How Can I Register C Class Member Functions as C Callbacks?

Registering C Class Member Functions as C Callback Functions

When working with C libraries that require callback functions, we may encounter the need to register C class member functions. However, attempting to register a non-static member function as a callback leads to a compiler error.

Can We Register C Class Member Functions?

Yes, it is possible to register C class member functions as callback functions using static member functions.

Static Member Function Approach

Static member functions do not have an implicit first parameter of the class type. Thus, their signature matches the expected type of the callback function, allowing them to be registered successfully.

Example

class A {
public:
    static int e(int *k, int *j) {
        return 0;
    }
};

A::A() {
    register_with_library(A::e);
}

Alternative Approaches

• Function Pointers: Create a function pointer that points to the C member function and register it with the library.

int e(int *k, int *j) {
    A a;
    return a.e(k, j);
}

A::A() {
    register_with_library(e);
}

• Functors: Create a functor class that overrides the operator() to call the C member function. Register an instance of the functor as the callback.

struct EF {
    A* a;
    int operator()(int *k, int *j) {
        return a->e(k, j);
    }
};

A::A() {
    EF ef = {this};
    register_with_library(ef);
}

The choice of approach depends on the specific requirements of the C library and the desired level of encapsulation and reusability.

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