According to strict aliasing rules:
struct B { virtual ~B() {} };
struct D : public B { };
D d;
char *c = reinterpret_cast<char*>(&d);
A char* to any object of different type is valid. But now the question is, will it point to the same address of &d? what is the guarantee made by C++ Standard that it will return the same address?
c and &d do indeed have the same value, and if you reinterpret-cast c back to a D* you get a valid pointer that you may dereference. Furthermore, you can treat c as (pointer to the first element of) an opaque array char[sizeof(D)] -- this is indeed the main purpose of casting pointers to char pointers: To allow (de)serialization (e.g. ofile.write(c, sizeof(D));), although you should generally only do this for primitive types (and arrays thereof), since the binary layout of of compound types is not generally specified in a portable fashion.
As @Oli rightly points out and would like me to reinforce, you should really never serialize compound types as a whole. The result will almost never be deserializable, since the implementation of polymorphic classes and padding between data fields is not specified and not accessible to you.
Note that reinterpret_cast<char*>(static_cast<B*>(&d)) may be treated as an opaque array char[sizeof(B)] by similar reasoning.