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#include <stdlib.h>
#include <stdio.h>
#include <stddef.h>

typedef struct Person Person;
typedef struct Person {
    Person *base; // reference to itself to retain reference to original function
    char* first;
    char* last;
    void (*display)(const Person *self);
} Person;
Person* Person_new(char* first, char* last);

static void display(const Person *self) {
    printf("First: %s\n", self->first);
    printf("Last: %s\n", self->last);
}

Person* Person_new(char* first, char* last) {
    Person* self = malloc(sizeof(Person));
    self->first = first;
    self->last = last;
    self->display = display;

    self->base = self; // assigning reference to self to retain original functions in case they get overriden

    return self;
}

#define EMPLOYEE_BASE(obj) ((Person*) (obj))

typedef struct Employee Employee;
struct Employee {
    Person super;
    char* company;
    int salary;
    Person *super_ref;
};
Employee* Employee_new(char* first, char* last, char* company, int salary);

static void display2(const Person* self) {
    self->base->display(self->base);

    Employee *e = (Employee*) self;

    printf("Company: %s\n", e->company);
    printf("Salary: %d\n", e->salary);
}

Employee* Employee_new(char* first, char* last, char* company, int salary) {
    Employee* employee = malloc(sizeof(Employee));

    employee->super_ref = Person_new(first, last);
    employee->super = *employee->super_ref;
    employee->super.display = display2; // override


    employee->company = company;
    employee->salary = salary;

    return employee;
}


int main(void)
{
    Person* person = Person_new("John", "Doe");
    Employee* employee = Employee_new("Jane", "Doe", "Acme", 40000);
    person->display(person); 
    puts("---");
    EMPLOYEE_BASE(employee)->display(EMPLOYEE_BASE(employee));
}

#include <stdlib.h>
#include <stdio.h>
#include <stddef.h>

typedef struct Person Person;
typedef struct Person {
    Person *base; // reference to itself to retain reference to original function
    char* first;
    char* last;
    void (*display)(const Person *self);
    void (*destroy)(Person *self);
} Person;
Person* Person_new(char* first, char* last);

static void display(const Person *self) {
    printf("First: %s\n", self->first);
    printf("Last: %s\n", self->last);
}

static void destroy_person(Person *self)
{
    if(self == NULL)
        return;

    free(self);
}

Person* Person_new(char* first, char* last) {
    Person* self = malloc(sizeof(Person));
    self->first = first;
    self->last = last;
    self->display = display;
    self->destroy = destroy_person;

    self->base = self; // assigning reference to self to retain original functions in case they get overriden

    return self;
}

typedef struct Employee Employee;
struct Employee {
    Person super;
    char* company;
    int salary;
};
Person* Employee_new(char* first, char* last, char* company, int salary);

static void display2(const Person* self) {
    self->base->display(self->base);

    Employee *e = (Employee*) self;

    printf("Company: %s\n", e->company);
    printf("Salary: %d\n", e->salary);
}

static void destroy_employee(Person *self)
{
    if(self == NULL)
        return;

    if(self->base)
        self->base->destroy(self->base);

    free(self);
}

Person* Employee_new(char* first, char* last, char* company, int salary) {
    Employee* employee = malloc(sizeof(Employee));

    employee->super = *Person_new(first, last);
    employee->super.display = display2; // override
    employee->super.destroy = destroy_employee; // override


    employee->company = company;
    employee->salary = salary;

    return (Person*) employee;
}


int main(void)
{
    Person* person = Person_new("John", "Doe");
    Person* employee = Employee_new("Jane", "Doe", "Acme", 40000);
    person->display(person); 
    puts("---");
    employee->display(employee);

    person->destroy(person);
    employee->destroy(employee); 

}

edit3

The fact that my little changes do not have the problem of endless recursion in display2 kept me thinking about it and I've been running the code with the debugger to see why I didn't have the problem as well. And now I understand why my code does not have that problem, and your's shouldn't have that as well. In fact, I realized that there is a way of freeing the memory correctly without have a separate member like super_ref.

Let me explain:

In Employee_new you do:

employee->super = *Person_new(first, last);
employee->super.display = display2; // override

I told you that you would be leaking memory because you lose the original pointer that malloc returns. And that is true, however I failed to realize that you have in fact a pointer pointing to the original address that malloc returns: the base member of the Person struct. In Person_new you do

self->base = self;

and this solves the problem with the overloading and the freeing, even though we both didn't realize it at the time.

So again, when you do

employee->super = *Person_new(first, last);

you are dereferencing the pointer returned by malloc and making a copy of the object into another object. But I failed to realize is that employee->super->base points to the original location returned by malloc. So when you do

employee->super.display = display2; // override

you are indeed setting a new value to employee->super.display, but because employee->super is just a copy, the original does not change and with employee->super->base you get the orginal object.

That's why

static void display2(const Person* self) {
    self->base->display(self->base);
    ...
}

shouldn't do end in a recursion, because self->display points to display2, but self->base->display is still unchanged and points to display.

And now you can use the same behaviour for destroying the Employee object. Let's say you have destroy function pointer in Person which points to the destroy function:

void destroy_person(Person *person)
{
    if(person == NULL)
        return NULL;

    free(person);
}

and in Person_new you add:

self->destroy = destroy_person;

Now you can destroy a person object by doing person->destroy(person);.

