Mega Code Archive
Function - defines a subroutine that returns a value
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1 Function Name : Return type; {Directives;}
2 Function Name(Parameters) : Return type; {Directives;}
3 type Name = Function{(Parameters)} : Return type {of object};
Description
The Function keyword defines a subroutine that returns a value. See the SubRoutines tutorial for details on using functions.
Version 1
Defines a function that simply returns a data type. Only one value can be returned from a function.
Version 2
Defines a function that is passed one or more parameters, and returns a data type. Only one value can be returned from a function. However, using the out or var keyword before a parameter allows the parameter to be treated as variable to contain return values.
In both cases, the returned value is passed by assigning to the Result pseudo variable. Delphi creates this variable for you at the function start, with the correct return data type.
(Older Pascal code assigned to a variable with the same name as the function).
When a function is defined in a class, it is commonly called a Method.
The same name may be used for more than one function as long as the Overload directive is used. The other main directives, in the order that they should appear is given here:
Reintroduce : Redefines a suppressed function
Overload : Allows same name for 2 or more functions
Virtual : Can be redefined in a child class
Override : Redefines a parent class method
Abstract : Forces child class to implement
Version 3
Defines a function as a data type. This allows the function to be passed as a parameter, and used as a variable. The type definition defines just the profile of the function - and not the name.
A variable of such a type could be assigned the name of any function with that profile. When assigned, the variable name can be treated as if it were a function name. See the example code.
Further still, the Of Object option allows you to refer to an object method. Access to a variable of such type would then behave as if you were calling the object method directly. See the second example.
Related commands
Abstract Defines a class method only implemented in subclasses
Const Starts the definition of fixed data values
Out Identifies a routine parameter for output only
Override Defines a method that replaces a virtual parent class method
Procedure Defines a subroutine that does not return a value
Result A variable used to hold the return value from a function
Var Starts the definition of a section of data variables
Virtual Allows a class method to be overriden in derived classes
Dynamic Allows a class method to be overriden in derived classes
Example code : A simple example
// Full Unit code.
// -----------------------------------------------------------
// You must store this code in a unit called Unit1 with a form
// called Form1 that has an OnCreate event called FormCreate.
unit Unit1;
interface
uses
// The System unit does not need to be defined
Forms, Dialogs;
type
TForm1 = class(TForm)
procedure FormCreate(Sender: TObject);
end;
var
Form1: TForm1;
implementation
{$R *.dfm} // Include form definitions
Function GetSum(a, b : Integer) : Integer;
begin
// Add the two numbers together, and return this value
Result := a + b;
end;
// The main form On Create routine - our main program
procedure TForm1.FormCreate(Sender: TObject);
var
total : Integer;
begin
// Show the sum of a few number pairs
total := GetSum(1,2);
ShowMessageFmt('%d + %d = %d',[1,2,total]);
total := GetSum(62,444);
ShowMessageFmt('%d + %d = %d',[62,444,total]);
end;
end.
1 + 2 = 3
62 + 444 = 506
Example code : Illustrating a function and a function type
// Full Unit code.
// -----------------------------------------------------------
// You must store this code in a unit called Unit1 with a form
// called Form1 that has an OnCreate event called FormCreate.
unit Unit1;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs;
type
// The form class itself
TForm1 = class(TForm)
procedure FormCreate(Sender: TObject);
end;
var
Form1: TForm1;
implementation
{$R *.dfm}
// In line functions
Function MaxValue(a, b : Integer) : Integer;
begin
// Return the highest of 2 numbers
if a > b
then Result := a
else Result := b;
end;
Function MinValue(a, b : Integer) : Integer;
begin
// Return the lowest of 2 numbers
if a < b
then Result := a
else Result := b;
end;
// Main line code
procedure TForm1.FormCreate(Sender: TObject);
type
TNumberFunc = Function(a, b : Integer) : Integer;
var
numberFunc : TNumberFunc;
a, b, c : Integer;
begin
// Use the MaxValue function directly
a := 3;
b := 6;
c := MaxValue(a, b);
ShowMessage('Direct call to MaxValue :');
ShowMessageFmt('Max of %d and %d is %d',[a,b,c]);
// Now call it indirectly
numberFunc := MaxValue;
c := numberFunc(a, b);
ShowMessage('Indirect call to MaxValue :');
ShowMessageFmt('Max of %d and %d is %d',[a,b,c]);
// And call it again for the MinValue function
numberFunc := MinValue;
c := numberFunc(a, b);
ShowMessage('Indirect call to MinValue :');
ShowMessageFmt('Min of %d and %d is %d',[a,b,c]);
end;
end.
Direct call to MaxValue :
Max of 3 and 6 is 6
Indirect call to MaxValue :
Max of 3 and 6 is 6
Indirect call to MinValue :
Min of 3 and 6 is 3
Example code : Using a type of a function of a class
// Full Unit code.
// -----------------------------------------------------------
// You must store this code in a unit called Unit1 with a form
// called Form1 that has an OnCreate event called FormCreate.
unit Unit1;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs;
type
// Define a simple class
TSimple = class
private
name : string;
public
function GetName : string;
constructor Create(name : string);
end;
// The form class itself
TForm1 = class(TForm)
procedure FormCreate(Sender: TObject);
end;
var
Form1: TForm1;
implementation
{$R *.dfm}
// Create a simple object
constructor TSimple.Create(name: string);
begin
// Save the passed string
self.name := name;
end;
// Returns the simple name
function TSimple.GetName: string;
begin
Result := name;
end;
// Main line code
procedure TForm1.FormCreate(Sender: TObject);
type
TNameFunc = Function : string of object;
var
simple : TSimple;
nameFunc : TNameFunc;
begin
// Create a simple object
simple := TSimple.Create('Brian');
// Show the object name
ShowMessage('Name accessed directly = '+simple.GetName);
// Now refer to this method indirectly
nameFunc := simple.GetName;
// Show the object name
ShowMessage('Name accessed indirectly = '+nameFunc);
end;
end.
Name accessed directly = Brian
Name accessed indirectly = Brian
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