extconf.rb

require "mkmf"
$libs += " -lstdc++ "
create_makefile("human")

human.cxx

#include "human.hxx"

Human::Human(const char* name, int age) {
  this->_sig  = _signature;
  this->_name = name;
  this->_age  = age;
}

std::string Human::toString() const {
  std::ostringstream ret;
  ret << "Human [name="
      << this->_name
      << ", age="
      << this->_age
      << "]";
  return ret.str();
}

std::string Human::greet() const {
  std::ostringstream ret;
  ret << "My name is "
      << this->_name
      << ". I'm "
      << this->_age
      << " years old.";
  return ret.str();
}

human.hxx

#ifndef _INCLUDE_HUMAN_H_
#define _INCLUDE_HUMAN_H_

#include <string>
#include <sstream>

class Human
{
  private:
    static const int _signature = 0x123f3f7c;

  public:
    Human(const char* name, int age);
    virtual ~Human() {}

    bool isLegal() const { return this->_sig == _signature; }

    std::string getName() const { return this->_name; }
    int getAge() const { return this->_age; }
    std::string toString() const;
    std::string greet() const;

  private:
    int _sig;
    std::string _name;
    int _age;
};

#endif // _INCLUDE_HUMAN_H_

humanwrap.cxx

#include <new>
#include "ruby.h"
#include "human.hxx"

// function prototypes
extern "C" void Init_human();
static void   wrap_Human_free(Human* p);
static VALUE  wrap_Human_alloc(VALUE klass);
static VALUE  wrap_Human_init(VALUE self, VALUE _name, VALUE _age);
static VALUE  wrap_Human_toString(VALUE self);
static VALUE  wrap_Human_greet(VALUE self);
static VALUE  wrap_Human_getName(VALUE self);
static VALUE  wrap_Human_getAge(VALUE self);
static Human* getHuman(VALUE self);

static Human* getHuman(VALUE self) {
  Human* p;
  Data_Get_Struct(self, Human, p);
  return p;
}

static void wrap_Human_free(Human* p) {
  if (p->isLegal()) p->~Human();
  ruby_xfree(p);
}

static VALUE wrap_Human_alloc(VALUE klass) {
  return Data_Wrap_Struct(klass, NULL, wrap_Human_free, ruby_xmalloc(sizeof(Human)));
}

static VALUE wrap_Human_init(VALUE self, VALUE _name, VALUE _age) {
  const char* name = StringValuePtr(_name);
  int age = NUM2INT(_age);

  Human* p = getHuman(self);
  new (p) Human(name, age);

  return Qnil;
}

static VALUE wrap_Human_toString(VALUE self) {
  return rb_str_new2(getHuman(self)->toString().c_str());
}

static VALUE wrap_Human_greet(VALUE self) {
  return rb_str_new2(getHuman(self)->greet().c_str());
}

static VALUE wrap_Human_getName(VALUE self) {
  return rb_str_new2(getHuman(self)->getName().c_str());
}

static VALUE wrap_Human_getAge(VALUE self) {
  return INT2NUM(getHuman(self)->getAge());
}

void Init_human() {
  VALUE c = rb_define_class("Human", rb_cObject);

  rb_define_alloc_func(c, wrap_Human_alloc);
  rb_define_private_method(c, "initialize", RUBY_METHOD_FUNC(wrap_Human_init), 2);
  rb_define_method(c, "inspect", RUBY_METHOD_FUNC(wrap_Human_toString), 0);
  rb_define_method(c, "to_s", RUBY_METHOD_FUNC(wrap_Human_toString), 0);
  rb_define_method(c, "greet", RUBY_METHOD_FUNC(wrap_Human_greet), 0);
  rb_define_method(c, "name", RUBY_METHOD_FUNC(wrap_Human_getName), 0);
  rb_define_method(c, "age", RUBY_METHOD_FUNC(wrap_Human_getAge), 0);
}