Not an allowed type c++ form

#include <math.h>
#include <stdlib.h>
#include <vcl.h>
#pragma hdrstop
using namespace std;
#include "Unit1.h"
//---------------------------------------------------------------------------
#pragma package(smart_init)
#pragma resource "*.dfm"
TForm1 *Form1;
//---------------------------------------------------------------------------
const double eps = 0.01;
double a, b, t, x, y, t1, s;
double func(double a, double b, double e)
{
s = (a + b) / 2 ;
t1 = 7 * sin(2*s + e) ;
return t1;
}
 
void JustDoIt(double &a,double &b)
{
 t = (a + b) / 2.0;
                y = func(a, b,   - eps);
                x = func(a, b,  eps);
                if (y >= x)    a = t;
                else   b = t;
 
               if ( fabs(b - a) < eps) return;
               JustDoIt( a, b) ;
 
}
__fastcall TForm1::TForm1(TComponent* Owner)
        : TForm(Owner)
{
}
//---------------------------------------------------------------------------
 
void __fastcall TForm1::Button1Click(TObject *Sender)
{
double u;
a = 2;
b = 6;
switch (RadioGroup1->ItemIndex)
{
case 0: u=JustDoIt( a, b); break;
case 1: u=func(a,b,0); break;
}
Memo1->Lines->Add(AnsiString(FloatToStrF(u,ffFixed,8,5)));
}