c++ - Invalid conversion from Foo** to void** - why is implicit type conversion allowed to void* but not to void**?

Question

struct Foo {};
...
void * p = (Foo*)0; // OK
void ** pp = (Foo**)0; // Invalid conversion

As far as I recall, a pointer to any non-pointer type can be implicitly cast to void* in C++. Why then is the same not allowed for casting a ponter to pointer type to void**?

Answer 1

A pointer can be implicitly cast to void * because void * is the generic pointer. However, void ** isn't the generic pointer to pointer.

C FAQ 4.9 explains why there is no generic pointer to pointer type in C, I think it applies to C++ as well.