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,          


        

Answer 1

Briefly: It seems to me that the only one potentional problem is in that I can not found any info about "padding bytes" guarantees in C89, C2003 standarts....Do they have some extraordinary volatile or readonly behavior - I can not find even what does term "padding bytes" mean by the standarts...

Detailed:

For objects of POD types it is guaranteed by the C++2003 standard that:

• when you memcpy the contents of your object into an array of char or unsigned char, and then memcpy the contents back into your object, the object will hold its original value

• guaranteed that there will be no padding in the beginning of a POD object

• can break C++ rules about: goto statement, lifetime

For C89 there is also exist some guarantees about structures:

• When used for a mixture of union structures if structs have same begining, then first compoments have perfect mathing

• sizeof structures in C is equal to the amount of memory to store all the components, the place under the padding between the components, place padding under the following structures

• In C components of the structure are given addresses. There is a guarantee that the components of the address are in ascending order. And the address of the first component coincides with the start address of the structure. Regardless of which endian the computer where the program runs

So It seems to me that such rules is appropriate to C++ also, and all is fine. I really think that in hardware level nobody will restrict you from write in padding bytes for non-const object.