Parameter packs unable to resolve overloaded function

Question

#include <iostream>
#include <algorithm>
#include <memory>
#include <mutex>
#include <vector>
template <typename Observer>
class ObsNoti {
public:
    template <
            typename Ret,
            typename Class,
            typename... Args,
            typename = std::enable_if_t<std::is_same<Observer, Class>::value>>
    void notifyEveryone(Ret Class::*func, Args&&... args) {
        (obs.*func)(std::forward<Args>(args)...);
    }
    Observer obs;
};

class ContactObserver {
    public:
    virtual int onContactRetrieved() {
        std::cout<<"onContactRetrieved\n";
        return 0;
    }
    
    virtual int onContactRetrieved(bool bSucess) {
        std::cout<<"onContactRetrieved sucessfully\n";
        return 0;
    }
};

class MyClass : public ObsNoti<ContactObserver> {
    public:
    template<typename Func, bool,  typename... Args>
    inline void notifyObserver(Func &&func, bool b, Args&&... args) {
        ObsNoti<ContactObserver>::notifyEveryone(func , b, std::forward<Args>(args)...);
    }
    
    template<typename Func, typename... Args>
    inline void notifyObserver(Func &&func, Args&&... args) {
        ObsNoti<ContactObserver>::notifyEveryone(func , std::forward<Args>(args)...);
    }
};



int main() {
    MyClass my;
    my.notifyObserver(&ContactObserver::onContactRetrieved, true);
    return 0;
}

I am getting the following error:

/tmp/w7Rta4F1cT.cpp:53:22: error: no matching function for call to 'MyClass::notifyObserver(<unresolved overloaded function type>, bool)'
   53 |     my.notifyObserver(&ContactObserver::onContactRetrieved, true);
      |     ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/tmp/w7Rta4F1cT.cpp:37:17: note: candidate: 'template<class Func, bool <anonymous>, class ... Args> void MyClass::notifyObserver(Func&&, bool, Args&& ...)'
   37 |     inline void notifyObserver(Func &&func, bool b, Args&&... args) {
      |                 ^~~~~~~~~~~~~~
/tmp/w7Rta4F1cT.cpp:37:17: note:   template argument deduction/substitution failed:
/tmp/w7Rta4F1cT.cpp:53:22: note:   couldn't deduce template parameter 'Func'
   53 |     my.notifyObserver(&ContactObserver::onContactRetrieved, true);
      |     ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/tmp/w7Rta4F1cT.cpp:42:17: note: candidate: 'template<class Func, class ... Args> void MyClass::notifyObserver(Func&&, Args&& ...)'
   42 |     inline void notifyObserver(Func &&func, Args&&... args) {
      |                 ^~~~~~~~~~~~~~
/tmp/w7Rta4F1cT.cpp:42:17: note:   template argument deduction/substitution failed:
/tmp/w7Rta4F1cT.cpp:53:22: note:   couldn't deduce template parameter 'Func'
   53 |     my.notifyObserver(&ContactObserver::onContactRetrieved, true);
      |     ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I understand the problem here is that template is unable to resolve the function at compile time because of overloading. But, I am not sure of a correct way to implement it.

Any way to resolve this?

Answer 1

Your code have miltiple problems:

1. ObsNoti::notifyEveryone is defined to accept pointer to member variable, not pointer to member function.
2. Overload of ContactObserver::onContactRetrieved makes value &ContactObserver::onContactRetrieved ambiguous and this can't be resolved during template argument deduction.

To fix 1:

    template <
        typename Ret,
        typename Class,
        typename... FArgs,
        typename... Args>
    std::enable_if_t<std::is_same_v<Observer, Class>>
    notifyEveryone(Ret (Class::*func)(FArgs...), Args&&... args)
    {
        (obs.*func)(std::forward<Args>(args)...);
    }

To fix 2 you need use static_cast to select proper overload:

    my.notifyObserver(static_cast<int (ContactObserver::*)(bool)>(&ContactObserver::onContactRetrieved), true);

https://godbolt.org/z/azYGM3cjz
or prevent deduction of first argument by explicitly providing type:

    my.notifyObserver<int(ContactObserver::*)(bool)>(&ContactObserver::onContactRetrieved, true);

https://godbolt.org/z/E3avEbhr5