'No Match' Errors with a member callback function using std::tr1::function

Question

I am trying to create a callback function using str::tr1::function pointing to a public member function.

std::tr1::function < int (const string& , const MessageInfo* , const void* , const int , const void* ) > dssCallBack;
dssCallBack = &ABC::mDBtoDScallback;

This callback will be passed to a function inside the body of another function of class ABC. The signature of ABC::mDBtoDScallback is

int DataserviceSubscriber::mDBtoDScallback(const string& strTopic, const MessageInfo* messageInfo, const void* data, const int dataLen, const void* callback_data)

When I try to compile this, I get the following error from g++.

In file included from /usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/tr1/functional:56,
                 from ../src/bmrk/databus/ABC.hpp:17,
                 from ../src/bmrk/databus/ABC.cpp:1:
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/tr1_impl/functional: In static member function ‘static _Res std::tr1::_Function_handler<_Res(_ArgTypes ...), _Member _Class::*>::_M_invoke(const std::tr1::_Any_data&, _ArgTypes ...) [with _Class = ABC, _Member = int(const std::string&, const MessageInfo*, const void*, int, const void*), _Res = int, _ArgTypes = const std::string&, const MessageInfo*, const void*, int, const void*]’:
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/tr1_impl/functional:2005:   instantiated from ‘std::tr1::function<_Res(_ArgTypes ...)>::function(_Functor, typename __gnu_cxx::__enable_if<(! std::tr1::is_integral::value), std::tr1::function<_Res(_ArgTypes ...)>::_Useless>::__type) [with _Functor = int (ABC::*)(const std::string&, const MessageInfo*, const void*, int, const void*), _Res = int, _ArgTypes = const std::string&, const MessageInfo*, const void*, int, const void*]’

/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/tr1_impl/functional:1885:   instantiated from ‘typename __gnu_cxx::__enable_if<(! std::tr1::is_integral::value), std::tr1::function<_Res(_ArgTypes ...)>&>::__type std::tr1::function<_Res(_ArgTypes ...)>::operator=(_Functor) [with _Functor = int (ABC::*)(const std::string&, const MessageInfo*, const void*, int, const void*), _Res = int, _ArgTypes = const std::string&, const MessageInfo*, const void*, int, const void*]’
../src/bmrk/databus/dataservice_subscriber.cpp:266:   instantiated from here

/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/tr1_impl/functional:1714: error: no match for call to ‘(std::tr1::_Mem_fn<int (ABC::*)(const std::string&, const MessageInfo*, const void*, int, const void*)>) (const std::basic_string<char, std::char_traits<char>, std::allocator<char> >&, const MessageInfo*&, const void*&, int&, const void*&)’

/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/tr1_impl/functional:546: note: candidates are: _Res std::tr1::_Mem_fn<_Res (_Class::*)(_ArgTypes ...)>::operator()(_Class&, _ArgTypes ...) const [with _Res = int, _Class = ABC, _ArgTypes = const std::string&, const MessageInfo*, const void*, int, const void*]

/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/tr1_impl/functional:551: note:                 _Res std::tr1::_Mem_fn<_Res (_Class::*)(_ArgTypes ...)>::operator()(_Class*, _ArgTypes ...) const [with _Res = int, _Class = ABC, _ArgTypes = const std::string&, const MessageInfo*, const void*, int, const void*]

I am trying to see what I am doing wrong here but cant spot it. I tried to look up but could not find others with similar issues. I could use a C-style typedef but I wanted to use and keep things C++ style, in the process also getting used to some of the newer stuff in C++11.

Thanks.

