Accessing element of array of pointers to member functions

Question

I am having some troubles with member function pointers. How do I fix this, and why doesn't this work? The issue is inside main()... or so I am guessing!

#include <iostream>
#include <functional>

template<typename T>
using FnctPtr = void(T::*)(); // pointer to member function returning void

class Base {    
public:
    Base() : vtable_ptr{ &virtual_table[0] } {}
    void foo() {
        std::cout << "Base";
    }
    void x() { 
        std::cout << "X";
    }

 
private:
    // array of pointer to member function returning void
    inline static FnctPtr<Base> virtual_table[2] = { &Base::foo, &Base::x };

public:
    FnctPtr<Base>* vtable_ptr;
};

class Derived : public Base {
public:
    Derived() {
        vtable_ptr = reinterpret_cast<FnctPtr<Base>*>(&virtual_table[0]);
    }
    void foo() /* override */ {
        std::cout << "Derived";
    }

public:
    inline static FnctPtr<Derived> virtual_table[2] = { &Derived::foo, &Base::x };
};

int main() {
    Base* base = new Base();
    base->vtable_ptr[0](); // Issue here
    delete base;
}

Answer 1

The type of vtable_ptr is void (Base::**)() while the type of base->vtable_ptr[0] is void (Base::*)(). The syntax that you're using for the call is incorrect.

How do I fix this

The correct syntax of using the pointer to member function void (Base::*)() in your example would be as shown below:

(base->*(base->vtable_ptr[0]))();

Note that there was no need for the Derived class in the minimal reproducible example that you've provided.

Working demo