How To MQTT In CPP (C++)

Table of Contents

Set Up A Local Mosquitto Broker

A Data Consumer

The following program is an example of a simple data consumer built with the Paho Mosquitto C++ Library.

File: SimpleMqttConsumer.cpp

// Start by `#include`-ing the Mosquitto MQTT Library and other standard libraries.
#include <mqtt/client.h>  // Mosquitto client.
#include <ostream>  // std::cout.

// With the library header files included, continue by defining a main function.
int main()
{
    // In order to connect the mqtt client to a broker, 
    // Define an Ip address pointing to a broker. In this case, the localhost on port 1883.
    std::string ip = "localhost:1883";
    // Then, define an ID to be used by the client when communicating with the broker.
    std::string id = "consumer";

    // Construct a client using the Ip and Id, specifying usage of MQTT V5.
    mqtt::client client(ip, id, mqtt::create_options(MQTTVERSION_5));
    // Use the connect method of the client to establish a connection to the broker.
    client.connect();
    // In order to receive messages from the broker, specify a topic to subscribe to.
    client.subscribe("in");
    // Begin the client's message processing loop, filling a queue with messages.
    client.start_consuming();

    bool running = true;
    while (running)
    {
        // Construct a message pointer to hold an incoming message.
        mqtt::const_message_ptr messagePointer;

        // Try to consume a message, passing messagePointer by reference.
        // If a message is consumed, the function will return `true`, 
        // allowing control to enter the if-statement body.
        if (client.try_consume_message(&messagePointer))
        {
            // Construct a string from the message payload.
            std::string messageString = messagePointer -> get_payload_str();
            // Print payload string to console (debugging).
            std::cout << messageString << std::endl;

            // Perform processing on the string.
            // This is where message processing can be passed onto different
            // functions for parsing. 
            // Here, we break the loop and exit the program if a `quit` is received.
            if (messageString == "quit")
            {
                running = false;
            } 
        }
    }

    return 0;
}

// Start by `#include`-ing the Mosquitto MQTT Library and other standard libraries.

#include <mqtt/client.h> // Mosquitto client.

#include <ostream> // std::cout.

// With the library header files included, continue by defining a main function.

int main()

{

// In order to connect the mqtt client to a broker,

// Define an Ip address pointing to a broker. In this case, the localhost on port 1883.

std::string ip = "localhost:1883";

// Then, define an ID to be used by the client when communicating with the broker.

std::string id = "consumer";

// Construct a client using the Ip and Id, specifying usage of MQTT V5.

mqtt::client client(ip, id, mqtt::create_options(MQTTVERSION_5));

// Use the connect method of the client to establish a connection to the broker.

client.connect();

// In order to receive messages from the broker, specify a topic to subscribe to.

client.subscribe("in");

// Begin the client's message processing loop, filling a queue with messages.

client.start_consuming();

bool running = true;

while (running)

{

// Construct a message pointer to hold an incoming message.

mqtt::const_message_ptr messagePointer;

// Try to consume a message, passing messagePointer by reference.

// If a message is consumed, the function will return `true`,

// allowing control to enter the if-statement body.

if (client.try_consume_message(&messagePointer))

{

// Construct a string from the message payload.

std::string messageString = messagePointer -> get_payload_str();

// Print payload string to console (debugging).

std::cout << messageString << std::endl;

// Perform processing on the string.

// This is where message processing can be passed onto different

// functions for parsing.

// Here, we break the loop and exit the program if a `quit` is received.

if (messageString == "quit")

{

running = false;

}

return 0;

}

Compiling The Data Consumer

To compile the C++ source code into an executable file, use g++:

g++ SimpleMqttConsumer.cpp -lpaho-mqttpp3 -lpaho-mqtt3as -o simpleConsumer

Make note of the library links -lpaho-mqttpp3 and -lpaho-mqtt3as. The library for the Mosquitto C++ interface is -lpaho-mqttpp3 which in turn requires the Mosquitto C interface-lpaho-mqtt3as as a dependency.

A Data Producer

The following program is an example of a simple data producer built with the Paho Mosquitto C++ Library.

