CODEWITHSUNDEEP
pascalfreepascal
William
William

Reputation: 2935

How to reference an external function from inside a unit

I have two files. The first, a.pas, contains:

uses b;

function f(x: integer): integer;
begin
    f := x+1;
end;

begin
    writeln(g(10));
end.

The file b.pas contains:

unit b;

interface
function g(x: integer): integer;

implementation
function g(x: integer): integer;
begin
    g := f(x)*2;
end;

end.

Is it possible to somehow reference the function f defined in a.pas? I thought about using the forward keyword like this: (b.pas)

unit b;

interface
function g(x: integer): integer;

implementation
function f(x: integer): integer; forward;
function g(x: integer): integer;
begin
    g := f(x)*2;
end;

end.

But it doesn't work. It gives a "Forward declaration not solved" error.

I also thought about using the external keyword but, in order to use the external f function, b.pas requires a.pas to be already compiled (but a.pas require b.pas as well).

The only way to do this seems to be moving f (interface and implementation) to a new helper.pas file, and modifying b.pas like this:

unit b;

interface
function g(x: integer): integer;

implementation
uses helper;
function g(x: integer): integer;
begin
    g := f(x)*2;
end;

end.

I would prefer not having a helper.pas file. But maybe it's just impossible?

Upvotes: 0

Views: 360

Answers (1)

gammatester
gammatester

Reputation: 1141

With the current source code layout it is not possible. If you do not want to make a.pas a unit (why?), you can rewrite function g to have a function parameter:

{$ifdef FPC}
  {$mode delphi}
{$endif}

type
  tintfunc = function(x: integer): integer;

function g(x: integer; f: tintfunc): integer;
begin
  g := 2*f(x);
end;

function f(x: integer): integer;
begin
  f := x+1;
end; 

begin
  writeln(g(3,f));
end.

There are other methods (including global function pointers), but function parameters are IMO the most clean/safe way to implement it.

Upvotes: 1

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