Example code for my talk on object layouts in the C++ user group Karlsruhe (https://www.meetup.com/de-DE/C-User-Group-Karlsruhe/events/jphqdmyzgbnb/)

animals.hpp

// by Andreas Fried

#include <cstring>

#include <iostream>
#include <string>

class Animal {
protected:
    const std::string name;
public:
    Animal() : name("") { }
    Animal(std::string name) : name(name) { }

    virtual void make_a_noise() = 0;
};

class Dog : public Animal {
public:
    Dog() : Animal() { }
    Dog(std::string name) : Animal(name) { }

    virtual void make_a_noise() {
        std::cout << name << ": Wuff!" << std::endl;
    }
};

class Cat : public Animal {
public:
    Cat() : Animal() { }
    Cat(std::string name) : Animal(name) { }

    virtual void make_a_noise() {
        std::cout << name << ": Miau!" << std::endl;
    }
};

become.cpp

// by Andreas Fried

#include "animals.hpp"

template <class T> void become(void *obj) {
    T tmp;
    memcpy(obj, &tmp, sizeof(void*));
}

int main() {
    Animal *fiffi = new Dog("fiffi");

    fiffi->make_a_noise();
    become<Cat>(fiffi);
    fiffi->make_a_noise();
}

sub_object.cpp

#include <iostream>
#include <sys/mman.h>

class A {
public:
    int tmp = 111;
    virtual void print(){
        std::cout << "I belong to A" << std::endl;
    }
};

class B : public virtual A {
    int tmp = 444;
};

class NotA {
public:
    virtual void not_print(){
        std::cout << "Who's A?" << std::endl;
    }
};


void make_writable(void* ptr){
    size_t * aligned_ptr = (size_t *) ((size_t) ptr &~(4096-1));
    mprotect(aligned_ptr, 8000, PROT_WRITE|PROT_EXEC|PROT_READ );
}

template <typename T>
void change_name(T* obj, const std::string &name){
    void** vtbl = ((void***)obj)[0];
    void** type_info = (void**)vtbl[-1];
    make_writable(type_info);
    char* str = (char*)new std::string(std::move(name));
    type_info[0] = (void*)str;
}

template <typename T1, typename T2>
void change_vbase_ptr(T1 *obj, T2 *other){
    void** vtbl = ((void***)obj)[0];
    size_t * aligned_vtbl = (size_t *) ((size_t) vtbl &~(4096-1));
    mprotect(aligned_vtbl, 1024, PROT_WRITE|PROT_EXEC|PROT_READ );
    ((size_t*)vtbl)[-3] = (size_t)other - (size_t)obj;
}

void test_print(A *obj){
    obj->print();
}



int main(){
    B *b = new B();
    test_print(b);
    change_vbase_ptr(b, new NotA());
    test_print(b);

    A a;
    std::cout << typeid(a).name() << std::endl;
    change_name(&a, "Hello World!");
    std::cout << typeid(a).name() << std::endl;
}

type_name.cpp

#include <iostream>
#include <utility>
#include <sys/mman.h>
#include <cstring>

void make_writable(void* ptr){
    size_t * aligned_ptr = (size_t *) ((size_t) ptr &~(4096-1));
    mprotect(aligned_ptr, 8000, PROT_WRITE|PROT_EXEC|PROT_READ );
}

class A {
    virtual void blub(){}
};

template <typename T>
void change_name(T* obj, const std::string &name) {
    void **vtbl = ((void ***) obj)[0];
    void **type_info = (void **) vtbl[-1];
    make_writable(type_info);
    const char *str = (new std::string(name))->c_str();
    ((char **) type_info)[1] = (char *) str;
}

int main(){
    A a;
    std::cout << typeid(a).name() << std::endl;
    change_name(&a, "Hello World!");
    std::cout << typeid(a).name() << std::endl;
}