[ecfisa]

Hola JXJ.

Unit1.cpp :

#include <vcl.h>
#pragma hdrstop

#include "Unit1.h"
#include "Unit3.h"

#pragma package(smart_init)
#pragma resource "*.dfm"

TForm1 *Form1;

__fastcall TForm1::TForm1(TComponent* Owner)
  : TForm(Owner)
{
}

void __fastcall TForm1::Button1Click(TObject *Sender)
{
  inicializa();
  ShowMessage(DirUserDoc);
}

Unit3.h:

#ifndef Unit3H
#define Unit3H

#include <vcl>
#include <string>

  extern String DirUserDoc;
  void __fastcall inicializa();

#endif

Unit3.cpp:

#pragma hdrstop

#include "Unit3.h"
#include <string>

String DirUserDoc;

void __fastcall inicializa()
{
  DirUserDoc = "super mira nada mas";
}

#pragma package(smart_init)

Saludos