Callbacks in C++ for an Arduino project - c++

Question

I am working on a Particle project and coming from JS so I'm challenged by callbacks in C++. I am trying to refactor my Firebase code into a reusable class and for this I need callbacks:
void setup() {
firebase = new Firebase;
Serial.begin(9600);
firebase->subscribe();
firebase->setCallback(readCallback);
}
void readCallback(JsonObject& root)
{
r = root["r"];
g = root["g"];
b = root["b"];
Serial.printlnf("Yes! r=%d g=%d b=%d d=%d", r, g, b);
}
Firebase.h:
#ifndef Firebase_h
#define Firebase_h
#include <SparkJson.h>
class Firebase {
public:
Firebase();
void
subscribe(),
setCallback(void (*readCallback)(JsonObject& root));
private:
static void getDataHandler(const char *topic, const char *data);
void (*_readCallback)(JsonObject& root);
};
#endif
Firebase.m:
#include "Particle.h"
// This #include statement was automatically added by the Particle IDE.
#include <SparkJson.h>
#include "ArduinoJson.h"
#include "Firebase.h"
Firebase::Firebase(void) {
Serial.printlnf("Firebase instance created");
}
void Firebase::getDataHandler(const char *topic, const char *data) {
Serial.printlnf("getDataHandler invoked");
StaticJsonBuffer<256> jsonBuffer;
char *mutableCopy = strdup(data);
JsonObject& root = jsonBuffer.parseObject(mutableCopy);
free(mutableCopy);
Serial.printlnf("data received: %s", data);
// _readCallback(root);
}
void Firebase::subscribe() {
Serial.printlnf("Firebase subscribe");
Particle.subscribe("hook-response/test3rdata", getDataHandler, MY_DEVICES);
}
void Firebase::setCallback(void (*readCallback)(JsonObject& root))
{
Serial.printlnf("set callback");
_readCallback = readCallback;
}
When getDataHandler is static everything seems to work but naturally I am having trouble accessing the callback and I get:
invalid use of member 'Firebase::_readCallback' in static member
function
When it's not static I get for this line:
Particle.subscribe("hook-response/test3rdata", getDataHandler, MY_DEVICES);
the following error:
invalid use of non-static member function
When I try to bind it as advised here:
Particle.subscribe("hook-response/test3rdata", std::bind(&Firebase::getDataHandler,this), MY_DEVICES);
I get a mistype as Particle.subscribe does not expect a binded method:
no matching function for call to 'CloudClass::subscribe(const char
[25], std::_Bind_helper::type, Spark_Subscription_Scope_TypeDef)'
Is there a way around it?

Answer 1

You are getting this error because std::bind returns a function object that adheres to the signature void() and not void(char const*, char const*). The reason being, that you didn't specify any placeholders for those arguments. So a quick fix would be:
std::bind(&Firebase::getDataHandler, this, std::placeholders::_1, std::placeholders::_2)
Now the bind helper expects two parameters which it will forward to the bound member function.
Having said all that, there is no reason to use std::bind if a lambda will suffice. Lambdas are in fact superior in most regards. In C++14 there's virtually no reason to use std::bind at all. And even in C++11, your use case can be dealt with by a simple lambda:
Particle.subscribe("hook-response/test3rdata", [this](char const* a, char const* b) { getDataHandler(a, b); }, MY_DEVICES);