Segmentation fault after delete[] on base class pointer [duplicate]

Question

I have a snippet of code here, of which I don't understand why it results in a segmentation fault on line 22 (delete[] statement). Can you please explain this to me?

#include<iostream>
#include<memory>

class A {
  size_t a[1000];

  public:
    virtual ~A() { }
};

class B : public A {
  public:
    float b;
    virtual ~B() { }
};


int main(int argc, char** argv){

  A *b;

  b = new B[10];
  delete[] b;

  return 0;
}

Strangely, if class B doesn't have any member variables (i.e. I comment out the line "float b;") then the code just runs fine.

What's my mistake here?

Answer 1

Simply put, you have undefined behavior. You aren't providing delete[] with a pointer you got from new[]. You may think you do, but for the pointers to be the same in the array version, their static type has to match. You converted the pointer into a pointer to a base class.

Practically, when you don't have that added float, your implementation probably maintains sizeof(B) == sizeof(A). So the destructor and deallocation function invocations don't do anything immediately harmful. But it's just as undefined.