Obtaining a pointer to Lua object instance in C++

Question

I am using Luabind to expose a base class from C++ to Lua from which I can derive classes in Lua. This part works correctly and I am able to call C++ methods from my derived class in Lua.

Now what I want to do is obtain a pointer to the Lua-based instance in my C++ program.

C++ -> Binding

class Enemy {
private:
  std::string name;

public:
  Enemy(const std::string& n) 
    : name(n) { 
  }

  const std::string& getName() const { 
    return name; 
  }

  void setName(const std::string& n) { 
    name = n; 
  }

  virtual void update() { 
    std::cout << "Enemy::update() called.\n"; 
  }
};

class EnemyWrapper : public Enemy, public luabind::wrap_base {
public:
  EnemyWrapper(const std::string& n) 
    : Enemy(n) { 
  }

  virtual void update() { 
    call<void>("update"); 
  }

  static void default_update(Enemy* ptr) {
    ptr->Enemy::update();
  }

};

// Now I bind the class as such:
module(L)
[
class_<Enemy, EnemyWrapper>("Enemy")
  .def(constructor<const std::string&, int>())
    .property("name", &Enemy::getName, &Enemy::setName)
    .def("update", &Enemy::update, &EnemyWrapper::default_update)
];

Lua-based Derived Class

class 'Zombie' (Enemy)

function Zombie:__init(name)
    Enemy.__init(self, name)
end

function Zombie:update()
    print('Zombie:update() called.')
end

Now let's say I have the following object created from Lua:

a = Zombie('example zombie', 1)

How can I get a reference of that object as a pointer to the base class in C++?

My previous question stackoverflow.com/questions/1416797/… . You have to register the object with the Lua Registry (in the native side) — jameszhao00
– jameszhao00, Commented Sep 17, 2009 at 23:42
See my answer to jameszhao00's question. You use the Lua Registry to get an integer value that can refer to a specific Lua object. That integer value is analogous to a pointer, and can be used to retrieve the object later. — RBerteig
– RBerteig, Commented Sep 18, 2009 at 8:40

RBerteig · Accepted Answer · 2009-09-18 22:00:33Z

5

If in Lua you do

zombie = Zombie('example zombie', 1)

then you can get the value of the zombie like this:

object_cast<Enemy*>(globals(L)["zombie"]);

(object_cast and globals are members of the luabind namespace, L is your Lua state) This assumes you know the names of the variables you create in Lua.

You can always call a C++ method from Lua that takes a pointer to Enemy:

void AddEnemy(Enemy* enemy) { /* ... */ }
//...
module(L) [ def("AddEnemy", &AddEnemy) ]

and call it from Lua

a = Zombie("zombie", 1);
AddEnemy(a)

Be aware that if you do

AddEnemy(Zombie("temp zombie", 1));

Lua will delete the "temp zombie" after the method call and invalidate any pointers to the object.

edited Sep 18, 2009 at 22:00

RBerteig

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answered Sep 18, 2009 at 8:50

sbk

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2 Comments

Zack The Human Over a year ago

Thanks! This did the trick. Thanks also for the tip about the "temp zombie". Can this garbage collecting be worked around by using smart/shared pointers?

RBerteig Over a year ago

@sbk, I cleaned up the formatting. SO's Markdown apparently doesn't like code paragraphs near bullet lists, so I dropped the bullets in favor of having the code be clear. I put the Lua code in <pre> tags without indent for consistency with the C++ code.

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