Convert member function pointer to ordinary function pointer

Question

I am using the variadic call pattern shown here. It works fine for ordinary C functions. But I want to use it with a member function and a tuple that explicitly has an instance pointer as the first argument.

I tried something like this:

template<typename Ret, typename C, typename...Args>
int methodWrapper(const string& name, Ret (C::*func)(Args...), lua_State* L)
{
    return wrap<Ret,C*,Args...>::wrapFunc(name, func, L);
}

But it fails to convert the pointer type:

cannot initialize a parameter of type 'void (*)(Object *, float, float)' with an lvalue of type 'void (Object::*)(float, float)'

In this case, the wrap function is templated, so the type of the pointer has to match exactly.

template<typename Ret, typename...Args>
struct wrap{
    static int wrapFunc(const string& name, Ret (*func)(Args...), lua_State* L)
    {
        ...
    }
};

How to I explicitly convert a member function pointer into an ordinary function pointer that takes the this pointer as its first argument?

EDIT: for those that have linked me to this question, it is not the same. I do not have to pass a plain C function pointer to an existing API with a specific interface. And I am not trying to bind a member function to a specific object. This question also does not deal with variadic call or tuples.

Possible duplicate of Convert C++ function pointer to c function pointer — Paolo M
– Paolo M, Commented Dec 2, 2015 at 16:27
You should know that lua_CFunction has type extern "C" int (*) (lua_State *L). That you can pass int (*)(lua_State *L) instead is a bug in nearly every compiler and isn't standard conformant. — Simple
– Simple, Commented Dec 2, 2015 at 16:31
What about creating a lambda and passing that in? What does wrapFunc do with the function it receives? — Claudiu
– Claudiu, Commented Dec 2, 2015 at 16:40

stands2reason · Accepted Answer · 2015-12-02 18:40:33Z

0

std::mem_fn does this.

In order to make the template match: I had to explicitly declare the args tuple as a Class pointer followed by the parameter pack:

template<typename C, typename...Args>
struct wrapMethod<void,C,Args...>{
    static int wrap(const string& name, void (C::*func)(Args...), lua_State* L)
    {
        tuple<C*, Args...> args = cargs<C*,Args...>::getCArgs(L, name);

        //Return type (first template parameter) cannot be inferred.
        variadic_call<void>(mem_fn(func), args);

        return 0;
    }
};

answered Dec 2, 2015 at 18:40

stands2reason

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1 Comment

Deduplicator Over a year ago

Actually, it doesn't. It just converts a member-function-pointer into a callable.

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