Add an example (#423)

I had a really tough time figuring out how to do simple pub-sub with cppzmq; I finally figured it out so I'd like to add it.

examples/pubsub_multithread_inproc.cpp

@@ -0,0 +1,99 @@
+#include <future>
+#include <iostream>
+#include <string>
+
+#include "zmq.hpp"
+#include "zmq_addon.hpp"
+
+void PublisherThread(zmq::context_t *ctx) {
+    //  Prepare publisher
+    zmq::socket_t publisher(*ctx, zmq::socket_type::pub);
+    publisher.bind("inproc://#1");
+
+    // Give the subscribers a chance to connect, so they don't lose any messages
+    std::this_thread::sleep_for(std::chrono::milliseconds(20));
+
+    while (true) {
+        //  Write three messages, each with an envelope and content
+        publisher.send(zmq::str_buffer("A"), zmq::send_flags::sndmore);
+        publisher.send(zmq::str_buffer("Message in A envelope"));
+        publisher.send(zmq::str_buffer("B"), zmq::send_flags::sndmore);
+        publisher.send(zmq::str_buffer("Message in B envelope"));
+        publisher.send(zmq::str_buffer("C"), zmq::send_flags::sndmore);
+        publisher.send(zmq::str_buffer("Message in C envelope"));
+        std::this_thread::sleep_for(std::chrono::milliseconds(100));
+    }
+}
+
+void SubscriberThread1(zmq::context_t *ctx) {
+    //  Prepare subscriber
+    zmq::socket_t subscriber(*ctx, zmq::socket_type::sub);
+    subscriber.connect("inproc://#1");
+
+    //  Thread2 opens "A" and "B" envelopes
+    subscriber.set(zmq::sockopt::subscribe, "A");
+    subscriber.set(zmq::sockopt::subscribe, "B");
+
+    while (1) {
+        // Receive all parts of the message
+        std::vector<zmq::message_t> recv_msgs;
+        zmq::recv_result_t result =
+          zmq::recv_multipart(subscriber, std::back_inserter(recv_msgs));
+        assert(result && "recv failed");
+
+        std::cout << "Thread2: [" << recv_msgs[0].to_string_view() << "] "
+                  << recv_msgs[1].to_string_view() << std::endl;
+    }
+}
+
+void SubscriberThread2(zmq::context_t *ctx) {
+    //  Prepare our context and subscriber
+    zmq::socket_t subscriber(*ctx, zmq::socket_type::sub);
+    subscriber.connect("inproc://#1");
+
+    //  Thread3 opens ALL envelopes
+    subscriber.set(zmq::sockopt::subscribe, "");
+
+    while (1) {
+        // Receive all parts of the message
+        std::vector<zmq::message_t> recv_msgs;
+        zmq::recv_result_t result =
+          zmq::recv_multipart(subscriber, std::back_inserter(recv_msgs));
+        assert(result && "recv failed");
+
+        std::cout << "Thread3: [" << recv_msgs[0].to_string_view() << "] "
+                  << recv_msgs[1].to_string_view() << std::endl;
+    }
+}
+
+int main() {
+    /*
+     * No I/O threads are involved in passing messages using the inproc transport.
+     * Therefore, if you are using a ØMQ context for in-process messaging only you
+     * can initialise the context with zero I/O threads.
+     *
+     * Source: http://api.zeromq.org/4-3:zmq-inproc
+     */
+    zmq::context_t ctx(0);
+
+    auto thread1 = std::async(std::launch::async, PublisherThread, &ctx);
+
+    // Give the publisher a chance to bind, since inproc requires it
+    std::this_thread::sleep_for(std::chrono::milliseconds(10));
+
+    auto thread2 = std::async(std::launch::async, SubscriberThread1, &ctx);
+    auto thread3 = std::async(std::launch::async, SubscriberThread2, &ctx);
+    thread1.wait();
+    thread2.wait();
+    thread3.wait();
+
+    /*
+     * Output:
+     *   An infinite loop of a mix of:
+     *     Thread2: [A] Message in A envelope
+     *     Thread2: [B] Message in B envelope
+     *     Thread3: [A] Message in A envelope
+     *     Thread3: [B] Message in B envelope
+     *     Thread3: [C] Message in C envelope
+     */
+}