Woah(G:String):String; | + | |
| + | function Woah(G: String): String; | ||
begin | begin | ||
| − | + | Result := 'Woah ' + G; | |
end; | end; | ||
| − | // | + | // Overloaded function takes the two types of function |
| − | //pointers created above | + | // pointers created above |
| − | procedure Take(f: | + | procedure Take(f: TFuncNoArgsString); overload; |
begin | begin | ||
| − | + | WriteLn(f()); | |
end; | end; | ||
| − | procedure Take(f: | + | procedure Take(f: TFuncOneArgsString); overload; |
begin | begin | ||
| − | + | WriteLn(f('there!!!')); | |
end; | end; | ||
var | var | ||
| − | + | ptr: Pointer; | |
| − | + | List: TList; | |
begin | begin | ||
| − | // the "@" symbol turns the variable into a pointer. | + | // the "@" symbol turns the variable into a pointer. |
| − | // This must be done in order pass a function as a | + | // This must be done in order pass a function as a |
| − | // paramater. This also demonstrates that pascal | + | // paramater. This also demonstrates that pascal |
| − | // keeps track of the pointer type so the overloading works! | + | // 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.</delphi> | ||
Revision as of 12:25, 31 August 2010
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.
<delphi>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 // paramater. 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.</delphi>
