Procedural types allow you to treat procedures and functions as values that can be assigned to variables or passed to other procedures and functions. For example, suppose you define a function called Calc that takes two integer parameters and returns an integer:

function Calc(X,Y: Integer): Integer;

You can assign the Calc function to the variable F:

var F: function(X,Y: Integer): Integer;

F := Calc;

If you take any procedure or function heading and remove the identifier after the word procedure or function, what's left is the name of a procedural type. You can use such type names directly in variable declarations (as in the example above) or to declare new types:

type

  TIntegerFunction = function: Integer;
  TProcedure = procedure;
  TStrProc = procedure(const S: string);
  TMathFunc = function(X: Double): Double;
var
  F: TIntegerFunction;             { F is a parameterless function that returns an integer }
  Proc: TProcedure;                { Proc is a parameterless procedure }
  SP: TStrProc;                    { SP is a procedure that takes a string parameter }
  M: TMathFunc;                    { M is a function that takes a Double (real) parameter 

                                       and returns a Double }
procedure FuncProc(P: TIntegerFunction);  { FuncProc is a procedure whose only parameter 
                                              is a parameterless integer-valued function }

The variables above are all procedure pointers-that is, pointers to the address of a procedure or function. If you want to reference a method of an instance object (see Classes and objects), you need to add the words of object to the procedural type name. For example

type

  TMethod = procedure of object;
  TNotifyEvent = procedure(Sender: TObject) of object;

These types represent method pointers. A method pointer is really a pair of pointers; the first stores the address of a method, and the second stores a reference to the object the method belongs to. Given the declarations

type

  TNotifyEvent = procedure(Sender: TObject) of object;
  TMainForm = class(TForm)
    procedure ButtonClick(Sender: TObject);
    ...
  end;
var
  MainForm: TMainForm;
  OnClick: TNotifyEvent

we could make the following assignment.

OnClick := MainForm.ButtonClick;

Two procedural types are compatible if they have

the same calling convention,
        the same return value (or no return value), and
        the same number of parameters, with identically typed parameters in corresponding positions. (Parameter names do not matter.)

Procedure pointer types are always incompatible with method pointer types. The value nil can be assigned to any procedural type.
Nested procedures and functions (routines declared within other routines) cannot be used as procedural values, nor can predefined procedures and functions. If you want to use a predefined routine like Length as a procedural value, write a wrapper for it:

function FLength(S: string): Integer;

begin
  Result := Length(S);
end;

Procedural types in statements and expressions