The destroy for Employee will look similar to display2:

void destroy_employee(Person *self)
{
    if(self == NULL)
        return;

    if(self->base)
        self->base->destroy(self->base);

    free(self);
}

because self->base is still pointing to the original object, self->base->destroy points to destroy_person. Now in Employee_new you have to add

employee->super.display = display2; // override
employee->super.destroy = destroy_employee; // override

and in main you just do person->destroy(person) and employee->destroy(employee);. I've checked the code above (I've updated with the destroy functions) with valgrind and it told me that everything was freed correctly.

int Person_init(Person *self, char* first, char* last))
{
    if(self == NULL || first == NULL || last == NULL)
        return NULL;

    self->first = first;
    self->last = last;
    self->display = display;

    self->base = self; // assigning reference to self to retain original functions in case they get overriden

    return 1;
}

Person* Person_new(char* first, char* last) {
    Person* self = malloc(sizeof *self);

    if(self == NULL)
        return NULL;

    if(init_person(self, first, last) == 0)
    {
        free(self);
        return NULL;
    }

    return self;
}

Person* Employee_new(char* first, char* last, char* company, int salary) {
    Employee* employee = malloc(sizeof *employee);
    if(employee == NULL || char == NULL || company == NULL)
        return NULL;

    if(Person_init(&employee->super, first, last) == 0)
    {
        free(employee);
        return NULL;
    }
    employee->super.display = display; // override

    employee->company = company;
    employee->salary = salary;

    return (Person *)employee;
}

int Person_init(Person *self, char* first, char* last))
{
    if(self == NULL || first == NULL || last == NULL)
        return NULL;

    self->first = first;
    self->last = last;
    self->display = display;

    self->base = self; // assigning reference to self to retain original functions in case they get overriden

    return 1;
}

Person* Person_new(char* first, char* last) {
    Person* self = malloc(sizeof *self);

    if(self == NULL)
        return NULL;

    if(Person_init(self, first, last) == 0)
    {
        free(self);
        return NULL;
    }

    return self;
}

Person* Employee_new(char* first, char* last, char* company, int salary) {
    if(first == NULL || last == NULL || company == NULL)
        return NULL;

    Employee* employee = malloc(sizeof *employee);
    if(employee == NULL)
        return NULL;

    if(Person_init(&employee->super, first, last) == 0)
    {
        free(employee);
        return NULL;
    }
    employee->super.display = display; // override

    employee->company = company;
    employee->salary = salary;

    return (Person *)employee;
}

edit2

OP said in the comments*

I'm wondering what's wrong with the freeing the base pointer, I'm assigning it right away to the new value free(&employee->super); employee->super = *super;

Sorry, it seem to me you don't have a real understanding of memory managment and what the * dereferencing operator does.

First of all, you have no guarantee that after free(ptr) you can access the contents of ptr. After the free, ptr points to an invalid location and accessing/dereferencing it is undefined behaviour.

Secondly

employee->super = *Person_new(first, last);

is the same as doing

Person *tmp = Person_new(first, last);
employee->super = *tmp;

*tmp is dereferencing tmp, the type is Person not Person*. In C when you do an assigment with struct objects (not pointers) you are copying bit by bit the bit pattern into a new object. That means that the bit pattern of employee->super is the same as the bit pattern of *tmp, but they are not the sam object, the reside at different places in memory.

You can only call free(ptr) when ptr stores the address returned by either malloc or realloc. If you do free(&employee->super) you are passing a completely different address to that one that malloc returned in Person_new, this is undefined behaviour, you cannot do that.

There is no other way for you, you have to store the original pointer in the struct, that's the reason why I added the Person *super_ref in the struct, so that the original pointer is not lost.

We have a major problem, this implementation is causing recursion at

I made small changes to your code, I don't have this problem:

#include <stdlib.h>
#include <stdio.h>
#include <stddef.h>

typedef struct Person Person;
typedef struct Person {
    Person *base; // reference to itself to retain reference to original function
    char* first;
    char* last;
    void (*display)(const Person *self);
} Person;
Person* Person_new(char* first, char* last);

static void display(const Person *self) {
    printf("First: %s\n", self->first);
    printf("Last: %s\n", self->last);
}

Person* Person_new(char* first, char* last) {
    Person* self = malloc(sizeof(Person));
    self->first = first;
    self->last = last;
    self->display = display;

    self->base = self; // assigning reference to self to retain original functions in case they get overriden

    return self;
}

#define EMPLOYEE_BASE(obj) ((Person*) (obj))

typedef struct Employee Employee;
struct Employee {
    Person super;
    char* company;
    int salary;
    Person *super_ref;
};
Employee* Employee_new(char* first, char* last, char* company, int salary);

static void display2(const Person* self) {
    self->base->display(self->base);

    Employee *e = (Employee*) self;

    printf("Company: %s\n", e->company);
    printf("Salary: %d\n", e->salary);
}

Employee* Employee_new(char* first, char* last, char* company, int salary) {
    Employee* employee = malloc(sizeof(Employee));

    employee->super_ref = Person_new(first, last);
    employee->super = *employee->super_ref;
    employee->super.display = display2; // override


    employee->company = company;
    employee->salary = salary;

    return employee;
}


int main(void)
{
    Person* person = Person_new("John", "Doe");
    Employee* employee = Employee_new("Jane", "Doe", "Acme", 40000);
    person->display(person); 
    puts("---");
    EMPLOYEE_BASE(employee)->display(EMPLOYEE_BASE(employee));
}

It prints

$ ./b 
First: John
Last: Doe
---
First: Jane
Last: Doe
Company: Acme
Salary: 40000