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3.6 Procedural types

Free Pascal has support for procedural types, although it differs a little from the Turbo Pascal or Delphi implementation of them. The type declaration remains the same, as can be seen in the following syntax diagram:

_________________________________________________________________________________________________________
Procedural types

--procedural type-|-function header----------------------------------- procedure header of object ; call modifiers

--function header- function -formal parameter list :-result type----------

--procedure header procedure- formal parameter list--------------------

--call modifiers-|register ------------------------------------------ |-cdecl--| |-pascal--| |-stdcall--| |safecall-| --inline --
___________________________________________________________________

For a description of formal parameter lists, see chapter 11, page 478. The two following examples are valid type declarations:

Type TOneArg = Procedure (Var X : integer);  
     TNoArg = Function : Real;  
var proc : TOneArg;  
    func : TNoArg;

One can assign the following values to a procedural type variable:

  1. Nil, for both normal procedure pointers and method pointers.
  2. A variable reference of a procedural type, i.e. another variable of the same type.
  3. A global procedure or function address, with matching function or procedure header and calling convention.
  4. A method address.

Given these declarations, the following assignments are valid:

Procedure printit (Var X : Integer);  
begin  
  WriteLn (x);  
end;  
...  
Proc := @printit;  
Func := @Pi;

From this example, the difference with Turbo Pascal is clear: In Turbo Pascal it isn’t necessary to use the address operator (@) when assigning a procedural type variable, whereas in Free Pascal it is required. In case the -MDelphi or -MTP switches are used, the address operator can be dropped.

Remark: The modifiers concerning the calling conventions must be the same as the declaration; i.e. the following code would give an error:

Type TOneArgCcall = Procedure (Var X : integer);cdecl;  
var proc : TOneArgCcall;  
Procedure printit (Var X : Integer);  
begin  
  WriteLn (x);  
end;  
begin  
Proc := @printit;  
end.

Because the TOneArgCcall type is a procedure that uses the cdecl calling convention.

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