EDIT: As requested by Michael Burr, the call back is being called from a function as per the reference here http://en.cppreference.com/w/cpp/utility/functional/function

int ABC::subs_rt(const vector<string> &symbols, raw_callback_t raw_callback, void *app_data, Error *error)
{
    DBtoDS_callback_data cbData;
    cbData.subscriber_callback = raw_callback;
    cbData.raw_callback_app_data = app_data;
    cbData.err = error;

    // Perform processing on 'symbols'
    // dss is a member of class ABC and has been initialized in constructor
    dss->AddSubscriptionPrefix(symbols);
    b_cancel_subscription = false;

    std::tr1::function < int (const string& , const MessageInfo* , const void* , const int , const void* ) > dssCallBack;
    dssCallBack = &DataserviceSubscriber::mDBtoDScallback;

    dss->Subscribe(dssCallBack, static_cast<const void*>(&cbData));

    return 0;
}

The callback itself looks like

int ABC::mDBtoDScallback(const string& strTopic, const MessageInfo* messageInfo, const void* data, const int dataLen, const void* callback_data)
{
    const DBtoDS_callback_data* cbData = static_cast<const DBtoDS_callback_data*>(callback_data);

    if(0 == messageInfo) // Version 1
    {
        // Do callback Stuff
    }
    else // Version 2
    {
        Subscriber::timeval_t now;
        TimeUtils::now(now);
        std::string payload(static_cast<const char*>(data), dataLen);

        // Do callback Stuff
    }
}

The function int ABC::mDBtoDScallback is not static as WhozCraig guessed. Is that a problem? I can't make some of the variables used in this function static. Is there any way around this or do I have to use a C-style function pointer?

Thanks.

EDIT 2 : As per n.m. and WhozCraig's concern and per this link C++: Assigning a function to a tr1::function object

I changed the lines in ABC::subs_rt function to

std::tr1::function < int (const string& , const MessageInfo* , const void* , const int , const void* ) > dssCallBack;
//dssCallBack = std::tr1::bind(&ABC::mDBtoDScallback, this, std::tr1::placeholders::_1);
dssCallBack = std::tr1::bind(&ABC::mDBtoDScallback, this);

