pure virtual functions and runtime id

I am fairly new to polymorphism with c++ and am having trouble
figuring out an error message. I narrowed it down to the following
simple example:
------------------------------------------------------------
#include <stdio.h>
#include <stdlib.h>
#include <iostream>
#include <string>

using namespace std;

class Base {
  int temp1;
  //string temp2;
public:
  virtual void whoami() = 0;
};

class Derived1 : public Base {
public:
  virtual void whoami() {
    cout << "== Derived1 ==" << endl;
  }
};

class Derived2 : public Base {
public:
  virtual void whoami() {
    cout << "== Derived2 ==" << endl;
  }
};

int main(int argc, char *argv[]) {
  int t = 0;
  Base * var;

  if (t == 1) {
    Derived1 v;
    var = &v;
    var->whoami();
  } else if (t == 0) {
    Derived2 v;
    var = &v;
    var->whoami();
  }
  var->whoami();

  return 0;
}
----------------------------------------------------------------------

This code, as shown, compiles, runs and produces the expected result.
However, if I uncomment the 'string temp2;' line, the code runs to
produce the following error message:

==============
== Derived2 ==
pure virtual method called
terminate called without an active exception
Abort
==============

Could someone please explain this behavior and clue me in on how to
correct it? Thanks a lot.

J