How to Cast Between a void* Pointer and a Pointer to a Member Function in C ?

Casting between void* and a Pointer to Member Function

When working with C/C interoperability, one can encounter difficulties when attempting to cast between a void pointer and a pointer to a member function. This issue often arises when dealing with closures in languages like Lua, where user data is stored as void pointers.

Problem:

Casting void* to a pointer to a member function in C is invalid due to the fundamental differences in their memory representations. Pointers-to-members contain additional information not accessible through regular pointers.

Solution:

To workaround this, consider the following options:

1. Wrap the Member Function in a Free Function:

• Create a free function that accepts the object instance as its first argument and the member function parameters as subsequent arguments.
• Return the address of this free function instead of the pointer-to-member.

Code Snippet:

<code class="cpp">int call_int_function(lua_State *L) {
    void (*method)(T*, int, int) = reinterpret_cast<void 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;
}</t></void></code>

2. Use a Function Object:

• Utilize a technique similar to Boost.Bind's mem_fun to create a function object that wraps the member function.
• Template the function object to support different object types.

Other Considerations:

• Ensure control over the user data returned by lua_touserdata as it cannot contain a valid pointer-to-member.
• Consider wrapping the member function in a class method or static function if you require additional state.

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