Boost::signals2 - descruction of an object with the slot

Question

Consider this:

#include <boost/signals2.hpp>
#include <iostream>

struct object_with_slot
{
void operator()()
{
   std::cout << "Slot called!" << std::endl;
   member = 50500;
}
int member;
};


int main()
{
boost::signals2::signal<void ()> sig;

object_with_slot * ptr = new object_with_slot;
sig.connect(*ptr);

delete ptr;

sig();
}

Output is "Slot called!" and no crash or anything. That's why I have a few questions:

1) Why there is no crash?

2) Why there is no crash even if the slot function assigns something to object which doesn't exist?

3) How can I make the signal automatically track the lifetime of its slots? I mean when the slot is destroyed, it gets disconnected.

The question number 3 is the most important, as I need to implement observer pattern and very often lifetime of observers (slots) won't be static (for the whole time when app is running).

Answer 1

1) You're lucky. If not, you'll get a segmentation fault.

2) The memory was not overwritten in any way.

3) You could use slot::track to automatically disconnect when the tracked object gets deleted. Boost.Signals2 could track objects that are managed by boost::shared_ptr.

#include <boost/signals2.hpp>
#include <boost/shared_ptr.hpp>

struct object_with_slot
{
    void operator()()
    {
       std::cout << "Slot called!" << std::endl;
       member = 50500;
    }
    int member;
};

//
int main()
{
    typedef boost::signals2::signal<void ()> sig_type;
    sig_type sig;

    {
        boost::shared_ptr<object_with_slot> ptr(new object_with_slot);
        sig.connect(sig_type::slot_type(*ptr).track(ptr));

        // 'object_with_slot' managed by ptr is destroyed
    }

    sig(); // 'object_with_slot' not called here.

    return 0;
}

UPDATE:
Added code to track objects for std::shared_ptr and std::weak_ptr:

#include <memory>
#include <boost/signals2.hpp>

// added specializations for std::weak_ptr and std::shared_ptr
namespace boost
{
  namespace signals2
  {
    template<typename T> struct weak_ptr_traits<std::weak_ptr<T> >
    {
      typedef std::shared_ptr<T> shared_type;
    };

    template<typename T> struct shared_ptr_traits<std::shared_ptr<T> >
    {
      typedef std::weak_ptr<T> weak_type;
    };
  }
}

struct object_with_slot
{
    void operator()()
    {
       std::cout << "Slot called!" << std::endl;
       member = 50500;
    }
    int member;
};

//
int main()
{
    typedef boost::signals2::signal<void ()> sig_type;
    sig_type sig;

    std::shared_ptr<object_with_slot> ptr(new object_with_slot);
    sig.connect(sig_type::slot_type(*ptr).track_foreign(ptr)); // ptr is tracked

    sig();

    return 0;
}

Answer 2

1 and 2) In fact it is an undefined behaviour. You employed the dereference operator, now connect has the value of the object_with_slot, its address is free to be assigned by memory manager to any other process. By coincidence it is still a "valid address". And ptr is free to be assigned to any other value without cause memory leak.

Try something like this and you will see that explodes everytime

#include <boost/signals2.hpp>
#include <iostream>

struct object_with_slot
{
    object_with_slot()
    {
        member = new int(10);
    }

    ~object_with_slot()
    {
        delete member; //comment this line and everything works again
    }
    void operator()()
    {
        std::cout << "Slot called!" << std::endl;
        *member = 50500; //it was destroyed above
    }
    int *member;
};


int main()
{
    boost::signals2::signal<void ()> sig;

    object_with_slot * ptr = new object_with_slot;
    sig.connect(*ptr);

    delete ptr;
    ptr = 0x0;

    sig();
}

3) You can put another signal on destructor of object_with_slot, then it can notify when it is called.

Answer 3

Very dangerous examples are given. Take a look:

#include <iostream>
#include <memory>
#include <boost/signals2.hpp>

struct object_with_slot
{
    object_with_slot() {
        std::cout << "ctor\n";
    }

    object_with_slot(const object_with_slot &) {
        std::cout << "cctor\n";
    }

    ~object_with_slot() {
        std::cout << "dtor\n";
    }

    void operator()()
    {
       std::cout << "Slot called!" << std::endl;
       member = 50500;
    }
    int member;
};

//
int main()
{
    typedef boost::signals2::signal<void ()> sig_type;
    sig_type sig;

    std::shared_ptr<object_with_slot> ptr(new object_with_slot);
    sig.connect(sig_type::slot_type(*ptr).track_foreign(ptr)); // ptr is tracked

    sig();

    return 0;
}

How do you think, what does this code out (g++ 4.8.1, libboost 1.54)?

ctor
cctor
cctor
cctor
cctor
cctor
cctor
cctor
cctor
dtor
dtor
dtor
dtor
dtor
cctor
dtor
cctor
dtor
dtor
dtor
cctor
dtor
Slot called!
dtor
dtor

I don't think, that this behaviour was expected. Because we pass copy (by value) of *ptr (instance of object_with_slot) to the connect method. It might be solved, for example, by reference wrappers:

sig.connect(sig_type::slot_type(boost::ref(*ptr)).track_foreign(ptr)); // ptr is tracked

Be careful with templates and types.