I tried the commented and the uncommented option but I get this error now

In file included from /usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/tr1/functional:56,
                 from ../src/bmrk/databus/ABC.hpp:17,
                 from ../src/bmrk/databus/ABC.cpp:1:
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/tr1_impl/functional: In member function ‘typename std::tr1::result_of<_Functor(typename std::tr1::result_of<std::tr1::_Mu<_Bound_args, std::tr1::is_bind_expression::value, (std::tr1::is_placeholder::value > 0)>(_Bound_args, std::tr1::tuple<_UElements ...>)>::type ...)>::type std::tr1::_Bind<_Functor(_Bound_args ...)>::__call(const std::tr1::tuple<_UElements ...>&, std::tr1::_Index_tuple<_Indexes ...>) [with _Args = const std::basic_string<char, std::char_traits<char>, std::allocator<char> >&, const MessageInfo*&, const void*&, int&, const void*&, int ..._Indexes = 0, _Functor = std::tr1::_Mem_fn<int (ABC::*)(const std::string&, const MessageInfo*, const void*, int, const void*)>, _Bound_args = ABC*]’:
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/tr1_impl/functional:1191:   instantiated from ‘typename std::tr1::result_of<_Functor(typename std::tr1::result_of<std::tr1::_Mu<_Bound_args, std::tr1::is_bind_expression::value, (std::tr1::is_placeholder::value > 0)>(_Bound_args, std::tr1::tuple<_UElements ...>)>::type ...)>::type std::tr1::_Bind<_Functor(_Bound_args ...)>::operator()(_Args& ...) [with _Args = const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, const MessageInfo*, const void*, int, const void*, _Functor = std::tr1::_Mem_fn<int (ABC::*)(const std::string&, const MessageInfo*, const void*, int, const void*)>, _Bound_args = ABC*]’
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/tr1_impl/functional:1654:   instantiated from ‘static _Res std::tr1::_Function_handler<_Res(_ArgTypes ...), _Functor>::_M_invoke(const std::tr1::_Any_data&, _ArgTypes ...) [with _Res = int, _Functor = std::tr1::_Bind<std::tr1::_Mem_fn<int (ABC::*)(const std::string&, const MessageInfo*, const void*, int, const void*)>(ABC*)>, _ArgTypes = const std::string&, const MessageInfo*, const void*, int, const void*]’
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/tr1_impl/functional:2005:   instantiated from ‘std::tr1::function<_Res(_ArgTypes ...)>::function(_Functor, typename __gnu_cxx::__enable_if<(! std::tr1::is_integral::value), std::tr1::function<_Res(_ArgTypes ...)>::_Useless>::__type) [with _Functor = std::tr1::_Bind<std::tr1::_Mem_fn<int (ABC::*)(const std::string&, const MessageInfo*, const void*, int, const void*)>(ABC*)>, _Res = int, _ArgTypes = const std::string&, const MessageInfo*, const void*, int, const void*]’
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/tr1_impl/functional:1885:   instantiated from ‘typename __gnu_cxx::__enable_if<(! std::tr1::is_integral::value), std::tr1::function<_Res(_ArgTypes ...)>&>::__type std::tr1::function<_Res(_ArgTypes ...)>::operator=(_Functor) [with _Functor = std::tr1::_Bind<std::tr1::_Mem_fn<int (ABC::*)(const std::string&, const MessageInfo*, const void*, int, const void*)>(ABC*)>, _Res = int, _ArgTypes = const std::string&, const MessageInfo*, const void*, int, const void*]’
../src/bmrk/databus/ABC.cpp:266:   instantiated from here
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/tr1_impl/functional:1137: error: no match for call to ‘(std::tr1::_Mem_fn<int (ABC::*)(const std::string&, const MessageInfo*, const void*, int, const void*)>) (ABC*&)’
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/tr1_impl/functional:546: note: candidates are: _Res std::tr1::_Mem_fn<_Res (_Class::*)(_ArgTypes ...)>::operator()(_Class&, _ArgTypes ...) const [with _Res = int, _Class = ABC, _ArgTypes = const std::string&, const MessageInfo*, const void*, int, const void*]
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/tr1_impl/functional:551: note:                 _Res std::tr1::_Mem_fn<_Res (_Class::*)(_ArgTypes ...)>::operator()(_Class*, _ArgTypes ...) const [with _Res = int, _Class = ABC, _ArgTypes = const std::string&, const MessageInfo*, const void*, int, const void*]
make[1]: *** [ABC.lo] Error 1

RESOLUTION OF ISSUE: Given my requirements decided to make my callback a static member function and pass a pointer to the parent class object through the const void* callback_data. Being a static function it can access private functions of the class ABC and pass parameters to raw_callback. The help I got from all the comments was a big learning experience for me and eventually led me down towards the solution.

static int ABC::mDBtoDScallback(const string& strTopic, const MessageInfo* messageInfo, const void* data, const int dataLen, const void* callback_data)
{
    const DBtoDS_callback_data* cbData = static_cast<const DBtoDS_callback_data*>(callback_data);

    TimeUtils::now(now);
    std::string payload(static_cast<const char*>(data), dataLen);

    string symbol;
    string sym;
    int pri_s = 0;

    if(0 == messageInfo)
    {
        parse_topic(strTopic, symbol, pri_s, cbData->err);
    }
    else
    {
        symbol = messageInfo->key();
        pri_s = ( messageInfo->has_pri_s() ? messageInfo->pri_s() : 0 );
    }
    if (cbData->subs->symbols_need_translation())
    {
        cbData->subs->translate_symbol(cbData->subs->_translator, symbol, sym, false);
    }
    else
    {
        sym = symbol;
    }

    cbData->subscriber_callback(cbData->subs, sym, pri_s, cbData->subs->prod, payload, now, cbData->raw_callback_app_data, cbData->err);
}

Thank you.

Answer 1

As discussed in comments, the issue appears to be because your function object is expecting a static non-object-bound function, while you're actually passing it a member. From what I can see, there are several work-arounds for this, the simplest one you probably already are considering; using a static and pass your object's pointer as a parameter.

