v8-jsobjects.cc

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

const int kHeapObjectTag = 1;
const int kHeapObjectTagSize = 2;
const intptr_t kHeapObjectTagMask = (1 << kHeapObjectTagSize) - 1;

inline static bool HasHeapObjectTag(const char* value) {
  return ((reinterpret_cast<intptr_t>(value) & kHeapObjectTagMask) ==
          kHeapObjectTag);
}

class Map {
 public:
  enum InstanceType {
    JS_OBJECT,
    JS_ARRAY,
    JS_STRING
  };
  void set_instance_type(InstanceType instance_type) {
    instance_type_ = instance_type;
  }

  InstanceType instance_type() {
    return instance_type_;
  }
 private:
  InstanceType instance_type_;
};

const int kHeaderSize = sizeof(Map);
typedef char byte;
typedef char* Address;

class HeapObject {
 public:
  char value() {return reinterpret_cast<Address>(this)[0];}
  Map::InstanceType instance_type() {
    return reinterpret_cast<Map*>(
        reinterpret_cast<Address>(this) - kHeaderSize)->instance_type();
  }
  void Free() {
    auto top = reinterpret_cast<Address>(this) - kHeaderSize - kHeapObjectTag;
    free(top);
  }
 protected:
  static Address NewType(Map::InstanceType instance_type, size_t size) {
    auto allocated = reinterpret_cast<Address>(
        malloc(sizeof(byte) * (size + kHeaderSize + kHeapObjectTag)));
    auto map = reinterpret_cast<Map*>(allocated);
    map->set_instance_type(instance_type);
    return allocated + kHeaderSize + kHeapObjectTag;
  }
};

class JSObject: public HeapObject {
 public:
  static JSObject* New() {
    auto a = NewType(Map::JS_OBJECT, 1);
    a[0] = 'o';
    return reinterpret_cast<JSObject*>(a);
  }
};

class JSArray: public JSObject {
 public:
  static JSArray* New() {
    auto a = NewType(Map::JS_ARRAY, 1);
    a[0] = 'a';
    return reinterpret_cast<JSArray*>(a);
  }
};

class JSString: public JSObject {
 public:
  static JSString* New() {
    auto a = NewType(Map::JS_STRING, 1);
    a[0] = 's';
    return reinterpret_cast<JSString*>(a);
  }
};

int main() {
  JSObject* objects[] = {
    JSObject::New(),
    JSArray::New(),
    JSString::New()
  };

  for (int i = 0; i < 3; i++) {
    auto o = objects[i];
    switch (o->instance_type()) {
      case Map::JS_OBJECT:
        printf("%c\n", o->value());
        break;
      case Map::JS_ARRAY:
        printf("%c\n", o->value());
        break;
      case Map::JS_STRING:
        printf("%c\n", o->value());
        break;
    }
  }

  for (int i = 0; i < 3; i++) {
    objects[i]->Free();
  }
}