Why can it be dangerous to use this POD struct as a base class?

Question

I had this conversation with a colleague, and it turned out to be interesting. Say we have the following POD class

struct A { 
  void clear() { memset(this, 0, sizeof(A)); } 

  int age; 
  char type; 
};

clear is intended to clear all members, setting to 0 (byte wise). What could go wrong if we use A as a base class? There's a subtle source for bugs here.

Answer 1

The compiler is likely to add padding bytes to A. So sizeof(A) extends beyond char type (until the end of the padding). However in case of inheritance the compiler might not add the padded bytes. So the call to memset will overwrite part of the subclass.