Example of using virtual functions to dynamically construct a functor machine

dynamic_machine.cc

// Compile with -std=c++14

#include <array>
#include <memory>
#include <iostream>
#include <algorithm>

struct A {
    virtual void operator()() = 0;
    std::array<int,10> data;
};

struct B: public A {
    void operator()() override {
        for(auto &&a : data) {
            a = 1;
        }
    }
};

struct C: public A {
    void operator()() override {
        for(auto &&a : data) {
            a = 2;
        }
    }
};

static_assert(sizeof(A) == sizeof(B), "fail 1");
static_assert(sizeof(A) == sizeof(C), "fail 2");

int main() {
    // Allocate buffer and store it in a memory managing object
    auto buffer = std::make_unique<char[]>(2*sizeof(A));
    A* p = reinterpret_cast<A*>(buffer.get());

    // Use placement new to construct a B object and a C object
    new(p) B;
    new(p+1) C;

    // execute each functor
    std::for_each(p, p+2, [](auto &obj) {
        obj();
    });

    // output the state of data for each object
    std::for_each(p, p+2, [](auto &obj) {
        for(auto &&a : obj.data) {
            std::cout << a << " ";
        }
        std::cout << "\n";
    });
}