address.c

#include "ruby.h"

struct Address {
    char * town;
};

static VALUE address_alloc(VALUE klass) {
    struct Address* address = malloc(sizeof(struct Address));
    VALUE obj = Data_Wrap_Struct(klass, 0, free, address);
    
    return obj;
}

static VALUE wrap_address_get_town(VALUE self) {
    struct Address * address;
    Data_Get_Struct(self, struct Address, address);
    return rb_str_new2(address->town);
}

static VALUE address_initialize(VALUE self, VALUE town) {
    struct Address * address;
    Data_Get_Struct(self, struct Address, address);
    
    address->town = strdup(StringValueCStr(town));
    return self;
}

void Init_my_extension() {
    VALUE address_wrapper_class = rb_define_class("Address", rb_cObject);
    rb_define_alloc_func(address_wrapper_class, address_alloc);
    rb_define_method(address_wrapper_class, "initialize", address_initialize, 1);
    rb_define_method(address_wrapper_class, "town", wrap_address_get_town, 0);
}

extconf.rb

require 'mkmf'

create_makefile('my_extension')

ruby.rb

require './my_extension'

class Person
  attr_accessor :address
end


person1 = Person.new

person1.address = Address.new('London')

puts person1.address.town