C++ delete pointer twice [duplicate]

Question

I know that a "deleting the same memory twice" error can happen when two pointers address the same dynamically allocated object. If delete is applied to one of the pointers, then the object’s memory is returned to the free store. If we subsequently delete the second pointer, then the free store may be corrupted.

But why doesn't this code cause a run-time error?

 string *str_1 = new string;
  auto str_2 = str_1;
  *str_1 = "AAA";
  cout<<*str_2<<endl;
  delete str_1;
  delete str_2;  // No Error

    // Prints AAA

Answer 1

Deleting the same memory twice is undefined behaviour. Anything may happen, including nothing. It may e.g. cause a crash sometime later.

Answer 2

I compiled this program in g++ 4.9.1 and it gave me a runtime error:

*** Error in `./t': free(): invalid pointer: 0xbfa8c9d4 ***

You are trying to free something which is already freed. Hence, the error.