Non static member function as callback

[wezside]

This is a question and I hope this is the right place for this. I had a need for a C++ non static member function as callback. Being relatively new to C++ Google lead me to using a "trampoline" approach to get around this, i.e. declare a global callback function and then pass through a pointer to the instance of the class using void*. Then static_cast this pointer back to the correct class type. Much like this.

This however required me to add this second parameter to the dispatch signatures in the easywsclient.hpp class. Is there another way to do this without changing the code?

I'm using C++98 with GNU compiler.

[dhbaird]

Hi wezside! It is possible to bind a C++ method to a class instance and use it as a callback by using a "functor". No need to rewrite the dispatch signature, just need to bind the method to a class instance, for example,

class Functor {
    MyClass * instance;
  public:
    Functor(MyClass * instance) : instance(instance) {
    }
    void operator()(const std::string& message) {
        instance->memberFunction(message);
    }
};

int main() {
    // ...
    MyClass myClass;
    Functor functor(&myClass); // <-- a pre-C++11 way to bind instance+method
    webSocket->dispatch(functor); // <-- no need to change signature
    // ...
}

It is nicer if you can use a library that provides this binding capability for you though. Some libraries are:

http://www.boost.org/doc/libs/1_55_0/libs/bind/bind.html (if you do not have C++11)

I know you said you are using C++98, but if you had the luxury of C++11, then here are some more options:
http://en.cppreference.com/w/cpp/utility/functional/function (if you have C++11)
http://en.cppreference.com/w/cpp/utility/functional/bind (if you have C++11)

Hope this helps. Definitely feel free to ask if this raises another question :)

[wezside]

That's awesome. Didn't know about Functors and their use. Thanks!