Base classes packing/converting/creating object::with_zone support in C+...

[redboltz]

...+11.

[nobu-k]

I think it's perfect.

I thought MSGPACK_BASE had a problem regarding the compatibility of packed binaries at the first glance, but I noticed that your code beautifully solved it (and thanks to the frsyuki's original idea). One important characteristic of MSGPACK_DEFINE is that appending a new member at the end of it doesn't break the compatibility. For example, let's assume we have struct A.

// Version 1
struct A {
  int a;
  MSGPACK_DEFINE(a);
}

Then we add a new member b to it.

// Version 2
struct A {
  int a, b;
  MSGPACK_DEFINE(a, b);
}

The version 1 can still unpack the packed binary of the version 2's. b is simply ignored in the version 1. Then, look at the code proposed in this PR.

struct top {
    int t;
    MSGPACK_DEFINE(t);
};

struct mid1 : top {
    int m1;
    MSGPACK_DEFINE(MSGPACK_BASE(top), m1);
};

It seems like we cannot add a new member to top without breaking the compatibility of mid1. However, when packing mid1, top is serialized as an array. So, the original characteristic is preserved recursively. I like this simple solution.