Does the C++ spec allow an instance of a non-virtual class to include memory for a vtable pointer?

Question

Does the C++ spec allow an instance of a non-virtual class to include memory for a vtable pointer? I am asking this, because a colleague said he once used a C++ compiler where the following happened:

  class MyClass
  {

     public:

     HeaderStruct header; //This had extra words
     BodyStruct     message_body;
  };

He then changed the code to this, which got rid of the extra words:

  struct MyClass
  {

     HeaderStruct header; //This did not have extra words
     BodyStruct     message_body;
  };

None of these types was virtual or derived from anything virtual. So the theory was that perhaps this particular compiler allocated memory for a vptr for the class instances but not for the struct instances. So I'm just trying to determine if such compiler behavior is precluded by the spec.

Thanks!

Ken

Answer 1

I'm unable to find an online reference for it with a quick search, but I'm pretty sure a compiler is permitted to do ANYTHING with the layout of any class; in particular, in classes without any virtual methods, it's allowed to put a vftp or not, depending on how it's feeling that day, whether it's declared as class or struct (which are equivalent in C++ except for the default access specifier), the phase of the moon, or anything else. The only restriction that I'm aware of is that the top part of a derived object shall match the layout of its first (non-virtual) base class. And I'm not even sure it has to be the first one.

You should not at any time depend on a particular compiler's decision on the layout of an object. Many compilers put a vftp in all objects without exception, in order to provide run-time type information to debuggers, or just to make their own lives easier. Some don't. You have no reasonable way of knowing, except through the sizeof operator.