Convert C++ function pointer to c function pointer

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You can’t pass a non-static member function pointer as an ordinary function pointer. They’re not the same thing, and probably not even the same size.

You can however (usually) pass a pointer to a static member function through C. Usually when registering a callback in a C API, you also get to pass a “user data” pointer which gets passed back to your registered function. So you can do something like:

class MyClass
{

    void non_static_func(/* args */);

public:
    static void static_func(MyClass *ptr, /* other args */) {
        ptr->non_static_func(/* other args */);
    }
};

Then register your callback as

c_library_function(MyClass::static_func, this);

i.e. pass the instance pointer to the static method, and use that as a forwarding function.

Strictly speaking for total portability you need to use a free function declared extern "C" rather than a static member to do your forwarding (declared as a friend if necessary), but practically speaking I’ve never had any problems using this method to interface C++ code with GObject code, which is C callback-heavy.

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