ZeroMQ Message Corrupts Data - c++

Question

I'm sending a struct over a ZeroMQ connection. Two fields are correct, but one is corrupted. To illustrate the problem, my code tries to read the content of the zmq message and create a new struct.
Output:
$ ./client
id = 100
successful = 1
data_size = 356515840
Segmentation fault
client.cc
#include <string>
#include <iostream>
#include "zmq.hpp"
#include "struct.h"
int main () {
zmq::context_t context (1);
zmq::socket_t socket (context, ZMQ_REQ);
socket.connect ("tcp://localhost:5555");
query q(100);
q.data_size = 3;
q.data = "abc";
q.successful = true;
zmq::message_t request = q.generate_message();
query test_query((char *) request.data());
return 0;
}
struct.h
#include <arpa/inet.h>
#include "zmq.hpp"
struct query {
long id;
bool successful;
long data_size;
std::string data;
query (long id) {
this->id = id;
this->successful = false;
this->data_size = 0;
this->data = "";
}
query (char *start) {
id = *((long*) start);
start += sizeof(long);
std::cout << "id = " << id << std::endl;
successful = *((bool*) start);
start += sizeof(bool);
std::cout << "successful = " << successful << std::endl;
data_size = *((long *) start);
data_size = ntohl(data_size);
start += sizeof(long);
std::cout << "data_size = " << data_size << std::endl;
data = std::string(start, data_size);
}
long get_total_size() {
return 2 * sizeof(long) + sizeof(bool) + data_size;
}
// The string is encoded by calling data.c_str()
zmq::message_t generate_message() {
long size = get_total_size();
zmq::message_t msg(size);
memcpy((void *) msg.data(), this, size);
char *loc_of_data = (char *) msg.data() + 2 * sizeof(long) + sizeof(bool);
memcpy((void *) loc_of_data, data.c_str(), data_size);
return msg;
}
};

Answer 1

You're ignoring padding.
Because you are using memcpy, you're putting the same layout in the message as exists in your structure in memory. And long, bool, long in a structure has padding for alignment purposes.
I suggest a substructure for your "plain-old-data", and stop thinking about 2 * sizeof(long) + sizeof (bool).