How Can a Static C Class Member Function Be Used as a C Callback?

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

In C , it is possible to register a class member function as a C callback function, but only if the member function is declared as static.

Explanation:

Non-static member functions have an implicit first parameter of type class A* corresponding to the this pointer. However, C callback functions typically do not have this first parameter. To register a class member function as a callback, we must eliminate the implicit this pointer.

Solution:

Declare the member function as static. This means that the member function will not have access to the this pointer.

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

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

int
A::e(int *k, int *e)
{
  return 0;
}

Alternative Approaches:

In cases where it is not possible or desirable to use a static member function, alternative approaches include:

• Creating a plain C function: Define a C function that serves as a proxy for the class member function, and then register the C function as the callback.
• Using a function pointer: Create a function pointer that points to the class member function and then register the function pointer as the callback.
• Using a lambda expression: C 11 introduces lambda expressions, which provide a convenient way to define anonymous functions. You can use a lambda expression to implement the callback function.

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