Compile the following class
class Interface
{
virtual void doIt() = 0;
virtual ~Interface() = 0;
};
inline Interface::~Interface() {}
using gcc -fdump-class-hierarchy
.
gcc
emits
Class Interface size=4 align=4 base size=4 base align=4 Interface (0x1a779c0) 0 nearly-empty vptr=((& Interface::_ZTV9Interface) + 8u)
What is the significance of "nearly-empty"? What does it mean?
The C++ ABI provides a definition of "nearly empty" classes and an interesting discussion of how they affect vtable construction:
A class that contains a virtual pointer, but no other data except (possibly) virtual bases. In particular, it:
- has no non-static data members other than zero-width bitfields,
- has no direct base classes that are not either empty, nearly empty, or virtual,
- has at most one non-virtual, nearly empty direct base class, and
- has no proper base class that is empty, not morally virtual, and at an offset other than zero.
I ran across this while researching the effect of nearly empty virtual bases on object size, vtable size, and virtual call overhead.