invalid use of non-static member function

[dbabo]

Hello,
i can't figure out the error. Appreciate your advise:
error:
src/mqqt.cpp: In member function 'int AWS_MQQT::aws_mqqt_setup(...)':
src/mqqt.cpp:37:51: error: invalid use of non-static member function
while (subscribe(topic_name, mySubCallBackHandler) != 0) {
^

I removed parameters from functions to simplify the code:

< ======== AWS_IOT ======== >
typedef void (*pSubCallBackHandler_t)(char *topicName, int payloadLen, char *payLoad);

class AWS_IOT{

private:

public:
int connect(...);
int publish(...);
int subscribe(const char *subTopic, pSubCallBackHandler_t pSubCallBackHandler);
};

< ======= AWS_MQQT ==========>
class AWS_MQQT : public AWS_IOT {

private:
void mySubCallBackHandler (char *topicName, int payloadLen, char *payLoad);
public:
int aws_mqqt_setup(...);
};

int AWS_MQQT::aws_mqqt_setup(...) {
...
while ( connect(...) != 0 ) { ... }
...
while (subscribe(topic_name, mySubCallBackHandler) != 0) { ... }
}

<======== main.cpp ====== >

main.cpp:
...
AWS_MQQT* mqqt = new AWS_MQQT;
(*mqqt).aws_mqqt_setup(...);
...


thank you.

[TomCPP]

How do you set mySubCallBackHandler pointer?
If you assign some member/class function than remember to set this in the following way:
mySubCallBackHandler = &AWS_MQQT::some_function;

[ne555]

a member function operates on an object, which is passed as the this pointer
so you may see the member function void Bar::foo(int n); as void foo(Bar *this, int n);

AWS_MQQT::mySubCallBackHandler() does not respect the signature defined by `pSubCallBackHandler_t' because it has an extra AWS_MQQT parameter (the caller object) and then it can't be used for the `subscribe()'


> I removed parameters from functions to simplify the code
yes, but that code is full of ellipsis that causes other compile errors
your testcase should reproduce the error you are having

typedef void (*pSubCallBackHandler_t)(char *topicName, int payloadLen,
                                      char *payLoad);
class AWS_IOT {
private:
public:
  int subscribe(const char *subTopic,
                pSubCallBackHandler_t pSubCallBackHandler);
};

class AWS_MQQT : public AWS_IOT {
private:
  void mySubCallBackHandler(char *topicName, int payloadLen, char *payLoad);

public:
  int aws_mqqt_setup();
};

int AWS_MQQT::aws_mqqt_setup() {
  while (subscribe("", mySubCallBackHandler) != 0) {
  }
}

int main() {
  AWS_MQQT mqqt;
  mqqt.aws_mqqt_setup();
}

[dbabo]

@TomCPP:
Thank you for your reply. to your questions:
> How do you set mySubCallBackHandler pointer?
it is defined as :

void AWS_MQQT::mySubCallBackHandler (char *topicName, int payloadLen, char *payLoad) {
//	strncpy(rcvdPayload,payLoad,payloadLen);
//	rcvdPayload[payloadLen] = 0;
//	msgReceived = 1;
true;

};

>If you assign some member/class function then remember to set this in the following way:
mySubCallBackHandler = &AWS_MQQT::some_function;

it doesn't, as you see above. But i (think) i understand your point - i missed the pointer type of the function. If i change the call to :

while (subscribe(topic_name, &AWS_MQQT::mySubCallBackHandler) != 0) { 
...
}

it comes back with :

src/mqqt.cpp: In member function 'int AWS_MQQT::aws_mqqt_setup(const char*, const char*, const char*, const char*, const char*, const char*)':
src/mqqt.cpp:38:62: error: no matching function for call to 'AWS_MQQT::subscribe(const char*&, void (AWS_MQQT::*)(char*, int, char*))'
  while (subscribe(topic_name, &AWS_MQQT::mySubCallBackHandler) != 0) {
 
~  

[dbabo]

@ne555:
appreciate your reply.
>. a member function operates on an object, which is passed as this pointer
does it mean that i can't pass the child's member function to the parent's function?.. unless it is "static"?
And if can't, then what is the proper approach ?



>.. yes, but that code is full of ellipsis that causes other compile errors .your test case should reproduce the error you are having

I didn't think of that. Good point. I'll improve on this. thank you!

[ne555]

the prototypes don't match, you can't pass it.

#include <functional>
//function object
typedef std::function<void (char *topicName, int payloadLen, char *payLoad)> pSubCallBackHandler_t;

int AWS_MQQT::aws_mqqt_setup() {
	using namespace std::placeholders; //for _1, _2, _3
	auto callback = std::bind(&AWS_MQQT::mySubCallBackHandler, this, _1, _2, _3);
	//now callback is like a global function that would call this->mySubCallBackHandler(...)
	//so that's what we subscribe
	subscribe("", callback);
	return 0;
}

it would be similar to having an extra void *data parameter on the callback where you would pass the object.