void MainFunction(int hWnd,int SomeParam1,int SomeParam2)
{
 int AdditionalFunction()
 {
  return hWnd+2;
 }

 return AdditionalFunction();
}
    

int MainFunction(int hWnd,int SomeParam1,int SomeParam2) 
{
     return AdditionalFunction(hWnd);//Void functions cannot return stuff, since AdditionalFunction is an int make MainFunction an int.
}

int AdditionalFunction(int hWnd)
{
     return hWnd+2;//In oreder to use hWnd you need to pass it as a param or declare it in AdditionalFunction.
}
    

int MainFunction(int hWnd,int SomeParam1,int SomeParam2);//Notice the ;
int AdditionalFunction();
    

procedure foo(a: integer)

  function bar(): integer
  begin
    bar := 4 + a; { Notice it can access parameters and variables from its parent }
  end;

begin
  WriteLn(bar());
end;

{ bar can't be called from here }    

Unfortunately, it's only available for C (and not C++)