# Callback to a non-static C++ Member Function

**URL:** <https://community.particle.io/t/callback-to-a-non-static-c-member-function/29443>\
**Category:** Troubleshooting\
**Created:** [February 21, 2017, 12:23pm UTC](https://community.particle.io/t/callback-to-a-non-static-c-member-function/29443 "2017-02-21T12:23:01Z")\
**Posts on this page:** 10\
**Page:** 1

<div class="post-metadata">

**Author:** ![gusgonnet](https://sea2.discourse-cdn.com/flex026/user_avatar/community.particle.io/gusgonnet/32/27491_2.png) [@gusgonnet](https://community.particle.io/u/gusgonnet)\
**Post date:** [February 21, 2017, 12:23pm UTC](https://community.particle.io/t/callback-to-a-non-static-c-member-function/29443/1 "2017-02-21T12:23:01Z")

</div>

Hi guys,

I need help. I want to wrap [this mqtt library](https://github.com/hirotakaster/MQTT) in a class, and I’m getting this compilation issue:

```auto
running command$ particle compile photon /home/user/particle/mqttInClass --saveTo /home/user/particle/mqttInClass/firmware.bin
Compiling code for photon
Including:
    /home/user/particle/mqttInClass/MQTT.h
    /home/user/particle/mqttInClass/mqttClient.h
    /home/user/particle/mqttInClass/MQTT.cpp
    /home/user/particle/mqttInClass/mqttClient.cpp
attempting to compile firmware 
Compile failed. Exiting.
mqttClient.cpp: In constructor 'c_mqttClient::c_mqttClient()':
mqttClient.cpp:9:56: error: no match for call to '(MQTT) (const char [15], uint16_t, <unresolved overloaded function type>)'
  mqttCli("www.sample.com", uint16_t(1883), mqttCallback);
                                                        ^
make[1]: *** [../build/target/user/platform-6mqttClient.o] Error 1
make: *** [user] Error 2

```

This is the code that gets that issue:

**file mqttClient.cpp:**

```auto
#include "application.h"
#include "mqttClient.h"

void c_mqttClient::mqttCallback(char* topic, uint8_t* payload, unsigned int length) {
  Particle.publish("mqtt message received", "", 60, PRIVATE);
}

c_mqttClient::c_mqttClient() {
 mqttCli("www.sample.com", uint16_t(1883), mqttCallback);
}

```

**file mqttClient.h:**

```auto
#ifndef MQTTCLIENT_H
#define MQTTCLIENT_H

#include "application.h"
#include "MQTT.h"

class c_mqttClient {

    MQTT mqttCli;

 public:
    c_mqttClient();

    void mqttCallback(char* topic, byte* payload, unsigned int length);

};

#endif

```

* * *

I searched google, this community, c forums and I believe the issue is due to the following fact:  
**Pointers to non-static members are different to ordinary C function pointers since they need the this-pointer of a class object to be passed. Thus ordinary function pointers and non-static member functions have different and incompatible signatures!**  
[Source is here](http://www.newty.de/fpt/callback.html#static).

So I went ahead and tried the wrapper approach described [here](http://stackoverflow.com/questions/9027456/no-matching-function-error-when-using-attachinterrupt) (second answer) and [here](http://www.newty.de/fpt/callback.html#static). I ended up with this issue:

```auto

running command$ particle compile photon /home/user/particle/mqttInClass --saveTo /home/user/particle/mqttInClass/firmware.bin
Compiling code for photon
Including:
    /home/user/particle/mqttInClass/MQTT.h
    /home/user/particle/mqttInClass/mqttClient.h
    /home/user/particle/mqttInClass/MQTT.cpp
    /home/user/particle/mqttInClass/mqttClient.cpp
attempting to compile firmware 
Compile failed. Exiting.
mqttClient.cpp: In constructor 'c_mqttClient::c_mqttClient()':
mqttClient.cpp:19:63: error: no match for call to '(MQTT) (const char [15], uint16_t, void (&)(char*, uint8_t*, unsigned int))'
 mqttCli("www.sample.com", uint16_t(1883), mqttCallback_wrapper);
                                                               ^
make[1]: *** [../build/target/user/platform-6mqttClient.o] Error 1
make: *** [user] Error 2

