Using a C++ Class Member Function as a C Callback Function

Using a C++ class member function as a C callback function

You can do that if the member function is static.

Non-static member functions of class A have an implicit first parameter of type class A* which corresponds to this pointer. That's why you could only register them if the signature of the callback also had the first parameter of class A* type.

How to use a C++ member function as the callback function for a C framework

Does callback function need to be declared under extern "C"?

NO. extern "C" is necessary only when you are calling a C++ function directly, without the use of function pointers, from C. If function pointers are used, extern "C" is not required.

Can I use non-static member functions as a callback?

NO. Non-static member functions of class A have an implicit first parameter corresponding to this pointer.

Can I use static member functions as a callback?

YES, as long as signature matches with that of the callback.

Does it matter if the function is a template function or not?

NO, template function can be used as callbacks as long as the signature of the instantiated template matches with the callback.

Map C++ member function to C callback

Use the user_data pointer to give you back a pointer to the class that owns the callback. For example.

void MyCallback(char port, uint8_t interrupt_mask, uint8_t value_mask, void *user_data)
{
    static_cast<Example *>(user_data)->Callback(port, interrupt_mask, value_mask);
}

class Example
{
public:

    Example()
    {
        //setup callback here - i dont know what library you're using but you should be able to pass a this pointer as user_data
        init_lib(fictionalparameter1, fictionalparameter2, this);
    }

private:

     void   Callback(char port, uint8_t interrupt_mask, uint8_t value_mask)
     {
         //callback handler here...
     }

     friend void MyCallback(char port, uint8_t interrupt_mask, uint8_t value_mask, void *user_data);
};

How can I pass a class member function as a callback?

That doesn't work because a member function pointer cannot be handled like a normal function pointer, because it expects a "this" object argument.

Instead you can pass a static member function as follows, which are like normal non-member functions in this regard:

m_cRedundencyManager->Init(&CLoggersInfra::Callback, this);

The function can be defined as follows

static void Callback(int other_arg, void * this_pointer) {
    CLoggersInfra * self = static_cast<CLoggersInfra*>(this_pointer);
    self->RedundencyManagerCallBack(other_arg);
}

Calling C++ class methods from C using callbacks

The classical C way of passing callbacks is to pass two values: a pointer to the callback itself, and an opaque pointer which will be passed to the callback as an additional argument (take a look at qsort_r for example). When interfacing with C++, that opaque value may be used as instance pointer; you will only need to write a thin wrapper:

class B {
    void my_callback(int arg);
    static void my_callback_wrapper(int arg, void *u) {
        ((B*)u)->my_callback(arg);
    }
};

// or even:
extern "C" void my_callback_wrapper(int arg, void *u) {
    ((B*)u)->my_callback(arg);
}

and pass a pointer to the wrapper, along with a pointer to the object, to the C part. Be careful to use the exact same class type on both sides and not base/derived classes, for example.

Note that while it may be possible to get a pointer to the (non-static) method itself, on some compilers (tested on MSVC a long time ago) they have a special calling convention so that the pointer will not be compatible with any normal function pointer.

call a C++ method from a C callback

A library will usually allow you to provide a void * value as a "context", which is then passed to the callback. If the library doesn't provide that functionality (huh?), you can use a thread-local variable (also known as a global variable in single-threaded programs) instead.

This is how the code will often look like.

class A {
public:
    static void static_callback(void * ctx) {
        static_cast<A*>(ctx)->callback();
    }
    void callback();
};

extern void library_function(void (*callback)(void *), void * ctx);

int main()
{
    A a;
    library_function(&A::static_callback, &a);
}

How to pass a pointer to a member function to a C function?

Kindly refer to detailed topic about

How to Implement a Callback to a static C++ Member Function ?

How to Implement a Callback to a non-static C++ Member Function ?

http://www.newty.de/fpt/callback.html

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