仮想関数テーブルシミュレーション

vtbl.cpp

#include <iostream>

class drawable
{
    struct vtable
    {
        void(*draw)(void*);
    };

    template <typename T>
    struct vtable_initializer
    {
        static vtable vtbl;

        static void draw(void* this_)
        {
            static_cast<T*>(this_)->draw();
        }
    };
    void* this_;
    vtable* vptr_;

public:
    template <typename T>
    drawable(T& value)
        : this_{std::addressof(value)}, vptr_{&vtable_initializer<T>::vtbl}
    {}

    void draw() const
    {
        vptr_->draw(this_);
    }
};

template <typename T>
drawable::vtable drawable::vtable_initializer<T>::vtbl = {&drawable::vtable_initializer<T>::draw};

struct X {
    void draw() const
    {
        std::cout << "a\n";
    }
};

int main()
{
    X x;
    drawable d = x;
    d.draw();
}