File: SimpleMqttProducer.cpp

// Start by `#include`-ing the Mosquitto MQTT Library and other standard libraries.
#include <mqtt/client.h>  // Mosquitto client.
#include <ostream>  // std::cout.
#include <chrono>  // Time keeping.
#include <thread>  // Sleep.

// With the library header files included, continue by defining a main function.
int main()
{
    // In order to connect the mqtt client to a broker,
    // Define an Ip address pointing to a broker. In this case, the localhost on port 1883.
    std::string ip = "localhost:1883";
    // Then, define an ID to be used by the client when communicating with the broker.
    std::string id = "publisher";

    // Construct a client using the Ip and Id, specifying usage of MQTT V5.
    mqtt::client client(ip, id, mqtt::create_options(MQTTVERSION_5));
    // Use the connect method of the client to establish a connection to the broker.
    client.connect();
    // Initialize an empty message with specified topic.
    mqtt::message_ptr timeLeftMessagePointer = mqtt::make_message("consumer/in", "");

    // Create a loop for counting down from N seconds.
    for (int i = 10; i > 0; --i)
    {
        // Sleep for one second.
        std::this_thread::sleep_for(std::chrono::milliseconds(1000));
        // Configure Mqtt message to contain payload specifying time until end.
        timeLeftMessagePointer->set_payload("Time Left: " + std::to_string(i));
        // Publish the Mqtt message using the connected client.
        client.publish(timeLeftMessagePointer);
    }

    // After counting down, configure Mqtt message for sending the quit signal.
    timeLeftMessagePointer->set_payload("quit");
    // Send quit signal to listeners.
    client.publish(timeLeftMessagePointer);

    return 0;
}

// Start by `#include`-ing the Mosquitto MQTT Library and other standard libraries.

#include <mqtt/client.h> // Mosquitto client.

#include <ostream> // std::cout.

#include <chrono> // Time keeping.

#include <thread> // Sleep.

// With the library header files included, continue by defining a main function.

int main()

{

// In order to connect the mqtt client to a broker,

// Define an Ip address pointing to a broker. In this case, the localhost on port 1883.

std::string ip = "localhost:1883";

// Then, define an ID to be used by the client when communicating with the broker.

std::string id = "publisher";

// Construct a client using the Ip and Id, specifying usage of MQTT V5.

mqtt::client client(ip, id, mqtt::create_options(MQTTVERSION_5));

// Use the connect method of the client to establish a connection to the broker.

client.connect();

// Initialize an empty message with specified topic.

mqtt::message_ptr timeLeftMessagePointer = mqtt::make_message("consumer/in", "");

// Create a loop for counting down from N seconds.

for (int i = 10; i > 0; --i)

{

// Sleep for one second.

std::this_thread::sleep_for(std::chrono::milliseconds(1000));

// Configure Mqtt message to contain payload specifying time until end.

timeLeftMessagePointer->set_payload("Time Left: " + std::to_string(i));

// Publish the Mqtt message using the connected client.

client.publish(timeLeftMessagePointer);

}

// After counting down, configure Mqtt message for sending the quit signal.

timeLeftMessagePointer->set_payload("quit");

// Send quit signal to listeners.

client.publish(timeLeftMessagePointer);

return 0;

}

Compiling The Data Producer

To compile the C++ source code into an executable file, use g++:

g++ SimpleMqttProducer.cpp -lpaho-mqttpp3 -lpaho-mqtt3as -o simpleProducer

Communicating Between The Clients

With each of the client programs compiled in to executables, they can now be used to communicate between one another.

Start The Consumer

I n one terminal, execute the simpleConsumer program to begin listening.

./simpleConsumer

Start The Producer

With the Consumer started, open another terminal and execute the simpleProducer program to begin publishing.

./simpleProducer

The simpleProducer will begin publishing messages to a topic on the MQTT broker. These messages are picked up by the simpleConsumer and printed to the console.

Time Left: 10
Time Left: 9
Time Left: 8
Time Left: 7
Time Left: 6
Time Left: 5
Time Left: 4
Time Left: 3
Time Left: 2
Time Left: 1
quit

The final message, quit, triggers the simpleConsumer to stop looping and return control to the console.