How Can I Overcome the Challenge of Casting void* to a Member Function Pointer in C ?

Casting Challenges: void* to Member Function Pointer

In the pursuit of an easy-to-use C object binding library for Lua, the task of casting between void* and pointer to member function emerges as a formidable obstacle. While leveraging GCC 4.4, developers encounter the following dilemma:

<code class="cpp">void (T::*method)(int, int) = reinterpret_cast<void (T::*)(int, int)>(lua_touserdata(L, lua_upvalueindex(1)));</code>

Herein lies the crux of the issue. GCC unequivocally disapproves of attempts to directly cast void* to a pointer to member function, as evident from its vehement complaints.

Unlocking the Solution: Unveiling Member Function Wrappers

The key to circumventing this casting impasse rests in the realization that pointers-to-members cannot be seamlessly converted to void* or any conventional pointer types. Unlike typical pointers, which directly reference memory locations, pointers-to-members encapsulate more intricate details, necessitating an alternative approach.

The solution, as proposed by experienced C programming aficionados, involves embracing the concept of member function wrappers. By wrapping the member function within a standard function that takes the object as its inaugural argument, we unlock the ability to wield reinterpret_cast to transform void* into the desired function pointer.

An Illustrative Code Refactoring

To illustrate the power of this technique, consider the following revised version of the aforementioned function:

<code class="cpp">template <class T>
int call_int_function(lua_State *L) 
{
    void (*method)(T*, int, int) = reinterpret_cast<void (*)(T*, int, int)>(lua_touserdata(L, lua_upvalueindex(1)));
    T *obj = reinterpret_cast<T *>(lua_touserdata(L, 1));
   method(obj, lua_tointeger(L, 2), lua_tointeger(L, 3));
   return 0;
}</code>

By adopting this modified approach, we effectively sidestep the issue of casting between void* and pointer to member function, enabling us to reap the benefits of seamless Lua object binding without encountering any casting pitfalls.

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