```

**File mqttClient.cpp:**

```auto
#include "application.h"
#include "mqttClient.h"

void* pt2Object;

void c_mqttClient::mqttCallback_wrapper (char* topic, uint8_t* payload, unsigned int length){
   // explicitly cast to a pointer to c_mqttClient
   c_mqttClient* mySelf = (c_mqttClient*) pt2Object;

   // call member
   mySelf->mqttCallback(topic, payload, length);
}

void c_mqttClient::mqttCallback(char* topic, uint8_t* payload, unsigned int length) {
  Particle.publish("mqtt message received", "", 60, PRIVATE);
}

c_mqttClient::c_mqttClient() {
mqttCli("www.sample.com", uint16_t(1883), mqttCallback_wrapper);
}

```

**File mqttClient.h:**

```auto
#ifndef MQTTCLIENT_H
#define MQTTCLIENT_H

#include "application.h"
#include "MQTT.h"

class c_mqttClient {
    MQTT mqttCli;
 public:
    c_mqttClient();
    void mqttCallback(char* topic, uint8_t* payload, unsigned int length);
    static void mqttCallback_wrapper(char* topic, uint8_t* payload, unsigned int length);
};

#endif

```

I’ve been trying variations of this code for a couple of weeks now without success.  
Can someone spot what I’m doing wrong here by any chance?  
Thank you!  
Gustavo.  
PS: I do not expect “[www.sample.com](http://www.sample.com)” to work, I just wanted to simplify my code in this post (so I don’t need to add a byte array to define the mqtt broker ip)

---

<div class="post-metadata">

**Author:** ![ScruffR](https://sea2.discourse-cdn.com/flex026/user_avatar/community.particle.io/scruffr/32/6952_2.png) [@ScruffR](https://community.particle.io/u/ScruffR)\
**Post date:** [February 21, 2017, 1:07pm UTC](https://community.particle.io/t/callback-to-a-non-static-c-member-function/29443/2 "2017-02-21T13:07:21Z")

</div>

That is a common thing when trying to use a non-static class member.  
Since non-static members need to be called in their own context (object instance) you’d need to pass an _object pointer_ along with the non-static member reference to allow the resolution.

To get around this withouth having to create a new overload for `mqttCli()` that also takes an object pointer, you could declare the callback static (providing it does **_only_** use static fields and methods too).  
But judging by the looks of it, you may have to implement several overloads that will take a `this` pointer.

---

<div class="post-metadata">

**Author:** ![BulldogLowell](https://sea2.discourse-cdn.com/flex026/user_avatar/community.particle.io/bulldoglowell/32/4305_2.png) [@BulldogLowell](https://community.particle.io/u/BulldogLowell)\
**Post date:** [February 21, 2017, 1:19pm UTC](https://community.particle.io/t/callback-to-a-non-static-c-member-function/29443/3 "2017-02-21T13:19:03Z")

</div>

> [@gusgonnet](#):
>
> ```auto
> mqttClient.cpp:9:56: error: no match for call to '(MQTT) (const char [15], uint16_t, &lt;unresolved overloaded function type&gt;)'
> 
> ```

Gus, are you trying to call this MQTT constructor:

```cpp
MQTT(char* domain, uint16_t port, void (*callback)(char*,uint8_t*,unsigned int));

```

here:

```cpp
c_mqttClient::c_mqttClient() {
mqttCli("www.sample.com", uint16_t(1883), mqttCallback_wrapper);

```

---

<div class="post-metadata">

**Author:** ![gusgonnet](https://sea2.discourse-cdn.com/flex026/user_avatar/community.particle.io/gusgonnet/32/27491_2.png) [@gusgonnet](https://community.particle.io/u/gusgonnet)\
**Post date:** [February 21, 2017, 1:53pm UTC](https://community.particle.io/t/callback-to-a-non-static-c-member-function/29443/5 "2017-02-21T13:53:53Z")

</div>

> [@BulldogLowell](#):
>
> Gus, are you trying to call this MQTT constructor:

yes, I am trying to call that one

---

<div class="post-metadata">

**Author:** ![gusgonnet](https://sea2.discourse-cdn.com/flex026/user_avatar/community.particle.io/gusgonnet/32/27491_2.png) [@gusgonnet](https://community.particle.io/u/gusgonnet)\
**Post date:** [February 21, 2017, 1:58pm UTC](https://community.particle.io/t/callback-to-a-non-static-c-member-function/29443/6 "2017-02-21T13:58:23Z")

</div>

Hey, thanks for taking a look

> [@ScruffR](#):
>
> providing it does only use static fields and methods too

I think I need to use non-static vars...

> [@ScruffR](#):
>
> create a new overload for mqttCli() that also takes an object pointer

maybe that is something that I can try, thanks for the tip.

> [@ScruffR](#):
>
> But judging by the looks of it, you may have to implement several overloads that will take a this pointer.

may you have any material to read, any examples or whatever you think can help me move forward?

thanks again!

