Can C Class Member Functions Be Used as C Callbacks?

Using a C Class Member Function as a C Callback Function

Problem:

When attempting to register a C class member function as a callback function for a C library, the compiler throws an error indicating that the function's type does not match the expected signature.

Questions:

1. Is it possible to register a C class member function as a callback function?
2. Are there alternative approaches to solve this issue?

Answer:

1. Using a Static Member Function:

Yes, it is possible to register a C class member function as a callback function, but it must be a static member function. Static member functions do not have an implicit first parameter of type class A*, so their signature matches the expected callback signature. For example:

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

int main() {
    register_with_library(A::e);
    return 0;
}

2. Alternative Approaches:

Alternatively, you can use one of the following approaches:

• Create a Free Function: Create a free function with the appropriate signature and call the C class member function from within it. For example:

void e_wrapper(int *k, int *j) {
    A().e(k, j);
}

• Use a Function Pointer: Define a function pointer and assign the address of the C class member function to it. For example:

int (*ptr)(int *, int *) = &A::e;
register_with_library(ptr);

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