C++: POD and POD-wrapping objects

Question

Often I declare classes to wrap a single Plain Old Data value; simple classes without virtual functions, like:

class Velocity {
    int vel;
public:
    // functions to work with velocity ...
}

1. Working with such object is the same as working with POD values? -- Is there any difference in space or time usage, if in my code I used an int, instead of a Velocity? Does the standard say anything about this?

2. Can I cast a pointer or an array of such objects as a pointer/array of POD values and viceversa? -- Am I totally safe doing Velocity *v = reinterpret_cast< Velocity* >( int_pointer )?

Answer 1

Working with such object is the same as working with POD values?

No. You are not totally safe to use memcpy and friends on it (only allowed on PODs!).

Can I cast a pointer or an array of such objects as a pointer/array of POD values and viceversa?

If it is a POD, you are perfectly safe. But this is not a POD because it has a private data member.

---

Both of that said, in practice it will work fine for that class (and in C++0x, you are allowed to use such a class with private members with memcpy, because it allows it for all trivially copyable types, which includes your type and many other non-PODs).

Answer 2

Re "is the same as POD", not entirely in C++98. C++98 doesn't permit PODs to have private members. C++0x lifts this restriction (and a few others).

Re efficiency, or lack thereof, it's a Quality of Implementation issue.

Ask your compiler to optimize, then measure, if it matters.

Re casting: no, that has implementation defined effect. Don't do it.

Cheers & hth.,