---

<div class="post-metadata">

**Author:** ![BulldogLowell](https://sea2.discourse-cdn.com/flex026/user_avatar/community.particle.io/bulldoglowell/32/4305_2.png) [@BulldogLowell](https://community.particle.io/u/BulldogLowell)\
**Post date:** [February 21, 2017, 2:24pm UTC](https://community.particle.io/t/callback-to-a-non-static-c-member-function/29443/7 "2017-02-21T14:24:21Z")

</div>

Since there is a 1:1 dependency on the MQTT class, have you thought about a derived class?

C++ (11) makes instantiating the base class easy using [Delegation](https://en.wikipedia.org/wiki/C++11#Object_construction_improvement). If you plan on using only the one base class constructor, it will make it a lot easier, I would think.

here is an awkward example with two base classes:

```cpp
class Pig {
  public:
    Pig(){Serial.println("Oink");};
    Pig(int numOinks){for(int i = 0; i < numOinks; i++)Serial.println("Oink");};
    void oink(void){Serial.println("Oink");};
    void poo(){crap();};
  private:
    void crap(){Serial.println("Pig Poo");};
};

class Cow {
  public:
    Cow(){Serial.println("Moo");};
    Cow(int numMoos){for(int i = 0; i < numMoos; i++)Serial.println("Moo");};
    void moo(void){Serial.println("Moo");};
    void poo(){crap();};
  private:
    void crap(){Serial.println("Cow Poo");};
};

class Zoo : public Pig, public Cow { // Take note of the order here and how it affects the construction of the two base classes
  public:
    Zoo(){};
    Zoo(int numOinks, int numMoos) : Pig(numOinks), Cow(numMoos){}; // constructor may pass args to the base classes
    using Cow::poo;
};

void setup() 
{
  Serial.begin(9600);
  Zoo zoo(3, 1);
  Serial.print(F("Making my zoo Oink: "));
  zoo.oink();
  Serial.print(F("Making my zoo Moo: "));
  zoo.moo();
  Serial.print(F("and if my zoo poos, I get "));
  zoo.poo();
}

void loop() {
  // put your main code here, to run repeatedly:

}

```

---

<div class="post-metadata">

**Author:** ![gusgonnet](https://sea2.discourse-cdn.com/flex026/user_avatar/community.particle.io/gusgonnet/32/27491_2.png) [@gusgonnet](https://community.particle.io/u/gusgonnet)\
**Post date:** [February 21, 2017, 2:37pm UTC](https://community.particle.io/t/callback-to-a-non-static-c-member-function/29443/8 "2017-02-21T14:37:45Z")

</div>

looks promising, thanks for the example!  
will take a look later on.

thanks again James

---

<div class="post-metadata">

**Author:** ![gusgonnet](https://sea2.discourse-cdn.com/flex026/user_avatar/community.particle.io/gusgonnet/32/27491_2.png) [@gusgonnet](https://community.particle.io/u/gusgonnet)\
**Post date:** [February 22, 2017, 11:46am UTC](https://community.particle.io/t/callback-to-a-non-static-c-member-function/29443/9 "2017-02-22T11:46:04Z")

</div>

> [@BulldogLowell](#):
>
> Since there is a 1:1 dependency on the MQTT class, have you thought about a derived class?

Hi, I was looking at derived classes, but can't understand how they can help me fix my issue of assigning a callback to a non static member function.  
would you mind to elaborate a bit why delegation would help me here?  
thanks!

---

<div class="post-metadata">

**Author:** ![BulldogLowell](https://sea2.discourse-cdn.com/flex026/user_avatar/community.particle.io/bulldoglowell/32/4305_2.png) [@BulldogLowell](https://community.particle.io/u/BulldogLowell)\
**Post date:** [February 22, 2017, 11:48am UTC](https://community.particle.io/t/callback-to-a-non-static-c-member-function/29443/10 "2017-02-22T11:48:30Z")

</div>

Have you already taken a look at [this tutorial](http://allenchou.net/2012/04/easy-c-delegates/)? This is what I had in mind.

---

<div class="post-metadata">

**Author:** ![gusgonnet](https://sea2.discourse-cdn.com/flex026/user_avatar/community.particle.io/gusgonnet/32/27491_2.png) [@gusgonnet](https://community.particle.io/u/gusgonnet)\
**Post date:** [February 22, 2017, 12:53pm UTC](https://community.particle.io/t/callback-to-a-non-static-c-member-function/29443/11 "2017-02-22T12:53:33Z")

</div>

oh, there I see it… “the callback class”. Perfect! I’ll give it a shot, thanks a bunch!  
Gustavo.