MessagePack C++ - How to iterate through an unknown data structure?

I want to share structured data between C++ and Python languages using MessagePack like this one:

{
  "t" : [ [t00,...,t0N], ... , [tM0,...,tMN] ],
  "x" : [ x0,..,xN],
  "P" : [ [P00, ..., P0N], ..., [PM0,...,PMN] ]
}

The number of variables is optional so in some cases I will have for example only:

{
 "t" : [ [t00,...,t0N], ... , [tM0,...,tMN] ]
}

Decoding this in Python is pretty simple, my problem is to figure out how to decode this in C++ if I don't know in advance the structure of the data ? or the exact number of variables that I would have; is it possible to iterate the structure in these cases?

I managed to handle a "fixed" data structure ( always with the same number of variables ) defining a struct for example:

struct variables
{
   std::vector< std::vector<double> > t;
   std::vector< double > x;
   std::vector< std::vector<double> > P;
   MSPACK_DEFINE_MAP( t, x, P );
};

std::stringstream inBuffer;

.... (read data )

std::string str( inBuffer.str() );
msgpack::object_handle oh = msgpack::unpack( str.data(), str.size() );
msgpack::object deserialized = oh.get();

variables var;
deserialized.convert( var );

Is there a better way to accomplish this ?, how could manage optional variables that could not appear in the structure ?; I repeat the previous question: could I iterate an unknown data structure in C++?, how ?

Thanks in advance!

Regards, Ernesto

Solution

There are two ways to treat unknown data structure.

The first way is using parse/visitor mechanism. Here is an example:

#include <msgpack.hpp>
#include <sstream>
#include <iostream>

// This is a simple print example visitor.
// You can do any processing in your visitor.
struct my_visitor : msgpack::null_visitor {
    bool start_map_key() {
        processing_map_key = true;
        return true;
    }
    bool end_map_key() {
        processing_map_key = false;
        return true;
    }
    bool start_array(uint32_t size) {
        std::cout << "array (size:" << size << ")[" << std::endl;
        return true;
    }
    bool end_array() {
        std::cout << "]" << std::endl;
        return true;
    }

    bool visit_str(const char* v, uint32_t size) {
        if (processing_map_key) {
            std::cout << "map key:" << std::string(v, size) << std::endl;
        }
        return true;
    }
    bool visit_positive_integer(uint64_t v) {
        std::cout << "found value:" << v << std::endl;
        return true;
    }

    bool processing_map_key = false;
    std::string indent;
};


int main() {
    // create test data
    std::stringstream ss;
    msgpack::packer<std::stringstream> pk(ss);
    pk.pack_map(1);
    pk.pack("t");
    pk.pack_array(2);
    pk.pack_array(3);
    pk.pack(1);
    pk.pack(2);
    pk.pack(3);
    pk.pack_array(3);
    pk.pack(4);
    pk.pack(5);
    pk.pack(6);

    // print data (for debug)
    {
        auto oh = msgpack::unpack(ss.str().data(), ss.str().size());
        std::cout << oh.get() << std::endl;
    }

    // apply visitor
    {
        my_visitor mv;
        msgpack::parse(ss.str().data(), ss.str().size(), mv);
    }
}

Running demo: https://wandbox.org/permlink/3NrR4IMDIuLTk9e9

See https://github.com/msgpack/msgpack-c/wiki/v2_0_cpp_visitor.

The other way is using msgpack::type::variant or `msgpack::type::variant_ref. The former copies data, you can update it. The latter doesn't copy data. You cannot update it. This approach requires boost. So you need to define MSGPACK_USE_BOOST. I recommend defining as a compiler option.

// Boost is required
#define MSGPACK_USE_BOOST

#include <msgpack.hpp>
#include <sstream>
#include <iostream>

struct my_visitor:boost::static_visitor<void> {
    void operator()(uint64_t v) const {
        std::cout << "positive insteger:" << v << std::endl;
    }
    // const is required for map key because std::multimap's key (first) is const.
    void operator()(std::string const& v) const {
        std::cout << "string:" << v << std::endl;
    }
    void operator()(std::vector<msgpack::type::variant>& v) const {
        std::cout << "array found" << std::endl;
        for (auto& e : v) {
            boost::apply_visitor(*this, e);
        }
    }
    void operator()(std::multimap<msgpack::type::variant, msgpack::type::variant>& v) const {
        std::cout << "map found" << std::endl;
        for (auto& e : v) {
            std::cout << "key:" << std::endl;
            boost::apply_visitor(*this, e.first);
            std::cout << "value:" << std::endl;
            boost::apply_visitor(*this, e.second);
        }
    }
    template <typename T>
    void operator()(T const&) const {
        std::cout << "  match others" << std::endl;
    }
};

int main() {
    // create test data
    std::stringstream ss;
    msgpack::packer<std::stringstream> pk(ss);
    pk.pack_map(1);
    pk.pack("t");
    pk.pack_array(2);
    pk.pack_array(3);
    pk.pack(1);
    pk.pack(2);
    pk.pack(3);
    pk.pack_array(3);
    pk.pack(4);
    pk.pack(5);
    pk.pack(6);

    auto oh = msgpack::unpack(ss.str().data(), ss.str().size());
    std::cout << oh.get() << std::endl;

    msgpack::type::variant v = oh.get().as<msgpack::type::variant>();
    boost::apply_visitor(my_visitor(), v);
}

Running demo: https://wandbox.org/permlink/HQwJjfwW8rLEMi0d

See https://github.com/msgpack/msgpack-c/wiki/v2_0_cpp_variant

Here are exampless: https://github.com/msgpack/msgpack-c/blob/master/example/boost/msgpack_variant_capitalize.cpp https://github.com/msgpack/msgpack-c/blob/master/example/boost/msgpack_variant_mapbased.cpp

Both ways can treat unpredictable data structure. You need to do some visitor processing. If the data structure is predictable some extent, your original approach is also good way.