reinterpret_cast for almost pod data (is layout-compatibility enough)

Question

I am trying to learn about static_cast and reinterpret_cast.

If I am correct the standard (9.2.18) says that reinterpret_cast

Answer 1

I believe the following are valid because complex_base is POD

You are wrong. d[0] does not refer to the first member of a complex_base object. Its alignment may therefor not be good enough for a complex_base object, therefor such a cast is not safe (and not allowed by the text you quote).

Does the following complete a valid cast to complex even though complex is NOT POD?

cb1 and cb2 do not point to subobjects of an object of type complex, therefor the static_cast produces undefined behavior. Refer to 5.2.9p5 of C++03

If the lvalue of type "cv1 B" is actually a sub-object of an object of type D, the lvalue refers to the enclosing object of type D. Otherwise, the result of the cast is undefined.

It's not enough if merely the types involved fit together. The text talks about a pointer pointing to a POD-struct object and about an lvalue referring to a certain subobject. oth complex and complex_base are standard-layout objects. The C++0x spec says, instead of the text you quoted:

Is POD-ness requirement too strict?

This is a different question, not regarding your example code. Yes, requiring POD-ness is too strict. In C++0x this was recognized, and a new requirement which is more loose, "standard-layout" is given. I do think that both complex and complex_base are standard-layout classes, by the C++0x definition. The C++0x spec says, instead of the text you quoted:

A pointer to a standard-layout struct object, suitably converted using a reinterpret_cast, points to its initial member (or if that member is a bit-field, then to the unit in which it resides) and vice versa.

I interpret that as allowing to cast a pointer to a double, which actually points to a complex member (member by inheritance), to be casted to a complex*. A Standard-layout class is one that either has no base classes containing non-static data, or has only one base-class containing non-static data. Thus there is an unique "initial member".