Question

In single inheritances, is the size of a pointer to virtual table always equal to the size of a void*? Say,

class vft { virtual ~vft(); }
assert (sizeof(vft) == sizeof(void*));

Would that assertion always be true?

Answer 1

The C++ ISO Standard says nothing about virtual function table pointer in the first place. A Compiler may follow this mechanism to support runtime-polymorphism or can come up with any other which doesn’t even involve vptr. Its entirely upto the compiler writers. Since the Standard doesn’t say anything about vptr, how can it say about its size? No way. The conclusion is: what you’re doing (or assuming) isn’t gauranteed by the language. However, for a compiler, it might be always true.

^{As a sidenote, for your compiler, how can you conclude that sizeof(vft) will be equal to sizeof(vptr)? It could very well be that sizeof(vft) > sizeof(vptr). I don’t claim that though.}