class Class1
{
};
class class2
{
};
 
main()
{
 ....
.....
 if(var==true)
 class1 c1=createandmanipulate(file);
 else
 class2 c1=createandmanipulate(file);
......
.....
.....
}

#include <iostream>
 
using namespace std;
 
class Class1 { };
class Class2 { };
 
// Specialize two type traits for picking class 1 and class 2.
template <bool>
class ClassType { };
 
template <>
class ClassType<false> { };
 
// Overload createandmanipulate to use the type traits.
Class1 createandmanipulate( ClassType<true> ) {
  cout<<"Returning Class1\n";
  return Class1();
}
 
Class2 createandmanipulate( ClassType<false> ) {
  cout<<"Returning Class2\n";
  return Class2();
}
 
// It all works! :)
int main() {
  Class1 c1 = createandmanipulate( ClassType<true>() );
  Class2 c2 = createandmanipulate( ClassType<false>() );
 
  return 0;
}

class base { virtual ~base() = 0 {} };
 
class Class1 : public base {} ;
 
class class2 : public base {} ;
 
base* createandmanipulate( ifstream& file );
 
int main()
{
  //....
  //.....
  base* object = createandmanipulate(file);
  if( var==true )
  {
    class1* c1 = dynamic_cast<class1*>(object) ;
    assert(c1) ;
    // use c1
  }
  else
  {
    class2* c2 = dynamic_cast<class2*>(object) ;
    assert(c2) ;
    // use c2
  }
  //....
  //.....
  delete base ;
}