Bind a class function to setCallback function

[letmp]

I would like to bind a class function to the setCallback function. Therefor I declared the callback function like that:

class MyClass
{

  private:
  void callbackFunction(MqttClient::CallBack cb);
  ...
}

and try to pass it like that:

client->setCallback(std::bind(&MyClass::callbackFunction, this, std::placeholders::_1));

I want to do that because I want to access dynamic variables of MyClass.

The problem: I cannot define my callbackFunction with MqttClient::Callback as function parameter because the type MqttClient::CallBack is inaccessible.

Is there any solution how I can access variables of MyClass within the function body of my callbackFunction?

[hsaturn]

Hello

You can use lambda for this:

class CallBackClass
{
   public:
      void onPublish(const MqttClient* srce, const Topic& topic, const char* payload, size_t len);
     ...
};

client->setCallback([&instance](const MqttClient* srce, const Topic& topic, const char* payload, size_t len)->void
{
   instance.onPublish(source, topic, payload, len);
});

Of course, you must take care that the instance should still exist when exiting the scope of setting a callback this way.

[letmp]

Thanks for your quick answer! I already tried that, but I always get this error:

no suitable conversion function from "lambda [](const MqttClient *source, const Topic &topic, const char *payload, size_t len)->void" to "MqttClient::CallBack" exists

Any ideas what could cause that? Thanks for your effort!

[hsaturn]

Strange, I tried this solution before sending it to you... I remember that I had tried it with tinytest. I'm going to write this again and send this to you. Are you sure that you have the latest release ?

[hsaturn]

What I've done:

1 - open TinyMqtt examples / tinymqtt-test 2 - at line 682 (just after client->setCallback(onPublish);

          client->setCallback([&x](const MqttClient *srce, const Topic &topic, const char* p, size_t len)->void
          {
            x.onPublish(srce, topic, p, len);
          });

3 - At the begining of the file

class X
{
  public:
    void onPublish(const MqttClient* srce, const Topic &topic, const char* p, size_t len) {}
};

X x;

4 - Compiles fine !

Not tested, but pretty sure this'll work. Version of IDE : 2 Version of TinyMqtt : 0.9.8

see https://pastebin.com/SytPfQMX

[hsaturn]

Sorry, 0.9.8 was only existing on my PC, and will never exist for some reasons. I've just released the 0.9.9.

Anyway the lambda should work with any release. Best regards

[hsaturn]

Hello Can I close this issue or is it still alive ?

[letmp]

I will test it tomorrow. Thanks

[alranel]

I had this problem as well. If I recall correctly, you can use a lambda here only if it does not capture. In fact, according to the C++ standard, only non-capturing lambdas can be converted to function pointers. Maybe turning MqttClient::CallBack to std::function could be the best solution, although it would introduce some overhead. In my case, I created my clients as global variables and created static callbacks.

[hsaturn]

Hello Alranel and Letmp

@Alranel, you're right. Memory on embedded devices is the main issue we are facing. This is why I've designed TinyMqtt with a function callback instead of any other design (inheritance, std::function ...). This is also the reason why both MqttClient and MqttBroker classes do not make use of virtual.

@letmp, As I said in my exemple with tinytest, the X instance is at the begining of the file. So it has to be global ! You cannot use a local instance of any class and put a callback on it for two reasons:

• TinyMqtt class cannot store a pointer (the local instance) in any way, even with bind because in this case, bind would need some extra bytes that cannot be stored. My example works because x is global.
• Having a callback on scoped instances is dangerous: What if the scope is destroyed if a Mqtt message arrives.

The destructor of MqttClient take care about point 2.

I may have a solution anyway (to be continued...)

[hsaturn]

@letmp -> (please read the previous answer)

• The callback function must be callable in all situation, or this would lead to crashes. So either the function has to be static of a class, or the instance has to be global as said previously.
• I will not add extra bytes to MqttClient for solving this issue.

In few words, I won't force other users of TinyMqtt to support the overload that would be needed to achieve binding to a class.

But there is a solution to move the overhead from TinyMqtt to your side: -> The idea is that we already have a pointer on the client.

So it is possible to use this pointer to route the message to another instance.

Here is the solution for your problem https://pastebin.com/SwaTGUyR

This solution is not fully satisfying because it can crash if the MqttClient instance is destroyed when MqttReceiver::onPublish makes use of it. Also only the last registered MqttReceiver on a given MqttClient will receive the message.

Maybe I'll add the binding class to the library (which requires to write unit tests and an example for this).

[hsaturn]

Hello again

I've released the version 0.9.11 that should take care of this problem. See the example 'advanced'.

[letmp]

Thank you very much for your effort!