How Can I Use a C Class Member Function as a C Callback?

Using a C Class Member Function as a C Callback Function

When interacting with C libraries from C code, it becomes necessary to establish callbacks. This article explores the challenges encountered when attempting to register a C class member function as a callback for a C function with a specific signature, int a(int , int ).

The primary obstacle encountered is that non-static member functions possess an implicit first parameter of type class A*, representing the this pointer. This mismatch in parameter signature between the expected callback function and the provided C class member function leads to a compilation error.

Solution:

To overcome this issue, the member function must be declared static. This eliminates the implicit first parameter, allowing the C class member function to be registered as the callback.

Alternate Approach:

If the callback registration mechanism permits, an alternative approach is to create a wrapper function. This wrapper function can be a regular C function that forwards the arguments to the desired C class member function. By registering the wrapper function as the callback, the class member function can be indirectly invoked through the proxy.

Example:

Consider the example code below:

class A {
public:
   A();
   ~A();
   int e(int *k, int *j);

private:
   static int e_c(int *k, int *j) {
      A* this_ptr = reinterpret_cast<A*>(callback_user_data);
      return this_ptr->e(k, j);
   }
};

A::A()
{
   register_with_library(e_c)
}

In the above code, the e_c() function is a static wrapper function that casts the callback_user_data argument to an A* pointer, allowing access to the this pointer and subsequent invocation of the e() member function. This approach enables the C class member function to be indirectly registered as a callback, even though it doesn't match the exact signature of the callback function.

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