Alternatively, I don't know if this will work for your particular architecture, but the following was quickly snapped together on how to invoke a member function with a specified object using an std::mem_fn<> in conjunction with a std::function<>. This is a stand-alone sample but I hope you can see how it may assist you.

#include <iostream>
#include <functional>
using namespace std;

class MyClass
{
public:
    MyClass() {}

    int CallMe(void *p, int a, float f)
    {
        // use params herere
        cout << "CallMe fired : " << this << " !\n" << endl;
        return 0;
    }
};

int main()
{
    MyClass obj, obj2;
    std::function<int(MyClass*,void*,int,float)> fn(std::mem_fn(&MyClass::CallMe));

    cout << "Invoking CallMe with " << &obj << " object..." << endl;
    fn(&obj, NULL, 1, 2.0);

    cout << "Invoking CallMe with " << &obj2 << " object..." << endl;
    fn(&obj2, NULL, 1, 2.0);

    return 0;
}

Output

Invoking CallMe with 0x7fff5fbff7d8 object...
CallMe fired : 0x7fff5fbff7d8 !

Invoking CallMe with 0x7fff5fbff7d0 object...
CallMe fired : 0x7fff5fbff7d0 !

Note

The read on how member function objects work, provided in C++11, on cppreference.com is fascinating to me. Not that anyone really cares about that.I have no idea if they properly dispatch to virtuals, etc, but I'm absolutely amazed by them.

I hope you find this helpful, but I'm fully prepared to delete it if one of the std-lib experts that truly understands the depths of std::function, std::bind, and std::men_fn pony up more concise explanations (or rips this snippet to shreds). Honestly, there are probably better samples on SO somewhere on invoking members through std::function<>, but the simplicity of using std::men_fn<> with std::function<> I have to say is rather captivating.

---

Virtual Dispatch

After seeking DeadMG's comment I was genuinely curious if virtual dispatching worked. I thought it stood a good chance, since we're providing a this pointer, but was not holding my breath, since clearly we pass the MyClass::CallMe member by-address to the constructor of our std:::mem_fn<>.

The updated source, and resulting run, were interesting. Notice the same function object used in both instances. Whether it is intended to work this way (and it appears to, btw) I cannot say, but it was interesting to me, none-the-less.

#include <iostream>
#include <functional>
using namespace std;

class MyClass
{
public:
    MyClass() {}

    virtual int CallMe(void *p, int a, float f)
    {
        // use params herere
        cout << "MyClass::CallMe fired : " << this << endl;
        return 0;
    }
};

class MyDerived : public MyClass
{
public:
    MyDerived() {}

    virtual int CallMe(void *p, int a, float f)
    {
        // use params herere
        cout << "MyDerived::CallMe fired : " << this << endl;
        return 0;
    }
};

int main()
{
    MyClass obj;
    MyDerived obj2;

    std::function<int(MyClass*,void*,int,float)> fn(std::mem_fn(&MyClass::CallMe));

    cout << "Invoking CallMe with " << &obj << " object..." << endl;
    fn(&obj, NULL, 1, 2.0);

    cout << "Invoking CallMe with " << &obj2 << " object..." << endl;
    fn(&obj2, NULL, 1, 2.0);

    return 0;
}

Output

Invoking CallMe with 000000000021FB58 object...
MyClass::CallMe fired : 000000000021FB58
Invoking CallMe with 000000000021FB78 object...
MyDerived::CallMe fired : 000000000021FB78

Answer 2

The source is simple, if you know where to look. Consider the following code:

std::tr1::function < int (const string& , const MessageInfo* , const void* , const int , const void* ) > dssCallBack;
dssCallBack = &ABC::mDBtoDScallback;
dssCallBack(std::string(), nullptr, nullptr, 0, nullptr);

What is this? You never provided it. So there's no way that std::function could possibly make this work- you're trying to invoke a member but did not provide an object. What is std::function supposed to do, magically decide what this should be?

The two solutions are, bind a this using std::bind, or pass this as an argument, as in std::mem_fn.