how to pack multi-key map with msgpack-c

问题:

I am trying to use msgpack (in c version, not c++) to replace our own serialization method, which is in primary xml based. It is quite straight forward to pack some ordinary data. However, we have a lot of k-v based structures like

struct Table {
  struct Key {
    // Multi-keys
    int key1;
    int key2;
  };
  struct Attr {
    // Attributes
    int attr1;
    bool attr2;
    char[8] attr3;
  };
}

How to pack multi-key table with msg_pack_map in msgpack-c? (unfortunately, our system is exception disabled, so I cannot use the c++ version "msgpack.hpp")

my code snippet:

  msgpack_sbuffer sbuf;
  msgpack_sbuffer_init(&sbuf);
  msgpack_packer pk;
  msgpack_packer_init(&pk, &sbuf, msgpack_sbuffer_write);
  msgpack_packer_map(&pk, 10) // 10 pairs
  for(int i= 0; i<10; ++i) {
    // key
    msgpack_pack_array(&pk, 2);
    msgpack_pack_int(&pk, i);
    msgpack_pack_int(&pk, 100+i);
    // attr
    msgpack_pack_array(&pk, 3);
    msgpack_pack_int(&pk, 1);
    msgpack_pack_true(&pk);
    msgpack_pack_str(&pk, 7);
    msgpack_pack_str_body(&pk, "example");
}

I assume in msgpack, we have to use msgpack_pack_array to pack struct.

Is my code right, or is there any better way to do that?

回答1:

Yes, you are right. There are two subtle mistake in your code snippet. msgpack_packer_map should be msgpack_pack_map. msgpack_pack_str_body requires a length of the string as the third argument.

I updated your code snippet, and added test codes.

See:

http://melpon.org/wandbox/permlink/RuZKLmzwStHej5TP

#include <msgpack.h>
#include <msgpack.hpp>
#include <map>
#include <iostream>

struct Key {
    // Multi-keys
    int key1;
    int key2;
    MSGPACK_DEFINE(key1, key2);
};

inline bool operator<(Key const& lhs, Key const& rhs) {
    if (lhs.key1 < rhs.key1) return true;
    if (lhs.key1 > rhs.key1) return false;
    if (lhs.key2 < rhs.key2) return true;
    return false;
}

struct Attr {
    // Attributes
    int attr1;
    bool attr2;
    std::string attr3;
    MSGPACK_DEFINE(attr1, attr2, attr3);
};

int main() {
    msgpack_sbuffer sbuf;
    msgpack_sbuffer_init(&sbuf);
    msgpack_packer pk;
    msgpack_packer_init(&pk, &sbuf, msgpack_sbuffer_write);
    msgpack_pack_map(&pk, 10); // 10 pairs
    for(int i= 0; i<10; ++i) {
        // key
        msgpack_pack_array(&pk, 2);
        msgpack_pack_int(&pk, i);
        msgpack_pack_int(&pk, 100+i);
        // attr
        msgpack_pack_array(&pk, 3);
        msgpack_pack_int(&pk, 1);
        msgpack_pack_true(&pk);
        msgpack_pack_str(&pk, 7);
        msgpack_pack_str_body(&pk, "example", 7);
    }

    {
        auto upd = msgpack::unpack(sbuf.data, sbuf.size);

        std::cout << upd.get() << std::endl;
        auto tbl = upd.get().as<std::map<Key, Attr>>();
    }
    msgpack_sbuffer_destroy(&sbuf);
}

That is a c++ code but I use C API in the packing part. You can check the unpacked msgpack object. It is also converted as C++ map successfully.

Here is C API documentation: https://github.com/msgpack/msgpack-c/wiki/v1_1_c_overview